- Email: [email protected]
Hydrocracking of asphaltene with metal catalysts supported on SBA-15
Applied Catalysis A: General 252 (2003) 193–204
Hydrocracking of asphaltene with metal catalysts supported on SBA-15 Enkhsaruul Byambajav, Yasuo Ohtsuka∗ Research Center for Sustainable Materials Engineering, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan Received 12 April 2003; received in revised form 19 May 2003; accepted 19 May 2003
Abstract Hydrocracking of petroleum asphaltene with Fe and Ni catalysts loaded on SBA-15 supports has been carried out with a stainless autoclave at 573 K in pressurized H2 . Asphaltene conversion with 10 wt.% Fe catalysts with average pore diameters of 4.5–15 nm increases with increasing pore diameter up to 12 nm and reaches about 70%, but levels off beyond this value. Maltene yield shows the same dependency on the diameter and the highest value of 40% at 12–15 nm. When metal loading in the Fe catalyst with the pore diameter of 12 nm is varied between 4 and 30 wt.%, asphaltene conversion and maltene yield are the largest at 4 and 10% Fe, respectively, indicating the presence of the optimum Fe loading for maltene formation. The use of the 10% Ni catalyst in place of the 10% Fe catalyst lowers the conversion slightly but improves selectivity to maltene greatly. This means that the Ni is suitable for selective conversion of asphaltene to maltene, probably because of higher hydrogenation ability. The X-ray diffraction (XRD) measurements of toluene-insoluble (TI) fractions recovered after hydrocracking and the analyses of SO2 evolved during the temperature-programmed oxidation (TPO) reveal the formation of Fe–S and Ni–S phases as surface species. The catalysis of asphaltene conversion by the sulfided forms is discussed. © 2003 Elsevier B.V. All rights reserved. Keywords: Hydrocracking of asphaltene; Iron and nickel catalysts; SBA-15 supports
1. Introduction Declining reserves of sweet crude oils and environmental issues of sulfur and particulate matter emissions have been increasing the importance of catalytic upgrading of heavy oils and petroleum residues to middle distillates. Such oils and residues include significant amounts of asphaltenes, which are the heaviest components and are frequently responsible for catalyst deactivation in the hydrotreating processes [1–4]. Since the molecular sizes of asphaltene micelles and ∗ Corresponding author. Tel.: +81-22-217-5653; fax: +81-22-217-5655. E-mail address: [email protected] (Y. Ohtsuka).
aggregates exceed more than 2 nm, according to earlier research about asphaltene molecules [5–8], it has been accepted that the mesopores (defined as the pores with the diameters of 2–50 nm) in catalyst supports for hydroprocessing of heavy residues play crucial roles in efficient cracking of asphaltenes and consequent avoidance of catalyst deactivation [2,9–14]. On the other hand, recently developed mesoporous silica materials, such as FSM-16 [15], MCM-41 [16], and SBA-15 [17], have been suggested as sites and spaces of adsorption, separation, synthesis and reaction for large molecules. It is of interest to utilize such materials as catalyst supports for hydroprocessing of heavy residues, because the mesopores may allow asphaltene micelles and aggregates to access to
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00469-1
194
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
catalytically active sites and consequently enable efficient production of light distillates with coke formation suppressed. Only a few papers have been reported on hydrocracking of vacuum gas oils [18,19] and hydrodesulfurization of residual oils [20] with Ni–Mo and Co–Mo catalysts supported on MCM-41 and SBA-15. The mesopores with the diameters of 2–4 nm may be too small to crack asphaltene molecules in the residues [20]. Since SBA-15 can readily provide large pore diameters of more than 5 nm and possess high hydrothermal stability [17], compared with FSM-16 and MCM-41 having the same type of hexagonal array of uniform mesopores, the present authors have been working on the utilization of SBA-15 as supports of metal catalysts for asphaltene cracking [21–23]. We have shown that, when 10% Fe is loaded on four kinds of SBA-15 support with different pore diameters of 5–15 nm, asphaltene conversion at 573 K during cracking with the Fe/SBA-15 catalysts in an inert atmosphere increases with increasing average pore diameter up to 12 nm, but levels off beyond this value [23]. The present work, therefore, focuses first on examining whether such a dependency of asphaltene cracking with 10% Fe/SBA-15 catalysts on the pore diameter takes place under pressurized H2 or not. Since almost the same dependency is observed, the SBA-15 support with the diameter of 12 nm is mainly used, and the effects of Fe loading and metal type on hydrocracking of asphaltene at 5.0 MPa are then investigated. The changes in pore structures and chemical forms of SBA-15 supported catalysts before and after hydrocracking are finally investigated to make clear not only the structural stability of SBA-15 support but also the surface states of catalyst components.
2. Experimental 2.1. Synthesis of SBA-15 and addition of catalyst components to SBA-15 Four kinds of SBA-15 with average pore diameters of 5–15 nm were synthesized according to the method reported by Stucky and co-workers [17]. The SBA-15 with the diameter of 12 nm was usually used, unless
otherwise described. The synthesis method has been reported in detail elsewhere [17,24] and is thus simply explained below. An aqueous solution of a triblock copolymer (EO20 PO70 EO20 : EO, ethylene oxide; PO, propylene oxide) as a template, tetraethyl orthosilicate as a SiO2 source, HCl, and 1,3,5-trimethylbenzene (TMB) was first stirred at 308 K for 24 h and then subjected to post-synthesis heat treatment at 370–380 K. To synthesize the SBA-15 with the pore diameter of 12 nm, we used the 1.0 weight ratio of TMB/the template and 48 h heat treatment. As-synthesized SBA-15 was finally calcined in a stream of air at 773 K for 6 h and then used as a catalyst support. The addition of Fe or Ni nitrate to the calcined SBA-15 was performed by the impregnation method with an ethanol solution of Fe(NO3 )3 ·9H2 O or Ni(NO3 )2 ·6H2 O, respectively. A mixture of SBA-15 and the solution was stirred at 313 K for 1 h, followed by removal of ethanol under vacuum at 313 K. The material recovered was calcined again in the same manner as above and then used as a catalyst. These are denoted as Fe/SBA-15 or Ni/SBA-15 throughout this paper, Fe or Ni loading in the catalyst being 10 wt.% as the metal, unless otherwise stated. 2.2. Preparation of asphaltene and its mixture with catalyst Asphaltene, soluble in toluene but insoluble in n-heptane, was recovered from vacuum residue of crude oil from the Middle East by the precipitation method with 20-fold volume of n-heptane under ultrasonic irradiation, followed by filtration and drying under vacuum at 363 K; the recovery yield of asphaltene was 9.0 ± 0.5 wt.%. The elemental analysis is: C, 84.0; H, 7.8; N, 0.9; S, 5.3; O (by difference), 2.0 wt.%; Ni, 140; V, 400 ppm by weight. Asphaltene was first mixed with catalyst in toluene at room temperature under ultrasonic irradiation. After removal of toluene and subsequent dryness at 363 K, this mixture was then used in all hydrocracking runs, as reported earlier [21–23]. The weight ratio of asphaltene and catalyst was 1.0, unless otherwise described. 2.3. Asphaltene cracking and product separation Approximately 0.5 g of the mixture and about 1 g of tetralin as a solvent were first charged into a
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
stirred stainless autoclave. After complete replacement with pressurized H2 at the initial pressure of 5.0 MPa, the autoclave was heated at 5 K/min up to 573 K and then soaked for 1 h during stirring. Solid products after hydrocracking were separated by the Soxhlet method using n-heptane and toluene as solvents into maltene (heptane-soluble), recovered asphaltene (heptane-insoluble but toluene-soluble), and coke plus catalyst (toluene-insoluble, TI). Most runs were repeated two times. Asphaltene conversion and maltene yield are calculated by [1 − (amount of asphaltene recovered)/(amount of asphaltene fed)] and (amount of maltene)/(amount of asphaltene fed), respectively, and are expressed in wt.%. Coke yield is also estimated from the TI fraction recovered on the basis of the assumption that the weight of the catalyst added is unchanged. 2.4. Characterization The N2 adsorption isotherms of SBA-15 supports and Fe/SBA-15 and Ni/SBA-15 catalysts were measured by the conventional method. The surface area was estimated by the BET method. The pore size distribution, pore volume, and average pore diameter were determined by the BJH method, the diameter being calculated by using the volume and total surface of mesopores [17]. The X-ray diffraction (XRD) measurements were carried out by using Ni-filtered Cu K␣ radiation to make clear the crystalline forms and dispersion states of catalyst components. To examine the reducibility of fresh Fe/SBA and Ni/SBA-15 catalysts after air calcination, we performed the temperature-programmed reduction (TPR) run; the sample was heated at 10 K/min up to 1273 K under flowing 67.5 vol.% H2 /Ar, and the amount of H2 consumed in this process was analyzed on-line with a thermal conductivity detector. In addition, to clarify the sulfur forms of catalyst components in TI fractions recovered after hydrocracking, we carried out the temperature-programmed oxidation (TPO) experiments by heating at 3 K/min up to 1173 K in a stream of 10 vol.% O2 /He. The SO2 evolved was on-line determined with a gas chromatograph equipped with a chemi-luminescence detector.
