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Mesoporous NiO–Al2O3 catalyst for high pressure partial oxidation of methane to syngas
Applied Catalysis A: General 392 (2011) 86–92
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Mesoporous NiO–Al2 O3 catalyst for high pressure partial oxidation of methane to syngas Junpei Horiguchi a , Yasukazu Kobayashi a , Seishiro Kobayashi a , Yuichiro Yamazaki a , Kohji Omata a,∗ , Daisuke Nagao b , Mikio Konno b , Muneyoshi Yamada c a b c
Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba 6-6-07, Aramaki, Aoba-ku, Sendai 980-8579, Japan Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, Aoba 6-6-07, Aramaki, Aoba-ku, Sendai 980-8579, Japan Akita National College of Technology, 1-1 Iijima-Bunkyo-cho, Akita 011-8511, Japan
a r t i c l e
i n f o
Article history: Received 9 April 2010 Received in revised form 24 October 2010 Accepted 26 October 2010 Available online 3 November 2010 Keywords: Partial oxidation of methane Syngas formation Oxidative reforming of methane NiO–Al2 O3 catalyst Short contact time reaction Mesoporous
a b s t r a c t Mesoporous NiO–Al2 O3 catalysts were applied to catalytic partial oxidation of methane at 650 ◦ C, 1 MPa with a feed composition of CH4 /O2 /N2 /Ar = 5.2 vol.%/2.6 vol.%/4.9 vol.%/balance. In order to investigate the effect of mesopore of the catalyst on the catalytic activity, we used various types of organic reagents, such as P123, F127, PEG6000, and some carboxylic acids, to prepare bulk NiO–Al2 O3 catalysts. Among the organic reagents P123, F127, and PEG6000 were effective for the formation of mesopores. As a result of activity tests, the catalyst prepared by using P123 showed the highest oxidative activity and selectivity; the values were higher than those of NiO catalyst supported on ␣-Al2 O3 or ␥-Al2 O3 . Results of pulse reaction indicated that the oxidation tolerance of NiO–Al2 O3 prepared with P123 was high under reaction conditions. The effect of pore size of the catalyst was predominant because in the mesopores the diffusion values of the reactant gases are different and the CH4 /O2 ratio in the pores was kept high. These factors contributed to the high selectivity of the mesoporous NiO–Al2 O3 catalysts. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Catalytic partial oxidation of methane to synthesis gas has attracted increasing attention in recent years. This process is suitable for methanol synthesis and Fischer–Tropsch synthesis under high pressures (typically, 1–3 MPa). It is well known that Ni and Ru are catalytically active for this reaction [1]. From the economical view point, however, the development of nickel-based catalysts is preferred. Research has revealed that the active species is Ni metal under the reaction conditions [2], and studies aimed at increasing the dispersion of nickel precursor on typically Al2 O3 supports are conducted as one of the ways of improving the activity. To increase the dispersion one of the proven methods in the field of catalyst development is to use supports with high surface area, but it is difficult to attain both high surface area and high stability at high temperature when using Al2 O3 as a catalyst support. Recently mesoporous Ni–Al oxide was prepared by the one-pot synthesis, this material gives high BET surface area of 385 m2 /g and the improved thermal stability [3]. The similar oxides were first applied for the partial oxidation of methane into synthesis gas at atmospheric pressure [4]. It was reported that a relatively
∗ Corresponding author. E-mail address: [email protected] (K. Omata). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.10.028
strong interaction between the nickel species and alumina support resulted in finely dispersed nickel particles on the catalyst surface and the strong interaction prevented the coke formation. For the short contact time reaction at high pressure, high tolerance against the oxidation of Ni species under reaction conditions is also essential for the stable activity, in addition to the suppression of coke formation. Thus, in the present study, NiO–Al2 O3 catalyst prepared by using various organic reagents was applied to the high pressure partial oxidation of methane to syngas, and the effect of mesoporous structure was investigated to achieve high activity and selectivity at the short contact time region of 0.1 ms. 2. Experimental 2.1. Catalyst preparation Nickel–aluminum oxide catalysts (NiO–Al2 O3 ) were prepared by using a procedure similar to that reported by Morris et al. [3]. An appropriate amount of nickel nitrate was dissolved in 20.0 ml of ethanol with approximately 2.0 g of an organic reagent such as P123 ((EO)20 (PO)70 (EO)20 triblock copolymer, BASF Co.), F127 ((EO)100 (PO)65 (EO)100 triblock copolymer, BASF Co.), PEG6000 (Wako Pure Chemical Industries, Ltd.), lauric acid (Wako) or citric acid monohydrate (Wako) in order to prepare catalysts with various pore structures. After being stirred for 4 h at 200 rpm, the
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solution was mixed with a solution of a suitable amount of aluminum isopropoxide diluted with 20.0 ml ethanol and 3.2 ml of 65% nitric acid. The combined solution was stirred for 5 h at 300 rpm. The mixture was evaporated at 60 ◦ C without stirring and the solvent was removed before 48 h. Finally the resulting sample was heated in air to 700 ◦ C at 1◦ /min, and held at the temperature for 4 h. The organic reagent is shown in the parentheses of the catalyst nomenclature. When nickel species were supported on the support by the conventional impregnation method, a slash was used between Ni and the support for the nomenclature. 2.2. Catalytic activity test The catalytic activity tests for catalytic partial oxidation of methane were carried out in a fixed-bed reactor. A reactor tube made of pure Al2 O3 (i.d. = 3 mm, Nikkato Corp., SSA-S) was inserted in a stainless steel tube. Both ends of the alumina tube was sealed with O-rings and the space between the tubes was filled with reduced copper metal sphere (Wako, 150–250 m). In each test a total 10 mg catalyst diluted with 9.67 mg ␣-Al2 O3 , which showed no activity, was used to prevent hot spot formation during a reaction. Additionally 150 mg of quartz sand (150–250 m) was placed on the upper side of catalyst bed to promote heat transfer. The reaction mixture consisted of 5.2 vol.% CH4 , 2.6 vol.% O2 , 4.9 vol.% N2 and Ar balance, and the reciprocal of space velocity (SV−1 , ms gcat/cm3 ) was defined as the contact time (ms). It was set from 5 to 0.07 ms. Such large space velocity was necessary to investigate the catalysis as an oxidative reforming catalyst because the catalyst works as a steam reforming and/or a CO2 reforming catalyst after O2 conversion reaches at 100%. Thus, the selectivity at O2 conversion <100% should be observed. The catalysts were pretreated in hydrogen flow at 650 ◦ C with a heating rate of 10◦ /min, and held for 1 h under 0.1 MPa, and then the reactant gas was introduced into the reactor at 650 ◦ C under 1 MPa. The product gas was analyzed by a GC-TCD (Simadzu, GC-8A). The CH4 conversion, O2 conversion, H2 yield, CO yield, H2 selectivity and CO selectivity were defined as Eqs. (1)–(6) where CH4 ratio and N2 ratio are the ratios of these gases in the feed. An asterisk is used for the integrated peak area of GC-TCD chromatogram at the inlet.
