direct synthesis of methane from glycerol by using silica

1. Introduction

Biomass-derived energy sources including bio-gasoline, biodiesel, and biogas are renewable resources with high potential for solving environmental problems. Biodiesel can be manufactured through the trans-esterification of vegetable oils with alcohol. However, ca. 10 wt% of glycerol is simultaneously generated as a surplus waste product1),2). Consequently, many researchers have made efforts for the use of glycerol to provide useful chemicals including acrylic acid, aryl alcohol, propanediol and propene through catalytic con-versions2)～10).

Hydrogen production from glycerol through the steam-reforming over metal catalysts11)～16) has been a potential renewable alternative to fossil resources. The steam-reforming of glycerol takes place through multi-ple reactions (Eqs. (2) and (3)) according to stoichio-metric Eq. (1). In addition, the methanation (Eqs. (4) and (5)) as well as the coke formation could take place in parallel17).

C3H8O3 + 3H2O! 7H2 + 3CO2 (1)

C3H8O3 ! 4H2 + 3CO (2)

CO + H2O! H2 + CO2 (3)

CO + 3H2 ! CH4 + H2O (4)

CO2 + 4H2 ! CH4 + 2H2O (5)

The carbon atoms of glycerol must be emitted as CO2 gas during the formation of hydrogen. Such genera-tion of CO2 leads to the equilibrium limitation in the water-gas shift (WGS) reaction (Eq. (3)). However, subsequent transformation of CO and CO2 to methane would allow the use of the carbon atoms of glycerol as an energy resource, since methane is widely used as an energy source in the industry, energy, and transportation sectors. Although an adequate supply of hydrogen is generally required for the methanation of CO and CO2

18)～20), hydrogen produced during the steam-reforming of glycerol is directly available. Therefore, our target in this study is to develop a catalytic process for the direct synthesis of methane from glycerol for the use of biomass-derived carbon species as an energy resource.

Nickel catalysts have been recognized as a prime candidate for the direct synthesis of methane from glyc-erol because of their high catalytic activities for the steam-reforming, the WGS reaction, and the methana-tion of CO and CO2

18)～22). Over a nickel catalyst,

311Journal of the Japan Petroleum Institute, 60, (6), 311-321 (2017)

J. Jpn. Petrol. Inst., Vol. 60, No. 6, 2017

[Regular Paper]

Direct Synthesis of Methane from Glycerol by Using Silica-modified Nickel Catalyst

Hiroyuki IMAI＊, Manami YAMAWAKI, and Xiaohong LI

Faculty of Environmental Engineering, The University of Kitakyushu, 1-1 Hibikino, Wakamatsu-ku, Kitakyushu 808-0135, JAPAN

(Received April 6, 2017)

Direct synthesis of methane from glycerol has been carried out over silica-modified nickel catalysts in the pres-ence of water without the supply of hydrogen. The catalysts were synthesized by co-loading nickel species with silica on γ-Al2O3 by using a gel containing nickel and silica sources. In the conversion of glycerol over the Ni_

SiO2/Al2O3 catalyst, glycerol was firstly decomposed to CO and H2; subsequently, methane was produced through the methanation of CO and CO2 with H2 generated during the reaction. Higher reaction temperatures and pres-sures resulted in the enhancement of the methane formation. In addition, the yield of methane was increased by increasing the water content in the glycerol aqueous solution. 20 wt%Ni-20 wt%SiO2/Al2O3 gave the maximum space-time yield (STY) of methane of 122 mol kg–1 h–1 with the feed of 50 wt% glycerol aqueous solution at 673 K under the pressure of 0.3 MPa. Co-loading of nickel species with silica on γ-Al2O3 was effective in sup-pressing the sintering of nickel particles. The coke deposition on the catalyst was significantly suppressed by modifying nickel particles with silica. The amount of deposited coke was decreased by increasing the amount of silica; the amount of coke deposited on Ni-30 wt%SiO2/Al2O3 was one-third of that on conventional Ni/γ-Al2O3.