195
3. Results and discussion 3.1. Pore properties of fresh Fe/SBA-15 and Ni/SBA-15 catalysts Pore size distribution for all catalysts was obtained by analyzing the N2 desorption isotherms observed. Fig. 1 shows the typical results for SBA-15 support with average pore diameter of 12 nm and for three kinds of Fe catalysts supported on the SBA-15. The support alone, denoted as 0 wt.% Fe in Fig. 1, provided an asymmetric peak at around 12 nm. Although the peak shape and position were almost unchanged when 4–30 wt.% Fe was loaded, the peak height was smaller at higher Fe loading, which means some decrease in pore volume by the incorporation of Fe particles inside the mesopores. Table 1 summarizes pore properties and surface areas of all catalysts used in the present work. Their average pore diameters (Dp ) were in the range 4.5–15 nm. The pore volume (Vp ) at a constant loading of 10 wt.% Fe increased with increasing pore diameter and reached 2.4 cm3 /g at the largest diameter
Fig. 1. Pore size distribution for SBA-15 support and Fe/SBA-15 catalysts with different iron loadings.
196
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
Table 1 Pore properties and surface areas of all fresh Fe/SBA-15 and Ni/SBA-15 catalysts used in this work Metal loading
Fresh catalyst a
4 wt.% Fe 10 wt.% Fe 10 wt.% Fe 10 wt.% Fe 10 wt.% Fe 30 wt.% Fe 10 wt.% Ni
After mixing with asphaltene b
Dp (nm)
Vp
11.6 4.5 7.6 11.8 14.9 11.5 11.7
2.3 0.97 1.8 2.3 2.4 1.5 2.1
(cm3 /g)
SBET 800 700 730 680 650 520 620
c
(m2 /g)
Vp (cm3 /g)
SBET (m2 /g)
Vp d (cm3 /g)
n.a.e 0.29 0.79 0.70 0.68 n.a. 0.75
n.a. 110 200 180 140 n.a. 150
– 0.68 1.0 1.6 1.7 – 1.4
a
Average pore diameter. Pore volume. c Surface area. d Decrease in pore volume by mixing. e Not analyzed. b
of 15 nm, though the surface area (SBET ) was maximal at 7.6 nm. When the effect of Fe loading on the pore volume was examined at a constant pore diameter of 12 nm, the lowest volume was observed at the highest loading of 30 wt.% Fe, as indicated in Fig. 1. The same tendency with the surface area was provided in Table 1. The pore volume and surface area of the 10% Ni catalyst were close to those of the 10% Fe catalyst. The lower volume and smaller area observed at higher metal loading mean that the corresponding amounts of catalyst particles are held inside the mesopores. Table 1 also shows the changes in the properties of 10% Fe and 10% Ni catalysts before and after mixing with asphaltene in toluene. When asphaltene was mixed with each catalyst, the shape and position of the peak observed in the pore size distribution were almost unchanged, but the pore volume and surface area decreased remarkably, as shown in Table 1. It is evident that the asphaltene is present inside the mesopores of the catalysts. The decrease in the volume by mixing asphaltene, denoted as Vp , is also provided in Table 1. The value was 0.68 cm3 /g for the 10% Fe catalyst with average pore diameter of 4.5 nm, and it increased with increasing pore diameter up to 12 nm but seemed to level off beyond this value. The Vp for the 10% Ni catalyst with the diameter of 12 nm was almost the same as that for the corresponding 10% Fe catalyst. These observations point out that the amount of asphaltene molecules held inside the mesopores of the 10% Fe catalyst after mixing is higher at the larger pore diameter.
3.2. Crystalline forms of fresh Fe/SBA-15 and Ni/SBA-15 catalysts Fig. 2 shows the XRD profiles for Fe and Ni catalysts with average pore diameter of 12 nm. Since all the catalysts were air-calcined at 773 K after impregnation using Fe(NO3 )3 or Ni(NO3 )2 solution, the nitrate can be expected to be transformed to Fe2 O3 and
Fig. 2. XRD profiles for Fe/SBA-15 and Ni/SBA-15 catalysts.
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
NiO, respectively. However, no diffraction lines of Fe2 O3 could be detected with the 4 and 10% Fe. Other 10% Fe catalysts with the pore diameters of 4.5, 7.6, and 15 nm also revealed the absence of any signals of Fe2 O3 in the XRD profiles [23]. It is probable that Fe2 O3 particles on all of these Fe catalysts are too fine (probably < 5 nm [25]) to be detected by XRD. As shown in Fig. 2, on the other hand, very small peaks of ␣-Fe2 O3 were detectable with the 30% Fe, and distinct signals of NiO were observed with the 10% Ni. Although the former diffraction lines were too weak for the average crystalline size to be calculated by the Debye–Scherrer method, the average size of NiO was estimated to 8.0 nm, which was lower than the pore diameter (12 nm) of the 10% Ni catalyst. These observations indicate that all of these oxide particles are present inside the mesopores, irrespective of the kind of metal, and that NiO is more readily crystallized than ␣-Fe2 O3 . In the previous small angle X-ray scattering measurements of calcined SBA-15 [24], the strong (1 0 0), weak (1 1 0) and (2 0 0) peaks were detectable at very low angles (2θ (Cu K␣) of <2◦ ) with average pore diameters of 4–6 nm, but no significant scattering signals were observed with the diameter of 12 nm, showing less organized pore structures of SBA-15 with larger pore diameters.
197
Fig. 3. Typical examples for material balances during hydrocracking of asphaltene at 573 K and 5.0 MPa.
3.3. Performances of Fe/SBA-15 and Ni/SBA-15 catalysts in asphaltene hydrocracking Typical examples for material balances during hydrocracking with 10% Fe/SBA-15 catalysts at 573 K are illustrated in Fig. 3, where yields of recovered asphaltene, maltene and coke are plotted as a function of average pore diameter of catalyst or weight ratio of catalyst/feed asphaltene. The sum of all of the yields was almost independent of the diameter and the ratio, and it was in the range of 100–106 wt.%. The balances for all runs in the present work always fell within the reasonable range of 95–107 wt.%. Fig. 4 shows the effect of average pore diameter of the 10% Fe catalyst on the performance in hydrocracking of asphaltene. Asphaltene conversion increased almost linearly with increasing pore diameter up to 12 nm and reached approximately 70%, but it leveled off beyond 12 nm. Maltene yield exhibited a quite similar dependency on the diameter and provided
Fig. 4. Performances of 10% Fe/SBA-15 with different pore diameters in hydrocracking of asphaltene.
198
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
the largest value of about 40% at 12–15 nm. Selectivity to maltene was also higher at a larger diameter. The Fe catalysts with larger pore diameters of 12–15 nm were thus suitable for maltene formation from asphaltene under the present conditions; then we could use the catalyst with the diameter of 12 nm in most further experiments. Such a dependency of asphaltene conversion on the pore diameter of the 10% Fe catalyst was also observed in the cracking of asphaltene in an atmospheric inert gas [23]. The conversion (66%) at the diameter of 12 nm in the inert gas was almost the same as that (69%) in Fig. 4, whereas maltene yield increased drastically from 7% without H2 to 39% in this work. Since the difference between asphaltene conversion and maltene yield can be regarded as coke, as shown in Fig. 3, it is evident that pressurized H2 suppresses coke formation on the Fe catalyst remarkably and is thus essential for conversion of asphaltene to maltene. The relationship between average pore diameter and asphaltene conversion, observed in Fig. 4, may be explained by the amount of asphaltene molecules held inside the mesopores of the Fe catalyst in the process of mixing it with feed asphaltene in toluene [23]. Since the amount may be estimated by the decrease in the pore volume of the catalyst after mixing with the asphaltene, the value (denoted as Vp ) is summarized in Table 1. The Vp increased with increasing pore diameter up to 12 nm, but almost leveled off beyond this value. Such a trend was quite similar to the dependency of asphaltene conversion on the diameter (Fig. 4). This similarity strongly suggests that the higher conversion observed at the larger diameter arises from the presence of a larger amount of asphaltene molecules inside the mesopores. It is well known that the molecular size of asphaltene aggregates depends on type of solvent, kind of asphaltene, its concentration in solvent, and temperature [5–7,26,27]. The aggregate size of the asphaltene used in this study might be less than 12 nm under the present mixing conditions. Almost all the aggregates can be held inside the mesopores of Fe catalysts with the pore diameter of 12 nm or more, whereas some of them may remain outside the mesopores with the diameters of 4.5–7.6 nm. Such differences might be reflected in the dependency of asphaltene conversion on the pore diameter (Fig. 4).
Fig. 5. Asphaltene conversion and maltene yield in hydrocracking with Fe and Ni catalysts supported on SBA-15 with average pore diameter of 12 nm.
Fig. 5 shows the effect of Fe loading on asphaltene conversion and maltene yield when the Fe/SBA-15 catalyst with average pore diameter of 12 nm was used. Interestingly, the conversion was the highest (75%) at 4% Fe, and it decreased almost linearly with increasing Fe loading. On the other hand, maltene yield was the largest (40%) at 10% Fe. The most active 4% Fe catalyst provided coke as the main product, whereas the 10% Fe catalyst showed the largest selectivity to maltene, about 60%. The optimum Fe loading for maltene formation thus existed under the present conditions. The relationship between the loading and asphaltene conversion will be discussed later. As shown in Fig. 5, the use of the 10% Ni/SBA-15 in place of the 10% Fe/SBA-15 resulted in lower asphaltene conversion but higher maltene yield. Selectivity to maltene with the Ni catalyst reached about 80%, which was much higher than that (60%) with the Fe catalyst. These observations means that the Ni catalyst loaded on the SBA-15 support is more suitable for selective formation of maltene from asphaltene under the present conditions, compared with the Fe catalyst. This difference may originate from the higher hydrogenation ability of the former [2].