CH4 conversion (%) =
1−
O2 conversion (%) =
1−
CH4 /N2 CH∗4 /N∗2
O2 /N2 O∗2 /N∗2
H2 uptake, after H2 pretreatment at 650 ◦ C for 1 h, was also measured at 100 ◦ C using the same instrument. From the obtained H2 adsorption data, Ni particle sizes and dispersions were calculated. The morphology of the prepared catalyst was investigated by TEM measurement (HF-2000, Hitachi, Ltd., 200 kV). Small-angle X-ray scattering (SAXS) experiments were performed using CuK˛ radiation operated at 40 kV and 40 mA (RIGAKU Ultima IV). Diffraction intensities were recorded in the region of the scattering angle from 0◦ to 5◦ with a scan rate of 0.2◦ /min. The crystal structure of the catalyst was characterized by X-ray diffraction (Miniflex, Rigaku). CuK˛ radiation was used as an X-ray source. The X-ray tube was operated at 30 kV and 15 mA. Diffraction intensities were recorded from 20◦ to 80◦ at the rate of 1.0◦ /min, or from 62◦ to 67◦ at the rate of 0.1◦ /min. The observed diffraction patterns were assigned by referring to the Joint Committee on the Powder Diffraction Standards (JCPDS) cards. The reducibility of nickel oxide species was evaluated in H2 stream by temperature programmed reduction (TPR). A conventional flow reactor equipped with an on line GC-TCD (Shimadzu, GC-8A) was used for TPR measurements. About 150 mg of the catalyst was set in a quartz tube reactor (i.d.: 4 mm, length: 250 mm) and reduced in a stream of 7% H2 /Ar at the heating rate of 5 ◦ /min up to 1000 ◦ C. H2 consumption was continuously monitored with on-line GC-TCD to obtain TPR profiles of the catalysts. Each pulse reaction was conducted at 650 ◦ C after H2 pretreatment for 1 h. A reactant pulse (size 3.34 ml, CH4 /O2 /He = 40 vol.%/20 vol.%/40 vol.%) was injected 20 times, and then an oxidation pulse (size 3.34 ml, O2 /He = 1 vol.%/99 vol.%) was injected until no consumption of O2 pulse was observed. From the amounts of formed CO and consumed O2 during the oxidation pulse, the amount of Ni metal after the 20 pulses was estimated. The Knudsen number is a dimensionless number which is defined as the ratio of the molecular mean free path length to a representative length such as the pore diameter. The number is important because the manner of gas diffusion is determined by the number. Those of CH4 and O2 under the reaction conditions and in mesopore (the pore diameter was 3.6 nm) were about 100 and thus Knudsen diffusion coefficients were calculated using Eq. (7), D=
× 100
(1)
2 8RT r 3 M
1/2 (7)
(2)
where D is the diffusion coefficient (m2 /s), r the pore diameter (m), R the gas constant (Pa m3 /K/mol), T the temperature (K), and M is the molar weight (g/mol). 3. Result and discussion
× 100
87
H2 yield (%) =
H2 /H2 sensitivity × 100 N2 × CH4 ratio/N2 ratio × 2
(3)
CO yield (%) =
CO/CO sensitivity × 100 N2 × CH4 ratio/N2 ratio
(4)
3.1. Pore structure
H2 selectivity (%) =
H2 yield × 100 CH4 conversion
(5)
CO selectivity (%) =
CO yield × 100 CH4 conversion
(6)
2.3. Characterization Pores of the catalyst were analyzed using adsorption/desorption isotherms of N2 (AUTOSORB AS-1, Quantachrome). After the heat pretreatment at 200 ◦ C for 2 h under air, each adsorption isotherm of N2 was measured at liquid N2 temperature. From the adsorption data, the BET surface area, the total pore volume and the average pore size were calculated using ASWin software (Quantachrome).
Nitrogen adsorption/desorption isotherms are illustrated in Fig. 1. Isotherms of Ni supported on Al2 O3 prepared using P123 (NiO/Al2 O3 (P123)) and NiO/␥-Al2 O3 were compared with those of the bulk Ni–Al2 O3 . Isotherms of NiO–Al2 O3 (P123), NiO–Al2 O3 (F127), NiO–Al2 O3 (PEG), NiO/Al2 O3 (P123) and NiO/␥–Al2 O3 are categorized as type V with hysteresis loops which are characteristic for mesoporous material [5]. On the other hand, NiO–Al2 O3 (lauric), NiO–Al2 O3 (citric) and NiO–Al2 O3 (none) are not mesoporous materials but macroporous or nonporous materials, because no hysteresis was observed. It is known that mesoporous materials can be prepared by formation of a micelle complex composed of surfactant molecules and Si and/or Al cation. In a specific case, it was reported that mesoporous Al2 O3 and SiO2 are prepared by using triblock copolymer P123 or F127 as surfactant molecules for MCM-41 or SBA-16, respectively. In the
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Fig. 3. TEM images of NiO–Al2 O3 (P123) catalyst.
Fig. 1. Adsorption/desorption isotherms of NiO–Al2 O3 catalysts.
present case, it is suggested that P123, F127 and PEG interact strongly with both Al and Ni cation, leading to the formation of micelle complexes composed of surfactant molecules and Al and Ni cation under preparation condition. Among the mesoporous catalysts, two types of hysteresis were clearly observed. Type H1 hysteresis of NiO–Al2 O3 (P123) and NiO–Al2 O3 (F127) suggests the presence of cylindrical pore geometry and a pore size uniformity [5]. The result is in good agreement with the evidence of mesopores for NiO–Al2 O3 (P123) provided by SAXS patterns (in Fig. 2) and TEM images (in Fig. 3). Meanwhile, type H2 hysteresis was observed at NiO supported catalysts, i.e., NiO/Al2 O3 (P123) and NiO/␥-Al2 O3 . Generally the hysteresis suggests the presence of pores with narrow mouths as bottle-neck shape [5]. Such shape represents the plugging of the mouth of mesopores by large amounts of NiO loading on the supports after the impregnation procedure. Pore size distributions of catalysts were calculated by BJH method as shown in Fig. 4; and in Table 1 are summarized detailed adsorption parameters such as BET surface areas, pore volumes and average pore sizes. Pore size distributions of NiO–Al2 O3 (P123),
Fig. 2. Small-angle X-ray scattering pattern of the NiO–Al2 O3 (P123) catalyst.
NiO–Al2 O3 (F127) and NiO–Al2 O3 (PEG) were relativity narrow and centered at 3–5 nm as predicted from the presence of type H1 hysteresis. Particularly, NiO–Al2 O3 (P123) shows better and welldeveloped framework porosity with narrow pore size distribution. In agreement with the report that P123 with smaller polymer size results in smaller pore size than that obtained by F127, the pore size of NiO–Al2 O3 was also influenced by the organic template size. With regard to BET surface area, mesoporous materials were found to have relatively high BET surface areas. Particularly that of NiO–Al2 O3 (P123) was highest at 336 m2 /g. Consequently, these show relatively high pore volumes. Whereas the average pore sizes are same, the smaller BET surface area and smaller pore volume suggest that the pore mouths of Al2 O3 (P123) was plugged by NiO loading on NiO/Al2 O3 (P123) in comparison with those of NiO–Al2 O3 (P123). The result is in good agreement with those observed with the hysteresis.
Fig. 4. Pore size distribution of NiO–Al2 O3 catalysts.
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89
Table 1 BET surface areas, pore volumes and average pore sizes. BET surface area (m2 /g)
Catalyst NiO–Al2 O3 (P123) NiO–Al2 O3 (F127) NiO–Al2 O3 (PEG) NiO–Al2 O3 (lauric) NiO–Al2 O3 (citric) NiO–Al2 O3 (none) NiO/Al2 O3 (P123) NiO/␥-Al2 O3 NiO/␣-Al2 O3
336 263 274 85 174 141 267 202 10
Pore volume (cm3 /g)
Average pore size (nm)
0.60 0.56 0.64 0.08 0.10 0.15 0.48 0.63 –
3.6 4.3 4.7 1.9 1.1 2.2 3.6 5.2 –
Fig. 5. XRD patterns of NiO–Al2 O3 (P123) catalyst.
3.2. NiO species In the bulk NiO–Al2 O3 , NiO, NiAl2 O4 , and Al2 O3 were supposed to be present as precursors of catalyst. As shown in Fig. 5, however, X-ray diffraction pattern of NiO–Al2 O3 (P123) as a representative of NiO–Al2 O3 catalyst provided little information on the Ni species, probably because the particle sizes of these possible oxides are small. A similar result was also reported by Morris et al. [3]. Therefore, TPR (temperature programmed reduction) measurement was carried out in order to investigate metal–support interactions and the reducibility of the NiO species. It is known that the reduction temperature of NiO depends on the nature of the interactions between Ni and supports such as Al2 O3 or SiO2 . For example, the reduction temperature of NiAl2 O4 -like species formed during catalyst preparation by sol-gel method is above 800 ◦ C. Fig. 6 shows TPR profiles of the prepared catalysts with some template. As references, TPR profiles of NiO/Al2 O3 (P123), NiO/␥-Al2 O3 , NiO/␣-Al2 O3 , and bulk NiO were compared. TPR profiles in Fig. 6 show that bulk NiO is easily reduced by H2 at 350 ◦ C, and that NiO supported on ␣-Al2 O3 has weak interaction with alumina with respect to the reduction peak temperature. In our previous report [6], TPR profiles of NiO-K/␣-Al2 O3 catalysts
Fig. 6. TPR profiles of NiO–Al2 O3 catalysts, supported NiO catalysts, and NiO.
were deconvoluted into four peaks, which means four kinds of NiO species were included in the supported catalyst. With increasing surface area of the catalyst, NiO species are influenced by the alumina support and the reduction peak of each catalyst shifted to higher temperature. The supported catalyst and NiO–Al2 O3 catalysts prepared using carboxylic acids, showed two reduction peaks. The result suggests that NiO is not uniformly supported on, or included in, alumina. In other words, interaction between the NiO and alumina is not singular. On the contrary, mesoporous NiO–Al2 O3 catalysts prepared using P123, F127, and PEG, have only a single reduction peak at about 700–750 ◦ C. Using the same decon-
Table 2 Ni reduction degrees and dispersions. Catalyst
Ni reduction degree (%)
H2 adsorption (mol/g)
Ni dispersion (%)
NiO–Al2 O3 (P123) NiO–Al2 O3 (F127) NiO–Al2 O3 (PEG) NiO–Al2 O3 (lauric) NiO–Al2 O3 (citric) NiO–Al2 O3 (none) NiO/Al2 O3 (P123) NiO/␥-Al2 O3 NiO/␣-Al2 O3
63 100 100 67 98 94 100 97 100
21 56 70 15 9 36 32 13 33
4.8 8.1 10.0 3.4 1.4 5.7 6.9 2.7 3.9
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Fig. 7. Activity and selectivity of NiO–Al2 O3 catalysts and supported NiO catalysts. () CH4 conversion, (♦) O2 conversion, () H2 selectivity, and () CO selectivity. (a) NiO–Al2 O3 (P123), (b) NiO–Al2 O3 (F127), (c) NiO–Al2 O3 (PEG), (d) NiO–Al2 O3 (lauric), (e) NiO–Al2 O3 (citric), (f) NiO–Al2 O3 (none), (g) NiO/Al2 O3 (P123), (h) NiO/␥-Al2 O3 , and (i) NiO/␣-Al2 O3 .