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
199
Table 2 Effects of catalyst/asphaltene ratio and tetralin addition on the performance on 10% Fe catalysta,b Catalyst/asphaltene (weight ratio)
Tetralin added
0.25 0.50 1.0 1.0
Added Added Added Not added
a b
Asphaltene conversion (wt.%)
Maltene Yield (wt.%)
Selectivity (%)
50 54 69 68
40 42 39 31
80 78 57 46
Average pore diameter of 12 nm. At 573 K and 5.0 MPa.
Table 2 shows the effects of weight ratio of catalyst/feed asphaltene and coexistence of tetralin on the performance of 10% Fe catalyst with an average pore diameter of 12 nm. When the ratio decreased from the usual 1.0 to 0.50 and 0.25, asphaltene conversion was lower with the smaller ratio, whereas maltene yield was almost unchanged, which leads to the highest maltene selectivity of 80% at the lowest ratio of 0.25. The selectivity was equal to that (80%) observed with the 10% Ni catalyst. To decrease the amount of the Fe catalyst is therefore one of the key factors for selective formation of maltene from asphaltene. Tetralin was used as a solvent in asphaltene hydrocracking, unless otherwise stated. To examine the solvent effect, we compare the results with and without the solvent added in Table 2. The coexistence of tetralin had no significant effect of asphaltene conversion but increased maltene yield and selectivity to some extent. The increased selectivity may be ascribed to hydrogen-donating ability of tetralin, though the effect was small due to a low hydrocracking temperature of 573 K.
is transformed to Fe3 O4 but not reduced to metallic Fe even in pressurized H2 . With the Ni catalyst, on the other hand, weak but distinct XRD signals of NiO and metallic Ni could be detected, which means partial reduction of NiO to metallic Ni upon hydrocracking. Thus, Fig. 6 points out the different reducibility of the Fe and Ni catalysts. To examine this difference in more detail, the TPR runs of the fresh catalysts were carried out. The results are illustrated in Fig. 7, where the rate of H2 consumption normalized to unit weight of metal included is plotted against temperature. With the 4% Fe catalyst, the H2 consumption occurred at
3.4. Catalyst states after cracking To examine the changes in chemical forms of Fe and Ni catalysts after hydrocracking, we carried out the XRD measurements of TI fractions (coke plus catalyst) recovered. The results are shown in Fig. 6. No diffraction lines of Fe species were observed with a mixture of coke and the 4 or 10% Fe catalyst, though the XRD profile for the 4% Fe was not given in Fig. 6, whereas very small XRD peaks of Fe3 O4 were detectable with the mixture including the 30% Fe catalyst. These observations show that ␣-Fe2 O3 observed as the bulk species of the fresh 30% Fe catalyst (Fig. 2)
Fig. 6. XRD profiles for TI fractions (coke plus catalyst) recovered after hydrocracking with Fe/SBA-15 and Ni/SBA-15 catalysts.
200
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
Fig. 7. TPR profiles for fresh Fe/SBA-15 and Ni/SBA-15 catalysts.
Fig. 8. Profiles for SO2 evolved during TPO of TI fractions after hydrocracking with Fe/SBA-15 catalysts.
approximately 550 K, and the rate profile provided the main peak at 650 K and the small shoulder peak at 820 K, followed by the very slow consumption rate up to about 1100 K. As Fe loading increased to 10–30%, the rate was lower at higher Fe loading in the whole temperature range, and in particular the peak at 650 K became less clear at 30% Fe. According to earlier TPR research about 5–34% Fe catalysts loaded on conventional SiO2 supports [28,29], the main peak observed at 650 K and the following H2 consumption after 700 K may be identified to the transformation of ␣-Fe2 O3 to Fe3 O4 and the subsequent reduction to FeO and metallic Fe, respectively. The higher rate of H2 consumption at lower Fe loading in the temperature range of 550–900 K (Fig. 7) means that a smaller amount of Fe particles supported on SBA-15 can be reduced more readily. These observations suggest that the bulk species of 4–10% Fe/SBA-15 catalysts in the hydrocracking process are in more reduced forms than the Fe3 O4 (Fig. 6) observed with the 30% Fe. As shown in Fig. 7, the TPR profile for the 10% Ni catalyst exhibited two peaks at 635 and 770 K, suggesting the formation of two kinds of NiO species, which interact weakly and strongly with surface SiOH groups on the SBA-15, by analogy with the TPR results of Ni catalysts supported on conventional SiO2 [30,31]. The weakly interacting NiO alone may be
reduced to metallic Ni upon hydrocracking, because metallic Ni and NiO coexist as the XRD species after reaction, as observed in Fig. 6. The sulfur analysis in asphaltene and TI fraction (coke plus catalyst) recovered after hydrocracking revealed the retention of most of the sulfur converted in the TI fraction. It can thus be expected that the surfaces of Fe and Ni catalysts are sulfided in the hydrocracking process. Fig. 8 shows the profiles for SO2 evolved during the TPO runs of TI fractions recovered after hydrocracking with 4–30% Fe catalysts. A single and sharp peak of SO2 was observed at 725 K with feed asphaltene alone [23], and it can be identified as thiophenic forms that are the major sulfur ones in the asphaltene. The mixture of coke and the 4% or 10% Fe provided quite similar SO2 profiles and showed the main peak at 685 K, the shoulder at 600 K, and a small peak at 875 K. The former two signals may be attributed to thiophenic and non-thiophenic functionalities in the coke [32–35], and the latter peak may originate from the decomposition of Fe sulfates [33,35]. When the TI fraction after hydrocracking with the 30% Fe catalyst was subjected to the TPO run, as shown in Fig. 8, SO2 was formed in almost the same temperature region as that at lower loading of 4–10% Fe. The height of the SO2 peak, however, was much lower at 500–800 K and higher above 800 K with the
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
201
Table 3 Amounts of SO2 evolved during TPO of TI fractions recovered after hydrocracking Catalyst
4 wt.% Fe 10 wt.% Fe 30 wt.% Fe 10 wt.% Ni
Coke contenta (wt.%)
38 32 29 26
Total amount of SO2 evolvedb From coke fractiond
Conversionc (wt.%)
From metal sulfatese
230 200 110 130
Observed
Normalizedf
16 32 41 25
66 48 19 33
75 69 57 62
a
In TI fraction. Area (in arbitrary unit) estimated by integrating SO2 profile (Fig. 8). c Asphaltene conversion at 573 K in Fig. 5. d At 440–800 K in SO profile. 2 e At 800–1010 K in SO profile. 2 f Normalized to metal content in TI fraction. b
30% Fe catalyst. The TI fraction after reaction with the 10% Ni catalyst exhibited a similar TPO profile to that for the 10% Fe and showed the presence of Ni sulfates at 860 K, though this was not given in Fig. 8. The formation of Fe and Ni sulfates during TPO points out the presence of Fe–S and Ni–S phases in TI fractions, because it is likely that the sulfide species react with O2 in this process to form the corresponding sulfates, which are subsequently decomposed to evolve SO2 . Since Fe3 O4 and NiO are observed as the bulk forms after hydrocracking (Fig. 6), these oxides may be sulfided by reaction with H2 S evolved and/or directly with thiophenic sulfur forms in asphaltene during hydrocracking, according to Eqs. (1) and (2): Fe3 O4 (or FeO or Fe) + H2 S → Fe–S
(1)
NiO (or Ni) + H2 S → Ni–S
(2)
With the Fe catalysts, the more reduced forms (FeO and Fe), which are present probably as surface species at lower loading of 4–10% Fe, may also be sulfided, because smaller amounts of Fe particles supported can be reduced more readily, as shown in Fig. 7. In addition to NiO, metallic Ni detected as the bulk form of the 10% Ni catalyst (Fig. 6) may be transformed to Ni–S phases as well, according to Eq. (2). Total amounts of the SO2 evolved during the TPO runs are summarized in Table 3, where the value from coke fraction or metal sulfates is estimated separately by integrating the curve of the SO2 profile at 440–800 K or 800–1010 K, respectively, and is ex-
pressed in arbitrary units. Among the 4–30% Fe catalysts, the sum of the SO2 evolved from the coke was larger at its higher content in the TI fraction, whereas that from Fe sulfates increased with increasing Fe loading. When the latter value was normalized to Fe content in the TI and compared among the 4–30% Fe catalysts, as shown in Table 3, it was larger at lower Fe loading, which means that the proportion of Fe–S species in the catalyst increases in the sequence of 30% Fe < 10% Fe < 4% Fe. This order was the same as the dependency of asphaltene conversion on Fe loading (Table 3 and Fig. 5). Table 3 also reveals that the normalized value is lower with the 10% Ni than with the 10% Fe. This difference also corresponds to that in asphaltene conversion between the two. These observations may suggest that such sulfided species are catalytically active in asphaltene hydrocracking under the present conditions. Since no diffraction lines attributable to Fe–S and Ni–S phases were detectable after hydrocracking (Fig. 6), it is probable that the sulfided species are present at the outermost layers of the Fe and Ni catalysts. It has been reported that, when Fe and Ni catalysts impregnated on activated carbon are first sulfided and then used in hydrocracking of vacuum residue, the activity increases in the order of 1 wt.% Ni < 10 wt.% Fe < 1 wt.% Fe, and that the yields of maltene and coke are the highest at 1% Ni and 1% Fe, respectively [36]. These results are quite similar to the observations shown in Fig. 5, though feedstock, metal loading, and catalyst support are different. Since it is reasonable to
202
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
imagine that the size of catalyst particles supported is smaller at lower metal loading, smaller particles of Fe sulfide species may show higher hydrocracking activity and promote conversion of asphaltene to coke to a larger extent. As shown in Fig. 5, maltene yield increased in the order of 1% Fe ≤ 30% Fe < 10% Fe. Although higher hydrogenation activity of the presulfided Fe catalyst at larger loading of 10% Fe has been shown in hydrocracking of vacuum residue [36], the maltene yield in this work was lower with the 30% Fe than with the 10% Fe. The reason is not clear, but it might be related to the lowest reducibility of the 30% Fe catalyst (Fig. 7) and the smallest proportion of the sulfide species in it (Table 3). 3.5. Catalyst stability To examine the structural stability of Fe/SBA-15 and Ni/SBA-15 catalysts with 10% metal, we characterized them after hydrocracking and subsequent air calcination. A typical profile of the pore size distribution for the TI fraction containing the Fe catalyst is provided in Fig. 9, where the distribution for the fresh catalyst is also shown as a reference. The position of the asymmetric peak that appeared at a pore diameter of 12 nm was almost unchanged after hydrocracking, but the peak height, in other words, the pore volume was decreased considerably because of coke accumulation inside the mesopores. Fig. 9 also shows the results after air calcination at 773 K of TI fractions including the Fe and Ni catalysts. When the fractions were air-calcined to burn out the deposited coke, the pore size distribution observed was almost restored to the state of the fresh catalyst, irrespective of type of metal, though the pore volume after calcination was slightly lower in both cases. These results mean that pore structures of Fe/SBA-15 and Ni/SBA-15 catalysts are stable. Fig. 10 shows the XRD profiles after air calcination of the TI fractions. The Fe and Ni species in them can be expected to be transformed to ␣-Fe2 O3 and NiO in this process, respectively. No XRD peaks of ␣-Fe2 O3 could be detected with the Fe catalyst, as observed with the fresh one (Fig. 2), whereas the diffraction lines of NiO were detectable with the Ni catalyst. The average crystalline size of NiO was estimated to be 7.3 nm, which was nearly equal to that (8.0 nm) for the fresh catalyst (Fig. 2). The absence of ␣-Fe2 O3
Fig. 9. Changes in pore size distribution of Fe/SBA-15 and Ni/SBA-15 catalysts before hydrocracking, after hydrocracking and subsequent air calcination.
Fig. 10. XRD profiles for 10% Fe and 10% Ni catalysts after hydrocracking followed by air calcination.
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
and the unchanged crystalline size of NiO after air calcination indicate that no significant agglomeration of Fe or Ni particles takes place during hydrocracking and subsequent air calcination.
4. Conclusions Hydrocracking of oil-derived asphaltene in the coexistence of Fe/SBA-15 and Ni/SBA-15 catalysts with different pore diameters of 4.5–15 nm has been studied at 573 K under pressurized H2 . The conclusions are summarized as follows: 1. Asphaltene conversion and maltene yield at Fe loading of 10 wt.% increase with increasing pore diameter up to 12 nm and reach approximately 70 and 40%, respectively, but level off beyond 12 nm. 2. The dependency of the performance of the Fe catalyst with the pore diameter of 12 nm on metal loading of 4–30% shows the highest asphaltene conversion at 4% Fe and the largest maltene yield at 10% Fe, suggesting the optimum loading for maltene formation. 3. The use of the 10% Ni catalyst instead of the 10% Fe catalyst provides a slight decrease in asphaltene conversion but a remarkable improvement in selectivity to maltene, which means the larger effectiveness of the Ni for selective conversion of asphaltene to maltene. 4. The XRD analyses of TI fractions after hydrocracking and the SO2 measurements in the TPO runs reveal the presence of surface Fe–S and Ni–S phases, which may be catalytically active species. 5. Pore structures of the Fe and Ni catalysts are stable after hydrocracking and subsequent air calcination to burn up the coke accumulated inside the mesopores, and no significant agglomeration of catalyst particles takes place in these processes.
Acknowledgements This work was carried out partly as a research project of the Japan Petroleum Institute commissioned by the Petroleum Energy Center with a subsidy from the Ministry of International Trade and Industry of Japan.
203
References [1] T.F. Yen, G.V. Chilingarian (Eds.), Asphaltenes and Asphalts, vol. 1, Elsevier, Amsterdam, 1994, p. 1. [2] E. Furimsky, F.E. Massoth, Catal. Today 52 (1999) 381. [3] T. Kabe, A. Ishihara, W. Qian, Hydrodesulfurization and Hydrodenitrogenation, Kodansha/Wiley, Tokyo/New York, 1999, p. 325. [4] E.Y. Sheu, Energy Fuels 16 (2002) 74. [5] T.F. Yen, in: J.W. Burger (Ed.), Chemistry of Asphaltenes, Advances in Chemistry Series, vol. 195, American Chemical Society, Washington, DC, 1981, p. 39. [6] T.F. Yen, in: M. Grayson, J.I. Krochwitz (Eds.), Encyclopedia of Polymer Science and Engineering, Wiley, New York, 1988, p. 1. [7] D. Espinat, E. Rosenberg, M. Scarsella, L. Barre, D. Fenistein, D. Broseta, in: O.C. Mullins, E.Y. Sheu (Eds.), Structures and Dynamics of Asphaltenes, Plenum Press, New York, 1998, p. 145. [8] L. Artok, Y. Su, Y. Hirose, M. Hosokawa, S. Murata, M. Nomura, Energy Fuels 13 (1999) 287. [9] K.L. Gawrys, P.M. Spiecker, P.K. Kilpatrick, Am. Chem. Soc. Div. Petrol. Chem. 47 (2002) 332. [10] A. Miyauchi, Y. Inoue, PETROTECH 13 (1990) 44. [11] S. Inoue, T. Takatsuka, Y. Wada, S. Nakata, T. Ono, Catal. Today 43 (1998) 225. [12] K. Sano, Y. Nishimura, H. Shimada, Catal. Lett. 60 (1999) 83. [13] K. Sakanishi, T. Manabe, I. Watanabe, I. Mochida, J. Jpn. Petrol. Inst. 43 (2000) 10. [14] S. Terai, H. Fukuyama, K. Uehara, K. Fujimoto, J. Jpn. Petrol. Inst. 43 (2000) 17. [15] T. Yanagisawa, T. Shimizu, K. Kuroda, C. Kato, Bull. Chem. Soc. Jpn. 63 (1990) 988. [16] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [17] E. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [18] A. Corma, A. Martinez, V. Martinez-Soria, J.B. Monton, J. Catal. 153 (1995) 25. [19] S. Ahmed, Am. Chem. Soc. Div. Fuel Chem. 46 (2001) 591. [20] K.M. Reddy, B. Wei, C. Song, Catal. Today 43 (1998) 272. [21] E. Byambajav, Y. Ohtsuka, in: Proceedings of the 52nd Canadian Chemical Engineering Conference, 2002, p. 122. [22] E. Byambajav, R. Tanaka, Y. Ohtsuka, Am. Chem. Soc. Div. Fuel Chem. 48 (2003) 106. [23] E. Byambajav, Y. Ohtsuka, Fuel 82 (2003) 1571. [24] Y. Wang, M. Noguchi, Y. Takahashi, Y. Ohtsuka, Catal. Today 68 (2001) 4. [25] L.R. Radovic, P.L. Walker Jr., R.G. Jenkins, J. Catal. 82 (1983) 382. [26] J.N. Roux, D. Broseta, B. Demé, Langmuir 17 (2001) 5085. [27] R. Tanaka, R.W. Winans, J.E. Hunt, P. Thiyagarajan, S. Sato, T. Takanohashi, Am. Chem. Soc. Div. Fuel Chem. 46 (2001) 359. [28] H. Hayashi, L.Z. Chen, T. Tago, M. Kishida, K. Wakabayashi, Appl. Catal. A 231 (2002) 81.
204 [29] [30] [31] [32]
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
D.B. Bukur, C. Sivaraj, Appl. Catal. A 231 (2002) 201. R. Burch, A. Collins, Appl. Catal. 18 (1985) 389. P.S. Kumbhar, Appl. Catal. A 96 (1993) 241. E. Furimsky, Y. Yoshimura, Ind. Eng. Chem. Res. 26 (1987) 657. [33] R.B. LaCount, R.R. Anderson, S. Friedman, B.D. Blaustein, Fuel 66 (1987) 909.
[34] J. van Doorn, H.A.A. Barbolina, J.A. Moulijn, Ind. Eng. Chem. Res. 31 (1992) 101. [35] R.B. LaCount, D.G. Kern, W.P. King, R.B. LaCount Jr., D.J. Miltz Jr., A.L. Stewart, T.K. Trulli, D.K. Walker, R.K. Wicker, Fuel 72 (1993) 1203. [36] I. Nakamura, K. Fujimoto, Catal. Today 29 (1996) 245.