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91
Table 3 Coke and Ni0 /Nicat after CH4 /O2 /He pulse. Catalyst
Ni0 /Nicat (%)
Coke (mg/g-cat)
NiO–Al2 O3 (P123) NiO/␥-Al2 O3 NiO/␣-Al2 O3
63 0 50
0.4 0.0 2.6
volution method, roughly one peak was observed in these catalysts. Thus the chemical state was uniform. It is remarkable that even NiO supported on Al2 O3 (P123) is uniformly supported on alumina on these catalysts, judging from the single reduction peak. Table 2 summarizes the reduction degrees, H2 adsorptions and Ni dispersions for prepared catalysts. Their reduction degree was calculated from the total amount of H2 consumption for TPR. Reduction degrees of NiO–Al2 O3 (P123) and NiO–Al2 O3 (lauric) were 63% and 67%; these were smaller than of those of other catalysts. Whereas it is assumed that a part of NiO is not reduced with H2 due to existence as NiAl2 O4 , it is more probable that NiO is partially covered and isolated inside the alumina. 3.3. Catalytic performance in partial oxidation of methane Fig. 7 shows the activity and selectivity of the prepared catalysts as a function of contact time. Both CH4 conversion and O2 conversion decreased with decreasing contact time over each catalyst. Such a behavior is ordinary for the conventional catalytic reaction, while it is not the case in partial oxidation of methane. Because the reaction is highly exothermic, hot spots are easily formed in the catalyst bed and they can cause the rapid increase of conversions at short contact time. Thus, careful experiments were required in order to obtain the results like those in Fig. 7[7]. Whereas supported NiO catalysts (NiO/Al2 O3 (P123) and NiO/␥-Al2 O3 ) and bulk NiO catalysts (NiO–Al2 O3 (lauric), NiO–Al2 O3 (citric) and NiO–Al2 O3 (none)) were found to have low activity, NiO/␣-Al2 O3 showed relatively high CH4 conversion and syngas selectivity. It is remarkable that bulk type NiO catalyst with mesopores (NiO–Al2 O3 (P123), NiO–Al2 O3 (F127) and NiO–Al2 O3 (PEG)) showed quite high CH4 conversion under contact time 0.8 ms g-cat/cm3 . Particularly, NiO–Al2 O3 (P123) showed over 88% H2 selectivity and 78% CO selectivity at 0.1 ms. If the selectivities were determined by the equilibrium state of side reactions such as steam reforming of methane and dry reforming of methane, where steam and CO2 were formed from the combustion of methane, the selectivity can not be so high. Thus these selectivities are higher than those thermodynamically predicted values. Such high selectivity suggests some contribution of the direct route of syngas formation. 3.4. Oxidation tolerance of Ni metal Nickel metal species are active for a direct route of syngas formation from CH4 because C–H bonds can be cleaved on Ni metal. Thus, high tolerance of Ni metal species against the oxidation under reaction conditions in oxidative atmosphere is the most desired for Ni catalysts. CH4 /O2 and O2 pulse reactions were carried out in order to investigate the tolerance for oxidation of Ni metal. The result of the 20th pulse is illustrated in Fig. 8(a). Syngas formation was observed even after the 20 pulses. After the reactant CH4 /O2 pulse, O2 pulses were repeated until no consumption of O2 pulse was observed, as shown in Fig. 8(b). From the amount of formed CO and consumed O2 , the amount of Ni metal after the 20 pulses was estimated. In Table 3 are compared Ni metal and the amount of carbon deposition after 20 CH4 /O2 pulses on NiO–Al2 O3 (P123), NiO/␥Al2 O3 , and NiO/␣-Al2 O3 . Clearly NiO/␥-Al2 O3 was easily oxidized by CH4 /O2 pulses and therefore showed low activity and selectivity
Fig. 8. Pulse reaction for estimation of coke formation and Ni0 /Nicat ratio. (a) Products pattern after 20th CH4 /O2 /He pulse and (b) CO formation and O2 pulse behavior at repeated O2 /He pulses.
in Fig. 7. A larger part of Ni0 species remained after the pulse reaction in reduced NiO–Al2 O3 (P123) than in reduced NiO/␣-Al2 O3 . The tolerance against oxidation of Ni metal of NiO–Al2 O3 (P123) was enhanced; thus, high selectivity to CO and H2 were achieved. 3.5. On the role of mesopore From these observations, the presence of mesopores and high tolerance of Ni metal species against oxidation are to be the essential properties for partial oxidation of methane to syngas. The reducibility of NiO estimated by TPR, dispersion of Ni metal by H2 adsorption, BET surface area of the catalyst, and pore textures (size and volume) are, however, similar between the bulk NiO–Al2 O3 (P123) catalyst and the supported NiO/Al2 O3 (P123) catalyst. The quite low activity and low selectivity for syngas of the latter catalyst may not be justified. Considering the shape of mesopores, we think that NiO would be mainly located at the pore mouth of the supported NiO/Al2 O3 (P123) catalyst and active sites would be formed at the locations after reduction. If this is the case, the major difference between the two catalysts is the location of the active sites, because Ni species are well distributed in Al-O frame in the bulk NiO–Al2 O3 (P123) catalyst and thus the Ni metal on the surface in the mesopore is the active site. Reactant gas first should diffuse into the mesopores of NiO–Al2 O3 (P123) catalyst; in such small pores for Knudsen diffusion region, the diffusivity of CH4 and of O2 are different. The diffusion coefficients of CH4 and O2 in pores with
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diameter 3.6 nm under reaction conditions are 1.33 × 10−6 m2 /s and 0.94 × 10−6 m2 /s, respectively. If these gases diffuse independently into a straight pore (diameter 3.6 nm), the CH4 /O2 ratio of reaction gas (CH4 /O2 ratio = 2 at outside of pores) in the pore reaches 3.0 at 20 m depth from the inlet of the pore after 0.1 ms. In a real particle, the pore can not be a straight one and gases are converted to syngas during diffusion. So the calculated ratio is only a rough guideline, but it reveals that the ratio can be higher in the mesopores. It is well know that high CH4 /O2 is desirable for high syngas selectivity [8]. Using high CH4 /O2 ratio will increase the apparent tolerance of Ni metal species against the oxidation under reaction conditions. Thus, preparation of mesoporous bulk NiO–Al2 O3 catalysts resulted in high surface area catalyst, strong interaction between Al2 O3 and Ni species, formation of uniform Ni species, selective gas diffusion for high CH4 /O2 ratios, and apparent high tolerance of Ni metal species against the oxidation. The above combined factors were influential on the high activity and selectivity of NiO–Al2 O3 (P123) catalyst for partial oxidation of methane to syngas. 4. Conclusions Mesoporous bulk type NiO–Al2 O3 catalysts were applied to catalytic partial oxidation of methane to syngas at 650 ◦ C, 1 MPa. Among the various types of organic reagents, such as P123, F127,
PEG, and some carboxylic acids, the surfactants and PEG were effective to prepare mesoporous oxides. NiO–Al2 O3 catalyst prepared by using P123 gave the highest BET surface area of 340 m2 /g. The catalyst was reduced by H2 and then used for the reaction. High CO and H2 selectivities of 77% and 88%, respectively, were attained by the catalyst at the short contact region of 0.1 ms. High tolerance against the oxidation, which was also confirmed by pulse reaction, of Ni species under reaction conditions would originated from the mesopore structure, and would contribute to the high selectivity of mesoporous NiO–Al2 O3 catalysts. Mesoporous bulk type NiO–Al2 O3 catalyst was concluded to be a promising catalyst for partial oxidation of methane to syngas. Acknowledgment We acknowledge financial support from JSPS, Grant-in-Aid for Scientific Research (S), 17106011, 2005. References [1] [2] [3] [4] [5] [6]
B.C. Enger, R. Lødeng, A. Holmen, Appl. Catal. A: Gen. 346 (2008) 1–27. D. Dissanayake, M. Rosynek, K. Kharas, J. Lunsford, J. Catal. 132 (1991) 117–127. S. Morris, P. Fulvio, M. Jaroniec, J. Am. Chem. Soc. 130 (2008) 15210. P. Kim, Y. Kim, H. Kim, I. Song, J. Yi, App. Catal. A: Gen. 272 (2004) 157–166. M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169–3183. J. Horiguchi, S. Kobayashi, Y. Yamazaki, T. Nakanishi, D. Itabashi, K. Omata, M. Yamada, Appl. Catal. A: Gen. 377 (2010) 9–15. [7] Y. Boucouvalas, Z. Zhang, X.E. Verykios, Catal. Lett. 40 (1996) 189–195. [8] I. Balint, A. Miyazaki, K. Aika, J. Catal. 220 (2003) 74–83.
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Mesoporous NiO–Al2 O3 catalyst for high pressure partial oxidation of methane to syngas Junpei Horiguchi a , Yasukazu Kobayashi a , Seishiro Kobayashi a , Yuichiro Yamazaki a , Kohji Omata a,∗ , Daisuke Nagao b , Mikio Konno b , Muneyoshi Yamada c a b c
Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba 6-6-07, Aramaki, Aoba-ku, Sendai 980-8579, Japan Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, Aoba 6-6-07, Aramaki, Aoba-ku, Sendai 980-8579, Japan Akita National College of Technology, 1-1 Iijima-Bunkyo-cho, Akita 011-8511, Japan
a r t i c l e
i n f o
Article history: Received 9 April 2010 Received in revised form 24 October 2010 Accepted 26 October 2010 Available online 3 November 2010 Keywords: Partial oxidation of methane Syngas formation Oxidative reforming of methane NiO–Al2 O3 catalyst Short contact time reaction Mesoporous
a b s t r a c t Mesoporous NiO–Al2 O3 catalysts were applied to catalytic partial oxidation of methane at 650 ◦ C, 1 MPa with a feed composition of CH4 /O2 /N2 /Ar = 5.2 vol.%/2.6 vol.%/4.9 vol.%/balance. In order to investigate the effect of mesopore of the catalyst on the catalytic activity, we used various types of organic reagents, such as P123, F127, PEG6000, and some carboxylic acids, to prepare bulk NiO–Al2 O3 catalysts. Among the organic reagents P123, F127, and PEG6000 were effective for the formation of mesopores. As a result of activity tests, the catalyst prepared by using P123 showed the highest oxidative activity and selectivity; the values were higher than those of NiO catalyst supported on ␣-Al2 O3 or ␥-Al2 O3 . Results of pulse reaction indicated that the oxidation tolerance of NiO–Al2 O3 prepared with P123 was high under reaction conditions. The effect of pore size of the catalyst was predominant because in the mesopores the diffusion values of the reactant gases are different and the CH4 /O2 ratio in the pores was kept high. These factors contributed to the high selectivity of the mesoporous NiO–Al2 O3 catalysts. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Catalytic partial oxidation of methane to synthesis gas has attracted increasing attention in recent years. This process is suitable for methanol synthesis and Fischer–Tropsch synthesis under high pressures (typically, 1–3 MPa). It is well known that Ni and Ru are catalytically active for this reaction [1]. From the economical view point, however, the development of nickel-based catalysts is preferred. Research has revealed that the active species is Ni metal under the reaction conditions [2], and studies aimed at increasing the dispersion of nickel precursor on typically Al2 O3 supports are conducted as one of the ways of improving the activity. To increase the dispersion one of the proven methods in the field of catalyst development is to use supports with high surface area, but it is difficult to attain both high surface area and high stability at high temperature when using Al2 O3 as a catalyst support. Recently mesoporous Ni–Al oxide was prepared by the one-pot synthesis, this material gives high BET surface area of 385 m2 /g and the improved thermal stability [3]. The similar oxides were first applied for the partial oxidation of methane into synthesis gas at atmospheric pressure [4]. It was reported that a relatively
∗ Corresponding author. E-mail address: [email protected] (K. Omata). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.10.028
strong interaction between the nickel species and alumina support resulted in finely dispersed nickel particles on the catalyst surface and the strong interaction prevented the coke formation. For the short contact time reaction at high pressure, high tolerance against the oxidation of Ni species under reaction conditions is also essential for the stable activity, in addition to the suppression of coke formation. Thus, in the present study, NiO–Al2 O3 catalyst prepared by using various organic reagents was applied to the high pressure partial oxidation of methane to syngas, and the effect of mesoporous structure was investigated to achieve high activity and selectivity at the short contact time region of 0.1 ms. 2. Experimental 2.1. Catalyst preparation Nickel–aluminum oxide catalysts (NiO–Al2 O3 ) were prepared by using a procedure similar to that reported by Morris et al. [3]. An appropriate amount of nickel nitrate was dissolved in 20.0 ml of ethanol with approximately 2.0 g of an organic reagent such as P123 ((EO)20 (PO)70 (EO)20 triblock copolymer, BASF Co.), F127 ((EO)100 (PO)65 (EO)100 triblock copolymer, BASF Co.), PEG6000 (Wako Pure Chemical Industries, Ltd.), lauric acid (Wako) or citric acid monohydrate (Wako) in order to prepare catalysts with various pore structures. After being stirred for 4 h at 200 rpm, the
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solution was mixed with a solution of a suitable amount of aluminum isopropoxide diluted with 20.0 ml ethanol and 3.2 ml of 65% nitric acid. The combined solution was stirred for 5 h at 300 rpm. The mixture was evaporated at 60 ◦ C without stirring and the solvent was removed before 48 h. Finally the resulting sample was heated in air to 700 ◦ C at 1◦ /min, and held at the temperature for 4 h. The organic reagent is shown in the parentheses of the catalyst nomenclature. When nickel species were supported on the support by the conventional impregnation method, a slash was used between Ni and the support for the nomenclature. 2.2. Catalytic activity test The catalytic activity tests for catalytic partial oxidation of methane were carried out in a fixed-bed reactor. A reactor tube made of pure Al2 O3 (i.d. = 3 mm, Nikkato Corp., SSA-S) was inserted in a stainless steel tube. Both ends of the alumina tube was sealed with O-rings and the space between the tubes was filled with reduced copper metal sphere (Wako, 150–250 m). In each test a total 10 mg catalyst diluted with 9.67 mg ␣-Al2 O3 , which showed no activity, was used to prevent hot spot formation during a reaction. Additionally 150 mg of quartz sand (150–250 m) was placed on the upper side of catalyst bed to promote heat transfer. The reaction mixture consisted of 5.2 vol.% CH4 , 2.6 vol.% O2 , 4.9 vol.% N2 and Ar balance, and the reciprocal of space velocity (SV−1 , ms gcat/cm3 ) was defined as the contact time (ms). It was set from 5 to 0.07 ms. Such large space velocity was necessary to investigate the catalysis as an oxidative reforming catalyst because the catalyst works as a steam reforming and/or a CO2 reforming catalyst after O2 conversion reaches at 100%. Thus, the selectivity at O2 conversion <100% should be observed. The catalysts were pretreated in hydrogen flow at 650 ◦ C with a heating rate of 10◦ /min, and held for 1 h under 0.1 MPa, and then the reactant gas was introduced into the reactor at 650 ◦ C under 1 MPa. The product gas was analyzed by a GC-TCD (Simadzu, GC-8A). The CH4 conversion, O2 conversion, H2 yield, CO yield, H2 selectivity and CO selectivity were defined as Eqs. (1)–(6) where CH4 ratio and N2 ratio are the ratios of these gases in the feed. An asterisk is used for the integrated peak area of GC-TCD chromatogram at the inlet.