Hydrocracking of asphaltene with metal catalysts supported on SBA-15 Enkhsaruul Byambajav, Yasuo Ohtsuka∗ Research Center for Sustainable Materials Engineering, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan Received 12 April 2003; received in revised form 19 May 2003; accepted 19 May 2003
Abstract Hydrocracking of petroleum asphaltene with Fe and Ni catalysts loaded on SBA-15 supports has been carried out with a stainless autoclave at 573 K in pressurized H2 . Asphaltene conversion with 10 wt.% Fe catalysts with average pore diameters of 4.5–15 nm increases with increasing pore diameter up to 12 nm and reaches about 70%, but levels off beyond this value. Maltene yield shows the same dependency on the diameter and the highest value of 40% at 12–15 nm. When metal loading in the Fe catalyst with the pore diameter of 12 nm is varied between 4 and 30 wt.%, asphaltene conversion and maltene yield are the largest at 4 and 10% Fe, respectively, indicating the presence of the optimum Fe loading for maltene formation. The use of the 10% Ni catalyst in place of the 10% Fe catalyst lowers the conversion slightly but improves selectivity to maltene greatly. This means that the Ni is suitable for selective conversion of asphaltene to maltene, probably because of higher hydrogenation ability. The X-ray diffraction (XRD) measurements of toluene-insoluble (TI) fractions recovered after hydrocracking and the analyses of SO2 evolved during the temperature-programmed oxidation (TPO) reveal the formation of Fe–S and Ni–S phases as surface species. The catalysis of asphaltene conversion by the sulfided forms is discussed. © 2003 Elsevier B.V. All rights reserved. Keywords: Hydrocracking of asphaltene; Iron and nickel catalysts; SBA-15 supports
1. Introduction Declining reserves of sweet crude oils and environmental issues of sulfur and particulate matter emissions have been increasing the importance of catalytic upgrading of heavy oils and petroleum residues to middle distillates. Such oils and residues include significant amounts of asphaltenes, which are the heaviest components and are frequently responsible for catalyst deactivation in the hydrotreating processes [1–4]. Since the molecular sizes of asphaltene micelles and ∗ Corresponding author. Tel.: +81-22-217-5653; fax: +81-22-217-5655. E-mail address: [email protected] (Y. Ohtsuka).
aggregates exceed more than 2 nm, according to earlier research about asphaltene molecules [5–8], it has been accepted that the mesopores (defined as the pores with the diameters of 2–50 nm) in catalyst supports for hydroprocessing of heavy residues play crucial roles in efficient cracking of asphaltenes and consequent avoidance of catalyst deactivation [2,9–14]. On the other hand, recently developed mesoporous silica materials, such as FSM-16 [15], MCM-41 [16], and SBA-15 [17], have been suggested as sites and spaces of adsorption, separation, synthesis and reaction for large molecules. It is of interest to utilize such materials as catalyst supports for hydroprocessing of heavy residues, because the mesopores may allow asphaltene micelles and aggregates to access to
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00469-1
194
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
catalytically active sites and consequently enable efficient production of light distillates with coke formation suppressed. Only a few papers have been reported on hydrocracking of vacuum gas oils [18,19] and hydrodesulfurization of residual oils [20] with Ni–Mo and Co–Mo catalysts supported on MCM-41 and SBA-15. The mesopores with the diameters of 2–4 nm may be too small to crack asphaltene molecules in the residues [20]. Since SBA-15 can readily provide large pore diameters of more than 5 nm and possess high hydrothermal stability [17], compared with FSM-16 and MCM-41 having the same type of hexagonal array of uniform mesopores, the present authors have been working on the utilization of SBA-15 as supports of metal catalysts for asphaltene cracking [21–23]. We have shown that, when 10% Fe is loaded on four kinds of SBA-15 support with different pore diameters of 5–15 nm, asphaltene conversion at 573 K during cracking with the Fe/SBA-15 catalysts in an inert atmosphere increases with increasing average pore diameter up to 12 nm, but levels off beyond this value [23]. The present work, therefore, focuses first on examining whether such a dependency of asphaltene cracking with 10% Fe/SBA-15 catalysts on the pore diameter takes place under pressurized H2 or not. Since almost the same dependency is observed, the SBA-15 support with the diameter of 12 nm is mainly used, and the effects of Fe loading and metal type on hydrocracking of asphaltene at 5.0 MPa are then investigated. The changes in pore structures and chemical forms of SBA-15 supported catalysts before and after hydrocracking are finally investigated to make clear not only the structural stability of SBA-15 support but also the surface states of catalyst components.
2. Experimental 2.1. Synthesis of SBA-15 and addition of catalyst components to SBA-15 Four kinds of SBA-15 with average pore diameters of 5–15 nm were synthesized according to the method reported by Stucky and co-workers [17]. The SBA-15 with the diameter of 12 nm was usually used, unless
otherwise described. The synthesis method has been reported in detail elsewhere [17,24] and is thus simply explained below. An aqueous solution of a triblock copolymer (EO20 PO70 EO20 : EO, ethylene oxide; PO, propylene oxide) as a template, tetraethyl orthosilicate as a SiO2 source, HCl, and 1,3,5-trimethylbenzene (TMB) was first stirred at 308 K for 24 h and then subjected to post-synthesis heat treatment at 370–380 K. To synthesize the SBA-15 with the pore diameter of 12 nm, we used the 1.0 weight ratio of TMB/the template and 48 h heat treatment. As-synthesized SBA-15 was finally calcined in a stream of air at 773 K for 6 h and then used as a catalyst support. The addition of Fe or Ni nitrate to the calcined SBA-15 was performed by the impregnation method with an ethanol solution of Fe(NO3 )3 ·9H2 O or Ni(NO3 )2 ·6H2 O, respectively. A mixture of SBA-15 and the solution was stirred at 313 K for 1 h, followed by removal of ethanol under vacuum at 313 K. The material recovered was calcined again in the same manner as above and then used as a catalyst. These are denoted as Fe/SBA-15 or Ni/SBA-15 throughout this paper, Fe or Ni loading in the catalyst being 10 wt.% as the metal, unless otherwise stated. 2.2. Preparation of asphaltene and its mixture with catalyst Asphaltene, soluble in toluene but insoluble in n-heptane, was recovered from vacuum residue of crude oil from the Middle East by the precipitation method with 20-fold volume of n-heptane under ultrasonic irradiation, followed by filtration and drying under vacuum at 363 K; the recovery yield of asphaltene was 9.0 ± 0.5 wt.%. The elemental analysis is: C, 84.0; H, 7.8; N, 0.9; S, 5.3; O (by difference), 2.0 wt.%; Ni, 140; V, 400 ppm by weight. Asphaltene was first mixed with catalyst in toluene at room temperature under ultrasonic irradiation. After removal of toluene and subsequent dryness at 363 K, this mixture was then used in all hydrocracking runs, as reported earlier [21–23]. The weight ratio of asphaltene and catalyst was 1.0, unless otherwise described. 2.3. Asphaltene cracking and product separation Approximately 0.5 g of the mixture and about 1 g of tetralin as a solvent were first charged into a
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
stirred stainless autoclave. After complete replacement with pressurized H2 at the initial pressure of 5.0 MPa, the autoclave was heated at 5 K/min up to 573 K and then soaked for 1 h during stirring. Solid products after hydrocracking were separated by the Soxhlet method using n-heptane and toluene as solvents into maltene (heptane-soluble), recovered asphaltene (heptane-insoluble but toluene-soluble), and coke plus catalyst (toluene-insoluble, TI). Most runs were repeated two times. Asphaltene conversion and maltene yield are calculated by [1 − (amount of asphaltene recovered)/(amount of asphaltene fed)] and (amount of maltene)/(amount of asphaltene fed), respectively, and are expressed in wt.%. Coke yield is also estimated from the TI fraction recovered on the basis of the assumption that the weight of the catalyst added is unchanged. 2.4. Characterization The N2 adsorption isotherms of SBA-15 supports and Fe/SBA-15 and Ni/SBA-15 catalysts were measured by the conventional method. The surface area was estimated by the BET method. The pore size distribution, pore volume, and average pore diameter were determined by the BJH method, the diameter being calculated by using the volume and total surface of mesopores [17]. The X-ray diffraction (XRD) measurements were carried out by using Ni-filtered Cu K␣ radiation to make clear the crystalline forms and dispersion states of catalyst components. To examine the reducibility of fresh Fe/SBA and Ni/SBA-15 catalysts after air calcination, we performed the temperature-programmed reduction (TPR) run; the sample was heated at 10 K/min up to 1273 K under flowing 67.5 vol.% H2 /Ar, and the amount of H2 consumed in this process was analyzed on-line with a thermal conductivity detector. In addition, to clarify the sulfur forms of catalyst components in TI fractions recovered after hydrocracking, we carried out the temperature-programmed oxidation (TPO) experiments by heating at 3 K/min up to 1173 K in a stream of 10 vol.% O2 /He. The SO2 evolved was on-line determined with a gas chromatograph equipped with a chemi-luminescence detector.