CH4 conversion (%) =
1−
O2 conversion (%) =
1−
CH4 /N2 CH∗4 /N∗2
O2 /N2 O∗2 /N∗2
H2 uptake, after H2 pretreatment at 650 ◦ C for 1 h, was also measured at 100 ◦ C using the same instrument. From the obtained H2 adsorption data, Ni particle sizes and dispersions were calculated. The morphology of the prepared catalyst was investigated by TEM measurement (HF-2000, Hitachi, Ltd., 200 kV). Small-angle X-ray scattering (SAXS) experiments were performed using CuK˛ radiation operated at 40 kV and 40 mA (RIGAKU Ultima IV). Diffraction intensities were recorded in the region of the scattering angle from 0◦ to 5◦ with a scan rate of 0.2◦ /min. The crystal structure of the catalyst was characterized by X-ray diffraction (Miniflex, Rigaku). CuK˛ radiation was used as an X-ray source. The X-ray tube was operated at 30 kV and 15 mA. Diffraction intensities were recorded from 20◦ to 80◦ at the rate of 1.0◦ /min, or from 62◦ to 67◦ at the rate of 0.1◦ /min. The observed diffraction patterns were assigned by referring to the Joint Committee on the Powder Diffraction Standards (JCPDS) cards. The reducibility of nickel oxide species was evaluated in H2 stream by temperature programmed reduction (TPR). A conventional flow reactor equipped with an on line GC-TCD (Shimadzu, GC-8A) was used for TPR measurements. About 150 mg of the catalyst was set in a quartz tube reactor (i.d.: 4 mm, length: 250 mm) and reduced in a stream of 7% H2 /Ar at the heating rate of 5 ◦ /min up to 1000 ◦ C. H2 consumption was continuously monitored with on-line GC-TCD to obtain TPR profiles of the catalysts. Each pulse reaction was conducted at 650 ◦ C after H2 pretreatment for 1 h. A reactant pulse (size 3.34 ml, CH4 /O2 /He = 40 vol.%/20 vol.%/40 vol.%) was injected 20 times, and then an oxidation pulse (size 3.34 ml, O2 /He = 1 vol.%/99 vol.%) was injected until no consumption of O2 pulse was observed. From the amounts of formed CO and consumed O2 during the oxidation pulse, the amount of Ni metal after the 20 pulses was estimated. The Knudsen number is a dimensionless number which is defined as the ratio of the molecular mean free path length to a representative length such as the pore diameter. The number is important because the manner of gas diffusion is determined by the number. Those of CH4 and O2 under the reaction conditions and in mesopore (the pore diameter was 3.6 nm) were about 100 and thus Knudsen diffusion coefficients were calculated using Eq. (7), D=
× 100
(1)
2 8RT r 3 M
1/2 (7)
(2)
where D is the diffusion coefficient (m2 /s), r the pore diameter (m), R the gas constant (Pa m3 /K/mol), T the temperature (K), and M is the molar weight (g/mol). 3. Result and discussion
× 100
87
H2 yield (%) =
H2 /H2 sensitivity × 100 N2 × CH4 ratio/N2 ratio × 2
(3)
CO yield (%) =
CO/CO sensitivity × 100 N2 × CH4 ratio/N2 ratio
(4)
3.1. Pore structure
H2 selectivity (%) =
H2 yield × 100 CH4 conversion
(5)
CO selectivity (%) =
CO yield × 100 CH4 conversion
(6)
2.3. Characterization Pores of the catalyst were analyzed using adsorption/desorption isotherms of N2 (AUTOSORB AS-1, Quantachrome). After the heat pretreatment at 200 ◦ C for 2 h under air, each adsorption isotherm of N2 was measured at liquid N2 temperature. From the adsorption data, the BET surface area, the total pore volume and the average pore size were calculated using ASWin software (Quantachrome).
Nitrogen adsorption/desorption isotherms are illustrated in Fig. 1. Isotherms of Ni supported on Al2 O3 prepared using P123 (NiO/Al2 O3 (P123)) and NiO/␥-Al2 O3 were compared with those of the bulk Ni–Al2 O3 . Isotherms of NiO–Al2 O3 (P123), NiO–Al2 O3 (F127), NiO–Al2 O3 (PEG), NiO/Al2 O3 (P123) and NiO/␥–Al2 O3 are categorized as type V with hysteresis loops which are characteristic for mesoporous material [5]. On the other hand, NiO–Al2 O3 (lauric), NiO–Al2 O3 (citric) and NiO–Al2 O3 (none) are not mesoporous materials but macroporous or nonporous materials, because no hysteresis was observed. It is known that mesoporous materials can be prepared by formation of a micelle complex composed of surfactant molecules and Si and/or Al cation. In a specific case, it was reported that mesoporous Al2 O3 and SiO2 are prepared by using triblock copolymer P123 or F127 as surfactant molecules for MCM-41 or SBA-16, respectively. In the
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Fig. 3. TEM images of NiO–Al2 O3 (P123) catalyst.
Fig. 1. Adsorption/desorption isotherms of NiO–Al2 O3 catalysts.
present case, it is suggested that P123, F127 and PEG interact strongly with both Al and Ni cation, leading to the formation of micelle complexes composed of surfactant molecules and Al and Ni cation under preparation condition. Among the mesoporous catalysts, two types of hysteresis were clearly observed. Type H1 hysteresis of NiO–Al2 O3 (P123) and NiO–Al2 O3 (F127) suggests the presence of cylindrical pore geometry and a pore size uniformity [5]. The result is in good agreement with the evidence of mesopores for NiO–Al2 O3 (P123) provided by SAXS patterns (in Fig. 2) and TEM images (in Fig. 3). Meanwhile, type H2 hysteresis was observed at NiO supported catalysts, i.e., NiO/Al2 O3 (P123) and NiO/␥-Al2 O3 . Generally the hysteresis suggests the presence of pores with narrow mouths as bottle-neck shape [5]. Such shape represents the plugging of the mouth of mesopores by large amounts of NiO loading on the supports after the impregnation procedure. Pore size distributions of catalysts were calculated by BJH method as shown in Fig. 4; and in Table 1 are summarized detailed adsorption parameters such as BET surface areas, pore volumes and average pore sizes. Pore size distributions of NiO–Al2 O3 (P123),
Fig. 2. Small-angle X-ray scattering pattern of the NiO–Al2 O3 (P123) catalyst.
NiO–Al2 O3 (F127) and NiO–Al2 O3 (PEG) were relativity narrow and centered at 3–5 nm as predicted from the presence of type H1 hysteresis. Particularly, NiO–Al2 O3 (P123) shows better and welldeveloped framework porosity with narrow pore size distribution. In agreement with the report that P123 with smaller polymer size results in smaller pore size than that obtained by F127, the pore size of NiO–Al2 O3 was also influenced by the organic template size. With regard to BET surface area, mesoporous materials were found to have relatively high BET surface areas. Particularly that of NiO–Al2 O3 (P123) was highest at 336 m2 /g. Consequently, these show relatively high pore volumes. Whereas the average pore sizes are same, the smaller BET surface area and smaller pore volume suggest that the pore mouths of Al2 O3 (P123) was plugged by NiO loading on NiO/Al2 O3 (P123) in comparison with those of NiO–Al2 O3 (P123). The result is in good agreement with those observed with the hysteresis.
Fig. 4. Pore size distribution of NiO–Al2 O3 catalysts.
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Table 1 BET surface areas, pore volumes and average pore sizes. BET surface area (m2 /g)
Catalyst NiO–Al2 O3 (P123) NiO–Al2 O3 (F127) NiO–Al2 O3 (PEG) NiO–Al2 O3 (lauric) NiO–Al2 O3 (citric) NiO–Al2 O3 (none) NiO/Al2 O3 (P123) NiO/␥-Al2 O3 NiO/␣-Al2 O3
336 263 274 85 174 141 267 202 10
Pore volume (cm3 /g)
Average pore size (nm)
0.60 0.56 0.64 0.08 0.10 0.15 0.48 0.63 –
3.6 4.3 4.7 1.9 1.1 2.2 3.6 5.2 –
Fig. 5. XRD patterns of NiO–Al2 O3 (P123) catalyst.
3.2. NiO species In the bulk NiO–Al2 O3 , NiO, NiAl2 O4 , and Al2 O3 were supposed to be present as precursors of catalyst. As shown in Fig. 5, however, X-ray diffraction pattern of NiO–Al2 O3 (P123) as a representative of NiO–Al2 O3 catalyst provided little information on the Ni species, probably because the particle sizes of these possible oxides are small. A similar result was also reported by Morris et al. [3]. Therefore, TPR (temperature programmed reduction) measurement was carried out in order to investigate metal–support interactions and the reducibility of the NiO species. It is known that the reduction temperature of NiO depends on the nature of the interactions between Ni and supports such as Al2 O3 or SiO2 . For example, the reduction temperature of NiAl2 O4 -like species formed during catalyst preparation by sol-gel method is above 800 ◦ C. Fig. 6 shows TPR profiles of the prepared catalysts with some template. As references, TPR profiles of NiO/Al2 O3 (P123), NiO/␥-Al2 O3 , NiO/␣-Al2 O3 , and bulk NiO were compared. TPR profiles in Fig. 6 show that bulk NiO is easily reduced by H2 at 350 ◦ C, and that NiO supported on ␣-Al2 O3 has weak interaction with alumina with respect to the reduction peak temperature. In our previous report [6], TPR profiles of NiO-K/␣-Al2 O3 catalysts
Fig. 6. TPR profiles of NiO–Al2 O3 catalysts, supported NiO catalysts, and NiO.
were deconvoluted into four peaks, which means four kinds of NiO species were included in the supported catalyst. With increasing surface area of the catalyst, NiO species are influenced by the alumina support and the reduction peak of each catalyst shifted to higher temperature. The supported catalyst and NiO–Al2 O3 catalysts prepared using carboxylic acids, showed two reduction peaks. The result suggests that NiO is not uniformly supported on, or included in, alumina. In other words, interaction between the NiO and alumina is not singular. On the contrary, mesoporous NiO–Al2 O3 catalysts prepared using P123, F127, and PEG, have only a single reduction peak at about 700–750 ◦ C. Using the same decon-
Table 2 Ni reduction degrees and dispersions. Catalyst
Ni reduction degree (%)
H2 adsorption (mol/g)
Ni dispersion (%)
NiO–Al2 O3 (P123) NiO–Al2 O3 (F127) NiO–Al2 O3 (PEG) NiO–Al2 O3 (lauric) NiO–Al2 O3 (citric) NiO–Al2 O3 (none) NiO/Al2 O3 (P123) NiO/␥-Al2 O3 NiO/␣-Al2 O3
63 100 100 67 98 94 100 97 100
21 56 70 15 9 36 32 13 33
4.8 8.1 10.0 3.4 1.4 5.7 6.9 2.7 3.9
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Fig. 7. Activity and selectivity of NiO–Al2 O3 catalysts and supported NiO catalysts. () CH4 conversion, (♦) O2 conversion, () H2 selectivity, and () CO selectivity. (a) NiO–Al2 O3 (P123), (b) NiO–Al2 O3 (F127), (c) NiO–Al2 O3 (PEG), (d) NiO–Al2 O3 (lauric), (e) NiO–Al2 O3 (citric), (f) NiO–Al2 O3 (none), (g) NiO/Al2 O3 (P123), (h) NiO/␥-Al2 O3 , and (i) NiO/␣-Al2 O3 .