195
3. Results and discussion 3.1. Pore properties of fresh Fe/SBA-15 and Ni/SBA-15 catalysts Pore size distribution for all catalysts was obtained by analyzing the N2 desorption isotherms observed. Fig. 1 shows the typical results for SBA-15 support with average pore diameter of 12 nm and for three kinds of Fe catalysts supported on the SBA-15. The support alone, denoted as 0 wt.% Fe in Fig. 1, provided an asymmetric peak at around 12 nm. Although the peak shape and position were almost unchanged when 4–30 wt.% Fe was loaded, the peak height was smaller at higher Fe loading, which means some decrease in pore volume by the incorporation of Fe particles inside the mesopores. Table 1 summarizes pore properties and surface areas of all catalysts used in the present work. Their average pore diameters (Dp ) were in the range 4.5–15 nm. The pore volume (Vp ) at a constant loading of 10 wt.% Fe increased with increasing pore diameter and reached 2.4 cm3 /g at the largest diameter
Fig. 1. Pore size distribution for SBA-15 support and Fe/SBA-15 catalysts with different iron loadings.
196
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
Table 1 Pore properties and surface areas of all fresh Fe/SBA-15 and Ni/SBA-15 catalysts used in this work Metal loading
Fresh catalyst a
4 wt.% Fe 10 wt.% Fe 10 wt.% Fe 10 wt.% Fe 10 wt.% Fe 30 wt.% Fe 10 wt.% Ni
After mixing with asphaltene b
Dp (nm)
Vp
11.6 4.5 7.6 11.8 14.9 11.5 11.7
2.3 0.97 1.8 2.3 2.4 1.5 2.1
(cm3 /g)
SBET 800 700 730 680 650 520 620
c
(m2 /g)
Vp (cm3 /g)
SBET (m2 /g)
Vp d (cm3 /g)
n.a.e 0.29 0.79 0.70 0.68 n.a. 0.75
n.a. 110 200 180 140 n.a. 150
– 0.68 1.0 1.6 1.7 – 1.4
a
Average pore diameter. Pore volume. c Surface area. d Decrease in pore volume by mixing. e Not analyzed. b
of 15 nm, though the surface area (SBET ) was maximal at 7.6 nm. When the effect of Fe loading on the pore volume was examined at a constant pore diameter of 12 nm, the lowest volume was observed at the highest loading of 30 wt.% Fe, as indicated in Fig. 1. The same tendency with the surface area was provided in Table 1. The pore volume and surface area of the 10% Ni catalyst were close to those of the 10% Fe catalyst. The lower volume and smaller area observed at higher metal loading mean that the corresponding amounts of catalyst particles are held inside the mesopores. Table 1 also shows the changes in the properties of 10% Fe and 10% Ni catalysts before and after mixing with asphaltene in toluene. When asphaltene was mixed with each catalyst, the shape and position of the peak observed in the pore size distribution were almost unchanged, but the pore volume and surface area decreased remarkably, as shown in Table 1. It is evident that the asphaltene is present inside the mesopores of the catalysts. The decrease in the volume by mixing asphaltene, denoted as Vp , is also provided in Table 1. The value was 0.68 cm3 /g for the 10% Fe catalyst with average pore diameter of 4.5 nm, and it increased with increasing pore diameter up to 12 nm but seemed to level off beyond this value. The Vp for the 10% Ni catalyst with the diameter of 12 nm was almost the same as that for the corresponding 10% Fe catalyst. These observations point out that the amount of asphaltene molecules held inside the mesopores of the 10% Fe catalyst after mixing is higher at the larger pore diameter.
3.2. Crystalline forms of fresh Fe/SBA-15 and Ni/SBA-15 catalysts Fig. 2 shows the XRD profiles for Fe and Ni catalysts with average pore diameter of 12 nm. Since all the catalysts were air-calcined at 773 K after impregnation using Fe(NO3 )3 or Ni(NO3 )2 solution, the nitrate can be expected to be transformed to Fe2 O3 and
Fig. 2. XRD profiles for Fe/SBA-15 and Ni/SBA-15 catalysts.
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
NiO, respectively. However, no diffraction lines of Fe2 O3 could be detected with the 4 and 10% Fe. Other 10% Fe catalysts with the pore diameters of 4.5, 7.6, and 15 nm also revealed the absence of any signals of Fe2 O3 in the XRD profiles [23]. It is probable that Fe2 O3 particles on all of these Fe catalysts are too fine (probably < 5 nm [25]) to be detected by XRD. As shown in Fig. 2, on the other hand, very small peaks of ␣-Fe2 O3 were detectable with the 30% Fe, and distinct signals of NiO were observed with the 10% Ni. Although the former diffraction lines were too weak for the average crystalline size to be calculated by the Debye–Scherrer method, the average size of NiO was estimated to 8.0 nm, which was lower than the pore diameter (12 nm) of the 10% Ni catalyst. These observations indicate that all of these oxide particles are present inside the mesopores, irrespective of the kind of metal, and that NiO is more readily crystallized than ␣-Fe2 O3 . In the previous small angle X-ray scattering measurements of calcined SBA-15 [24], the strong (1 0 0), weak (1 1 0) and (2 0 0) peaks were detectable at very low angles (2θ (Cu K␣) of <2◦ ) with average pore diameters of 4–6 nm, but no significant scattering signals were observed with the diameter of 12 nm, showing less organized pore structures of SBA-15 with larger pore diameters.
197
Fig. 3. Typical examples for material balances during hydrocracking of asphaltene at 573 K and 5.0 MPa.
3.3. Performances of Fe/SBA-15 and Ni/SBA-15 catalysts in asphaltene hydrocracking Typical examples for material balances during hydrocracking with 10% Fe/SBA-15 catalysts at 573 K are illustrated in Fig. 3, where yields of recovered asphaltene, maltene and coke are plotted as a function of average pore diameter of catalyst or weight ratio of catalyst/feed asphaltene. The sum of all of the yields was almost independent of the diameter and the ratio, and it was in the range of 100–106 wt.%. The balances for all runs in the present work always fell within the reasonable range of 95–107 wt.%. Fig. 4 shows the effect of average pore diameter of the 10% Fe catalyst on the performance in hydrocracking of asphaltene. Asphaltene conversion increased almost linearly with increasing pore diameter up to 12 nm and reached approximately 70%, but it leveled off beyond 12 nm. Maltene yield exhibited a quite similar dependency on the diameter and provided
Fig. 4. Performances of 10% Fe/SBA-15 with different pore diameters in hydrocracking of asphaltene.
198
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
the largest value of about 40% at 12–15 nm. Selectivity to maltene was also higher at a larger diameter. The Fe catalysts with larger pore diameters of 12–15 nm were thus suitable for maltene formation from asphaltene under the present conditions; then we could use the catalyst with the diameter of 12 nm in most further experiments. Such a dependency of asphaltene conversion on the pore diameter of the 10% Fe catalyst was also observed in the cracking of asphaltene in an atmospheric inert gas [23]. The conversion (66%) at the diameter of 12 nm in the inert gas was almost the same as that (69%) in Fig. 4, whereas maltene yield increased drastically from 7% without H2 to 39% in this work. Since the difference between asphaltene conversion and maltene yield can be regarded as coke, as shown in Fig. 3, it is evident that pressurized H2 suppresses coke formation on the Fe catalyst remarkably and is thus essential for conversion of asphaltene to maltene. The relationship between average pore diameter and asphaltene conversion, observed in Fig. 4, may be explained by the amount of asphaltene molecules held inside the mesopores of the Fe catalyst in the process of mixing it with feed asphaltene in toluene [23]. Since the amount may be estimated by the decrease in the pore volume of the catalyst after mixing with the asphaltene, the value (denoted as Vp ) is summarized in Table 1. The Vp increased with increasing pore diameter up to 12 nm, but almost leveled off beyond this value. Such a trend was quite similar to the dependency of asphaltene conversion on the diameter (Fig. 4). This similarity strongly suggests that the higher conversion observed at the larger diameter arises from the presence of a larger amount of asphaltene molecules inside the mesopores. It is well known that the molecular size of asphaltene aggregates depends on type of solvent, kind of asphaltene, its concentration in solvent, and temperature [5–7,26,27]. The aggregate size of the asphaltene used in this study might be less than 12 nm under the present mixing conditions. Almost all the aggregates can be held inside the mesopores of Fe catalysts with the pore diameter of 12 nm or more, whereas some of them may remain outside the mesopores with the diameters of 4.5–7.6 nm. Such differences might be reflected in the dependency of asphaltene conversion on the pore diameter (Fig. 4).
Fig. 5. Asphaltene conversion and maltene yield in hydrocracking with Fe and Ni catalysts supported on SBA-15 with average pore diameter of 12 nm.
Fig. 5 shows the effect of Fe loading on asphaltene conversion and maltene yield when the Fe/SBA-15 catalyst with average pore diameter of 12 nm was used. Interestingly, the conversion was the highest (75%) at 4% Fe, and it decreased almost linearly with increasing Fe loading. On the other hand, maltene yield was the largest (40%) at 10% Fe. The most active 4% Fe catalyst provided coke as the main product, whereas the 10% Fe catalyst showed the largest selectivity to maltene, about 60%. The optimum Fe loading for maltene formation thus existed under the present conditions. The relationship between the loading and asphaltene conversion will be discussed later. As shown in Fig. 5, the use of the 10% Ni/SBA-15 in place of the 10% Fe/SBA-15 resulted in lower asphaltene conversion but higher maltene yield. Selectivity to maltene with the Ni catalyst reached about 80%, which was much higher than that (60%) with the Fe catalyst. These observations means that the Ni catalyst loaded on the SBA-15 support is more suitable for selective formation of maltene from asphaltene under the present conditions, compared with the Fe catalyst. This difference may originate from the higher hydrogenation ability of the former [2].