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Table 3 Coke and Ni0 /Nicat after CH4 /O2 /He pulse. Catalyst
Ni0 /Nicat (%)
Coke (mg/g-cat)
NiO–Al2 O3 (P123) NiO/␥-Al2 O3 NiO/␣-Al2 O3
63 0 50
0.4 0.0 2.6
volution method, roughly one peak was observed in these catalysts. Thus the chemical state was uniform. It is remarkable that even NiO supported on Al2 O3 (P123) is uniformly supported on alumina on these catalysts, judging from the single reduction peak. Table 2 summarizes the reduction degrees, H2 adsorptions and Ni dispersions for prepared catalysts. Their reduction degree was calculated from the total amount of H2 consumption for TPR. Reduction degrees of NiO–Al2 O3 (P123) and NiO–Al2 O3 (lauric) were 63% and 67%; these were smaller than of those of other catalysts. Whereas it is assumed that a part of NiO is not reduced with H2 due to existence as NiAl2 O4 , it is more probable that NiO is partially covered and isolated inside the alumina. 3.3. Catalytic performance in partial oxidation of methane Fig. 7 shows the activity and selectivity of the prepared catalysts as a function of contact time. Both CH4 conversion and O2 conversion decreased with decreasing contact time over each catalyst. Such a behavior is ordinary for the conventional catalytic reaction, while it is not the case in partial oxidation of methane. Because the reaction is highly exothermic, hot spots are easily formed in the catalyst bed and they can cause the rapid increase of conversions at short contact time. Thus, careful experiments were required in order to obtain the results like those in Fig. 7[7]. Whereas supported NiO catalysts (NiO/Al2 O3 (P123) and NiO/␥-Al2 O3 ) and bulk NiO catalysts (NiO–Al2 O3 (lauric), NiO–Al2 O3 (citric) and NiO–Al2 O3 (none)) were found to have low activity, NiO/␣-Al2 O3 showed relatively high CH4 conversion and syngas selectivity. It is remarkable that bulk type NiO catalyst with mesopores (NiO–Al2 O3 (P123), NiO–Al2 O3 (F127) and NiO–Al2 O3 (PEG)) showed quite high CH4 conversion under contact time 0.8 ms g-cat/cm3 . Particularly, NiO–Al2 O3 (P123) showed over 88% H2 selectivity and 78% CO selectivity at 0.1 ms. If the selectivities were determined by the equilibrium state of side reactions such as steam reforming of methane and dry reforming of methane, where steam and CO2 were formed from the combustion of methane, the selectivity can not be so high. Thus these selectivities are higher than those thermodynamically predicted values. Such high selectivity suggests some contribution of the direct route of syngas formation. 3.4. Oxidation tolerance of Ni metal Nickel metal species are active for a direct route of syngas formation from CH4 because C–H bonds can be cleaved on Ni metal. Thus, high tolerance of Ni metal species against the oxidation under reaction conditions in oxidative atmosphere is the most desired for Ni catalysts. CH4 /O2 and O2 pulse reactions were carried out in order to investigate the tolerance for oxidation of Ni metal. The result of the 20th pulse is illustrated in Fig. 8(a). Syngas formation was observed even after the 20 pulses. After the reactant CH4 /O2 pulse, O2 pulses were repeated until no consumption of O2 pulse was observed, as shown in Fig. 8(b). From the amount of formed CO and consumed O2 , the amount of Ni metal after the 20 pulses was estimated. In Table 3 are compared Ni metal and the amount of carbon deposition after 20 CH4 /O2 pulses on NiO–Al2 O3 (P123), NiO/␥Al2 O3 , and NiO/␣-Al2 O3 . Clearly NiO/␥-Al2 O3 was easily oxidized by CH4 /O2 pulses and therefore showed low activity and selectivity
Fig. 8. Pulse reaction for estimation of coke formation and Ni0 /Nicat ratio. (a) Products pattern after 20th CH4 /O2 /He pulse and (b) CO formation and O2 pulse behavior at repeated O2 /He pulses.
in Fig. 7. A larger part of Ni0 species remained after the pulse reaction in reduced NiO–Al2 O3 (P123) than in reduced NiO/␣-Al2 O3 . The tolerance against oxidation of Ni metal of NiO–Al2 O3 (P123) was enhanced; thus, high selectivity to CO and H2 were achieved. 3.5. On the role of mesopore From these observations, the presence of mesopores and high tolerance of Ni metal species against oxidation are to be the essential properties for partial oxidation of methane to syngas. The reducibility of NiO estimated by TPR, dispersion of Ni metal by H2 adsorption, BET surface area of the catalyst, and pore textures (size and volume) are, however, similar between the bulk NiO–Al2 O3 (P123) catalyst and the supported NiO/Al2 O3 (P123) catalyst. The quite low activity and low selectivity for syngas of the latter catalyst may not be justified. Considering the shape of mesopores, we think that NiO would be mainly located at the pore mouth of the supported NiO/Al2 O3 (P123) catalyst and active sites would be formed at the locations after reduction. If this is the case, the major difference between the two catalysts is the location of the active sites, because Ni species are well distributed in Al-O frame in the bulk NiO–Al2 O3 (P123) catalyst and thus the Ni metal on the surface in the mesopore is the active site. Reactant gas first should diffuse into the mesopores of NiO–Al2 O3 (P123) catalyst; in such small pores for Knudsen diffusion region, the diffusivity of CH4 and of O2 are different. The diffusion coefficients of CH4 and O2 in pores with
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diameter 3.6 nm under reaction conditions are 1.33 × 10−6 m2 /s and 0.94 × 10−6 m2 /s, respectively. If these gases diffuse independently into a straight pore (diameter 3.6 nm), the CH4 /O2 ratio of reaction gas (CH4 /O2 ratio = 2 at outside of pores) in the pore reaches 3.0 at 20 m depth from the inlet of the pore after 0.1 ms. In a real particle, the pore can not be a straight one and gases are converted to syngas during diffusion. So the calculated ratio is only a rough guideline, but it reveals that the ratio can be higher in the mesopores. It is well know that high CH4 /O2 is desirable for high syngas selectivity [8]. Using high CH4 /O2 ratio will increase the apparent tolerance of Ni metal species against the oxidation under reaction conditions. Thus, preparation of mesoporous bulk NiO–Al2 O3 catalysts resulted in high surface area catalyst, strong interaction between Al2 O3 and Ni species, formation of uniform Ni species, selective gas diffusion for high CH4 /O2 ratios, and apparent high tolerance of Ni metal species against the oxidation. The above combined factors were influential on the high activity and selectivity of NiO–Al2 O3 (P123) catalyst for partial oxidation of methane to syngas. 4. Conclusions Mesoporous bulk type NiO–Al2 O3 catalysts were applied to catalytic partial oxidation of methane to syngas at 650 ◦ C, 1 MPa. Among the various types of organic reagents, such as P123, F127,
PEG, and some carboxylic acids, the surfactants and PEG were effective to prepare mesoporous oxides. NiO–Al2 O3 catalyst prepared by using P123 gave the highest BET surface area of 340 m2 /g. The catalyst was reduced by H2 and then used for the reaction. High CO and H2 selectivities of 77% and 88%, respectively, were attained by the catalyst at the short contact region of 0.1 ms. High tolerance against the oxidation, which was also confirmed by pulse reaction, of Ni species under reaction conditions would originated from the mesopore structure, and would contribute to the high selectivity of mesoporous NiO–Al2 O3 catalysts. Mesoporous bulk type NiO–Al2 O3 catalyst was concluded to be a promising catalyst for partial oxidation of methane to syngas. Acknowledgment We acknowledge financial support from JSPS, Grant-in-Aid for Scientific Research (S), 17106011, 2005. References [1] [2] [3] [4] [5] [6]
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