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
199
Table 2 Effects of catalyst/asphaltene ratio and tetralin addition on the performance on 10% Fe catalysta,b Catalyst/asphaltene (weight ratio)
Tetralin added
0.25 0.50 1.0 1.0
Added Added Added Not added
a b
Asphaltene conversion (wt.%)
Maltene Yield (wt.%)
Selectivity (%)
50 54 69 68
40 42 39 31
80 78 57 46
Average pore diameter of 12 nm. At 573 K and 5.0 MPa.
Table 2 shows the effects of weight ratio of catalyst/feed asphaltene and coexistence of tetralin on the performance of 10% Fe catalyst with an average pore diameter of 12 nm. When the ratio decreased from the usual 1.0 to 0.50 and 0.25, asphaltene conversion was lower with the smaller ratio, whereas maltene yield was almost unchanged, which leads to the highest maltene selectivity of 80% at the lowest ratio of 0.25. The selectivity was equal to that (80%) observed with the 10% Ni catalyst. To decrease the amount of the Fe catalyst is therefore one of the key factors for selective formation of maltene from asphaltene. Tetralin was used as a solvent in asphaltene hydrocracking, unless otherwise stated. To examine the solvent effect, we compare the results with and without the solvent added in Table 2. The coexistence of tetralin had no significant effect of asphaltene conversion but increased maltene yield and selectivity to some extent. The increased selectivity may be ascribed to hydrogen-donating ability of tetralin, though the effect was small due to a low hydrocracking temperature of 573 K.
is transformed to Fe3 O4 but not reduced to metallic Fe even in pressurized H2 . With the Ni catalyst, on the other hand, weak but distinct XRD signals of NiO and metallic Ni could be detected, which means partial reduction of NiO to metallic Ni upon hydrocracking. Thus, Fig. 6 points out the different reducibility of the Fe and Ni catalysts. To examine this difference in more detail, the TPR runs of the fresh catalysts were carried out. The results are illustrated in Fig. 7, where the rate of H2 consumption normalized to unit weight of metal included is plotted against temperature. With the 4% Fe catalyst, the H2 consumption occurred at
3.4. Catalyst states after cracking To examine the changes in chemical forms of Fe and Ni catalysts after hydrocracking, we carried out the XRD measurements of TI fractions (coke plus catalyst) recovered. The results are shown in Fig. 6. No diffraction lines of Fe species were observed with a mixture of coke and the 4 or 10% Fe catalyst, though the XRD profile for the 4% Fe was not given in Fig. 6, whereas very small XRD peaks of Fe3 O4 were detectable with the mixture including the 30% Fe catalyst. These observations show that ␣-Fe2 O3 observed as the bulk species of the fresh 30% Fe catalyst (Fig. 2)
Fig. 6. XRD profiles for TI fractions (coke plus catalyst) recovered after hydrocracking with Fe/SBA-15 and Ni/SBA-15 catalysts.
200
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
Fig. 7. TPR profiles for fresh Fe/SBA-15 and Ni/SBA-15 catalysts.
Fig. 8. Profiles for SO2 evolved during TPO of TI fractions after hydrocracking with Fe/SBA-15 catalysts.
approximately 550 K, and the rate profile provided the main peak at 650 K and the small shoulder peak at 820 K, followed by the very slow consumption rate up to about 1100 K. As Fe loading increased to 10–30%, the rate was lower at higher Fe loading in the whole temperature range, and in particular the peak at 650 K became less clear at 30% Fe. According to earlier TPR research about 5–34% Fe catalysts loaded on conventional SiO2 supports [28,29], the main peak observed at 650 K and the following H2 consumption after 700 K may be identified to the transformation of ␣-Fe2 O3 to Fe3 O4 and the subsequent reduction to FeO and metallic Fe, respectively. The higher rate of H2 consumption at lower Fe loading in the temperature range of 550–900 K (Fig. 7) means that a smaller amount of Fe particles supported on SBA-15 can be reduced more readily. These observations suggest that the bulk species of 4–10% Fe/SBA-15 catalysts in the hydrocracking process are in more reduced forms than the Fe3 O4 (Fig. 6) observed with the 30% Fe. As shown in Fig. 7, the TPR profile for the 10% Ni catalyst exhibited two peaks at 635 and 770 K, suggesting the formation of two kinds of NiO species, which interact weakly and strongly with surface SiOH groups on the SBA-15, by analogy with the TPR results of Ni catalysts supported on conventional SiO2 [30,31]. The weakly interacting NiO alone may be
reduced to metallic Ni upon hydrocracking, because metallic Ni and NiO coexist as the XRD species after reaction, as observed in Fig. 6. The sulfur analysis in asphaltene and TI fraction (coke plus catalyst) recovered after hydrocracking revealed the retention of most of the sulfur converted in the TI fraction. It can thus be expected that the surfaces of Fe and Ni catalysts are sulfided in the hydrocracking process. Fig. 8 shows the profiles for SO2 evolved during the TPO runs of TI fractions recovered after hydrocracking with 4–30% Fe catalysts. A single and sharp peak of SO2 was observed at 725 K with feed asphaltene alone [23], and it can be identified as thiophenic forms that are the major sulfur ones in the asphaltene. The mixture of coke and the 4% or 10% Fe provided quite similar SO2 profiles and showed the main peak at 685 K, the shoulder at 600 K, and a small peak at 875 K. The former two signals may be attributed to thiophenic and non-thiophenic functionalities in the coke [32–35], and the latter peak may originate from the decomposition of Fe sulfates [33,35]. When the TI fraction after hydrocracking with the 30% Fe catalyst was subjected to the TPO run, as shown in Fig. 8, SO2 was formed in almost the same temperature region as that at lower loading of 4–10% Fe. The height of the SO2 peak, however, was much lower at 500–800 K and higher above 800 K with the
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
201
Table 3 Amounts of SO2 evolved during TPO of TI fractions recovered after hydrocracking Catalyst
4 wt.% Fe 10 wt.% Fe 30 wt.% Fe 10 wt.% Ni
Coke contenta (wt.%)
38 32 29 26
Total amount of SO2 evolvedb From coke fractiond
Conversionc (wt.%)
From metal sulfatese
230 200 110 130
Observed
Normalizedf
16 32 41 25
66 48 19 33
75 69 57 62
a
In TI fraction. Area (in arbitrary unit) estimated by integrating SO2 profile (Fig. 8). c Asphaltene conversion at 573 K in Fig. 5. d At 440–800 K in SO profile. 2 e At 800–1010 K in SO profile. 2 f Normalized to metal content in TI fraction. b
30% Fe catalyst. The TI fraction after reaction with the 10% Ni catalyst exhibited a similar TPO profile to that for the 10% Fe and showed the presence of Ni sulfates at 860 K, though this was not given in Fig. 8. The formation of Fe and Ni sulfates during TPO points out the presence of Fe–S and Ni–S phases in TI fractions, because it is likely that the sulfide species react with O2 in this process to form the corresponding sulfates, which are subsequently decomposed to evolve SO2 . Since Fe3 O4 and NiO are observed as the bulk forms after hydrocracking (Fig. 6), these oxides may be sulfided by reaction with H2 S evolved and/or directly with thiophenic sulfur forms in asphaltene during hydrocracking, according to Eqs. (1) and (2): Fe3 O4 (or FeO or Fe) + H2 S → Fe–S
(1)
NiO (or Ni) + H2 S → Ni–S
(2)
With the Fe catalysts, the more reduced forms (FeO and Fe), which are present probably as surface species at lower loading of 4–10% Fe, may also be sulfided, because smaller amounts of Fe particles supported can be reduced more readily, as shown in Fig. 7. In addition to NiO, metallic Ni detected as the bulk form of the 10% Ni catalyst (Fig. 6) may be transformed to Ni–S phases as well, according to Eq. (2). Total amounts of the SO2 evolved during the TPO runs are summarized in Table 3, where the value from coke fraction or metal sulfates is estimated separately by integrating the curve of the SO2 profile at 440–800 K or 800–1010 K, respectively, and is ex-
pressed in arbitrary units. Among the 4–30% Fe catalysts, the sum of the SO2 evolved from the coke was larger at its higher content in the TI fraction, whereas that from Fe sulfates increased with increasing Fe loading. When the latter value was normalized to Fe content in the TI and compared among the 4–30% Fe catalysts, as shown in Table 3, it was larger at lower Fe loading, which means that the proportion of Fe–S species in the catalyst increases in the sequence of 30% Fe < 10% Fe < 4% Fe. This order was the same as the dependency of asphaltene conversion on Fe loading (Table 3 and Fig. 5). Table 3 also reveals that the normalized value is lower with the 10% Ni than with the 10% Fe. This difference also corresponds to that in asphaltene conversion between the two. These observations may suggest that such sulfided species are catalytically active in asphaltene hydrocracking under the present conditions. Since no diffraction lines attributable to Fe–S and Ni–S phases were detectable after hydrocracking (Fig. 6), it is probable that the sulfided species are present at the outermost layers of the Fe and Ni catalysts. It has been reported that, when Fe and Ni catalysts impregnated on activated carbon are first sulfided and then used in hydrocracking of vacuum residue, the activity increases in the order of 1 wt.% Ni < 10 wt.% Fe < 1 wt.% Fe, and that the yields of maltene and coke are the highest at 1% Ni and 1% Fe, respectively [36]. These results are quite similar to the observations shown in Fig. 5, though feedstock, metal loading, and catalyst support are different. Since it is reasonable to
202
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
imagine that the size of catalyst particles supported is smaller at lower metal loading, smaller particles of Fe sulfide species may show higher hydrocracking activity and promote conversion of asphaltene to coke to a larger extent. As shown in Fig. 5, maltene yield increased in the order of 1% Fe ≤ 30% Fe < 10% Fe. Although higher hydrogenation activity of the presulfided Fe catalyst at larger loading of 10% Fe has been shown in hydrocracking of vacuum residue [36], the maltene yield in this work was lower with the 30% Fe than with the 10% Fe. The reason is not clear, but it might be related to the lowest reducibility of the 30% Fe catalyst (Fig. 7) and the smallest proportion of the sulfide species in it (Table 3). 3.5. Catalyst stability To examine the structural stability of Fe/SBA-15 and Ni/SBA-15 catalysts with 10% metal, we characterized them after hydrocracking and subsequent air calcination. A typical profile of the pore size distribution for the TI fraction containing the Fe catalyst is provided in Fig. 9, where the distribution for the fresh catalyst is also shown as a reference. The position of the asymmetric peak that appeared at a pore diameter of 12 nm was almost unchanged after hydrocracking, but the peak height, in other words, the pore volume was decreased considerably because of coke accumulation inside the mesopores. Fig. 9 also shows the results after air calcination at 773 K of TI fractions including the Fe and Ni catalysts. When the fractions were air-calcined to burn out the deposited coke, the pore size distribution observed was almost restored to the state of the fresh catalyst, irrespective of type of metal, though the pore volume after calcination was slightly lower in both cases. These results mean that pore structures of Fe/SBA-15 and Ni/SBA-15 catalysts are stable. Fig. 10 shows the XRD profiles after air calcination of the TI fractions. The Fe and Ni species in them can be expected to be transformed to ␣-Fe2 O3 and NiO in this process, respectively. No XRD peaks of ␣-Fe2 O3 could be detected with the Fe catalyst, as observed with the fresh one (Fig. 2), whereas the diffraction lines of NiO were detectable with the Ni catalyst. The average crystalline size of NiO was estimated to be 7.3 nm, which was nearly equal to that (8.0 nm) for the fresh catalyst (Fig. 2). The absence of ␣-Fe2 O3
Fig. 9. Changes in pore size distribution of Fe/SBA-15 and Ni/SBA-15 catalysts before hydrocracking, after hydrocracking and subsequent air calcination.
Fig. 10. XRD profiles for 10% Fe and 10% Ni catalysts after hydrocracking followed by air calcination.
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
and the unchanged crystalline size of NiO after air calcination indicate that no significant agglomeration of Fe or Ni particles takes place during hydrocracking and subsequent air calcination.
4. Conclusions Hydrocracking of oil-derived asphaltene in the coexistence of Fe/SBA-15 and Ni/SBA-15 catalysts with different pore diameters of 4.5–15 nm has been studied at 573 K under pressurized H2 . The conclusions are summarized as follows: 1. Asphaltene conversion and maltene yield at Fe loading of 10 wt.% increase with increasing pore diameter up to 12 nm and reach approximately 70 and 40%, respectively, but level off beyond 12 nm. 2. The dependency of the performance of the Fe catalyst with the pore diameter of 12 nm on metal loading of 4–30% shows the highest asphaltene conversion at 4% Fe and the largest maltene yield at 10% Fe, suggesting the optimum loading for maltene formation. 3. The use of the 10% Ni catalyst instead of the 10% Fe catalyst provides a slight decrease in asphaltene conversion but a remarkable improvement in selectivity to maltene, which means the larger effectiveness of the Ni for selective conversion of asphaltene to maltene. 4. The XRD analyses of TI fractions after hydrocracking and the SO2 measurements in the TPO runs reveal the presence of surface Fe–S and Ni–S phases, which may be catalytically active species. 5. Pore structures of the Fe and Ni catalysts are stable after hydrocracking and subsequent air calcination to burn up the coke accumulated inside the mesopores, and no significant agglomeration of catalyst particles takes place in these processes.
Acknowledgements This work was carried out partly as a research project of the Japan Petroleum Institute commissioned by the Petroleum Energy Center with a subsidy from the Ministry of International Trade and Industry of Japan.
203
References [1] T.F. Yen, G.V. Chilingarian (Eds.), Asphaltenes and Asphalts, vol. 1, Elsevier, Amsterdam, 1994, p. 1. [2] E. Furimsky, F.E. Massoth, Catal. Today 52 (1999) 381. [3] T. Kabe, A. Ishihara, W. Qian, Hydrodesulfurization and Hydrodenitrogenation, Kodansha/Wiley, Tokyo/New York, 1999, p. 325. [4] E.Y. Sheu, Energy Fuels 16 (2002) 74. [5] T.F. Yen, in: J.W. Burger (Ed.), Chemistry of Asphaltenes, Advances in Chemistry Series, vol. 195, American Chemical Society, Washington, DC, 1981, p. 39. [6] T.F. Yen, in: M. Grayson, J.I. Krochwitz (Eds.), Encyclopedia of Polymer Science and Engineering, Wiley, New York, 1988, p. 1. [7] D. Espinat, E. Rosenberg, M. Scarsella, L. Barre, D. Fenistein, D. Broseta, in: O.C. Mullins, E.Y. Sheu (Eds.), Structures and Dynamics of Asphaltenes, Plenum Press, New York, 1998, p. 145. [8] L. Artok, Y. Su, Y. Hirose, M. Hosokawa, S. Murata, M. Nomura, Energy Fuels 13 (1999) 287. [9] K.L. Gawrys, P.M. Spiecker, P.K. Kilpatrick, Am. Chem. Soc. Div. Petrol. Chem. 47 (2002) 332. [10] A. Miyauchi, Y. Inoue, PETROTECH 13 (1990) 44. [11] S. Inoue, T. Takatsuka, Y. Wada, S. Nakata, T. Ono, Catal. Today 43 (1998) 225. [12] K. Sano, Y. Nishimura, H. Shimada, Catal. Lett. 60 (1999) 83. [13] K. Sakanishi, T. Manabe, I. Watanabe, I. Mochida, J. Jpn. Petrol. Inst. 43 (2000) 10. [14] S. Terai, H. Fukuyama, K. Uehara, K. Fujimoto, J. Jpn. Petrol. Inst. 43 (2000) 17. [15] T. Yanagisawa, T. Shimizu, K. Kuroda, C. Kato, Bull. Chem. Soc. Jpn. 63 (1990) 988. [16] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [17] E. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [18] A. Corma, A. Martinez, V. Martinez-Soria, J.B. Monton, J. Catal. 153 (1995) 25. [19] S. Ahmed, Am. Chem. Soc. Div. Fuel Chem. 46 (2001) 591. [20] K.M. Reddy, B. Wei, C. Song, Catal. Today 43 (1998) 272. [21] E. Byambajav, Y. Ohtsuka, in: Proceedings of the 52nd Canadian Chemical Engineering Conference, 2002, p. 122. [22] E. Byambajav, R. Tanaka, Y. Ohtsuka, Am. Chem. Soc. Div. Fuel Chem. 48 (2003) 106. [23] E. Byambajav, Y. Ohtsuka, Fuel 82 (2003) 1571. [24] Y. Wang, M. Noguchi, Y. Takahashi, Y. Ohtsuka, Catal. Today 68 (2001) 4. [25] L.R. Radovic, P.L. Walker Jr., R.G. Jenkins, J. Catal. 82 (1983) 382. [26] J.N. Roux, D. Broseta, B. Demé, Langmuir 17 (2001) 5085. [27] R. Tanaka, R.W. Winans, J.E. Hunt, P. Thiyagarajan, S. Sato, T. Takanohashi, Am. Chem. Soc. Div. Fuel Chem. 46 (2001) 359. [28] H. Hayashi, L.Z. Chen, T. Tago, M. Kishida, K. Wakabayashi, Appl. Catal. A 231 (2002) 81.
204 [29] [30] [31] [32]
E. Byambajav, Y. Ohtsuka / Applied Catalysis A: General 252 (2003) 193–204
D.B. Bukur, C. Sivaraj, Appl. Catal. A 231 (2002) 201. R. Burch, A. Collins, Appl. Catal. 18 (1985) 389. P.S. Kumbhar, Appl. Catal. A 96 (1993) 241. E. Furimsky, Y. Yoshimura, Ind. Eng. Chem. Res. 26 (1987) 657. [33] R.B. LaCount, R.R. Anderson, S. Friedman, B.D. Blaustein, Fuel 66 (1987) 909.
[34] J. van Doorn, H.A.A. Barbolina, J.A. Moulijn, Ind. Eng. Chem. Res. 31 (1992) 101. [35] R.B. LaCount, D.G. Kern, W.P. King, R.B. LaCount Jr., D.J. Miltz Jr., A.L. Stewart, T.K. Trulli, D.K. Walker, R.K. Wicker, Fuel 72 (1993) 1203. [36] I. Nakamura, K. Fujimoto, Catal. Today 29 (1996) 245.