KeywordsGlycerol, Nickel catalyst, Silica modification, Alumina support, Methane synthesis, Reforming

DOI: doi.org/10.1627/jpi.60.311 ＊ To whom correspondence should be addressed. ＊ E-mail: [email protected]

glycerol is firstly decomposed to CO and H2; sub-sequently, CO is converted to CO2 through the WGS reaction to produce H2. Finally, CO and CO2 are transformed to methane by using H2 generated during the reaction. In comparison with noble metal cata-lysts, supported nickel catalysts are generally subject to the deactivation mainly due to the sintering of nickel particles and the coke deposition on the active sites during the reactions. Addition of various metals or metal oxides to nickel catalysts can improve their resistances to the sintering of nickel particles and coke deposi-tion23)～26). Furthermore, it has been reported that core-shell catalysts consisting of a nickel core covered with silica layers exhibited a higher stability compared with conventional nickel catalysts in the CO methana-tion and the dry-reforming of methane due to the sup-pression of both the sintering and coke deposition27)～30). Therefore, nickel catalysts with highly dispersed nickel particles protected by silica can be expected to improve both catalytic activity and catalyst life in the reforming of glycerol to methane.

In this study, we investigated the catalytic perfor-mances of silica-modified nickel catalysts in the direct synthesis of methane from glycerol in the gas phase. Steam was fed to the catalysts, whereas no hydrogen was supplied. A series of nickel catalysts were pre-pared by loading a gel containing nickel and silica sources on a γ-Al2O3 support. The effects of the mod-ification with silica on the catalytic properties as well as stability were investigated. We also investigated the effects of reaction parameters including temperatures, pressures, and feed compositions on the catalytic prop-erties to optimize reaction conditions.

2. Experimental

2. 1. Catalyst PreparationSilica-modified nickel catalysts were prepared using

the sol-gel method and the impregnation method as fol-lows. Ni(NO3)2・6H2O was dissolved in ethylene glycol (EG) by stirring at 313 K to produce an EG solution with a Ni/EG molar ratio of 2. Ethanol and tetraethyl orthosilicate (TEOS) were added to the EG solution, and the resulting mixture was stirred at 333 K to pro-vide a homogeneous gel. γ-Al2O3 powder was then added to the gel. The mixture was homogenized by stirring, and then dried at room temperature overnight. The resulting material was calcined in air at 723 K for 3 h. The silica-modified nickel catalyst is designated as Ni_SiO2/Al2O3.

A conventional nickel catalyst was prepared by im-pregnation method. A mixture of γ-Al2O3 and the Ni(NO3)2・6H2O aqueous solution was evaporated at 323 K to provide a homogenized powder. The result-ing material was calcined in air at 723 K for 3 h.

2. 2. CharacterizationThe structure of the catalysts was examined by X-ray

diffraction (XRD, Rigaku SmartLab). The diffractom-eter was operated at 40 kV and 20 mA using a Cu-Kα radiation source. XRD patterns were recorded over the angular range of 20-80º. Transmission electron microscopic (TEM) images of the catalysts were obtained on a JEM-3010 microscope (JEOL Ltd.) operating at 300 kV. The sample was mounted on a carbon-coated microgrid (Okenshoji Co.) without any metal coating. The amount of coke deposited during the reaction was determined from the thermogravimetric (TG) profile of weight loss from 473 to 1173 K measured with a thermogravimetric-differential thermal analyzer (TG-DTA, Rigaku ThermoPlus).2. 3. Methane Synthesis from Glycerol

The conversion of glycerol was carried out in a fixed-bed reactor of the pressurized flow type. First, 0.3 g of 355-710 μm pellets of the nickel catalyst was physi-cally mixed with 1.5 g of inert glass sand, and then the mixture was loaded into the reactor. The catalyst was reduced in a flow of H2 at 723 K for 3 h prior to the reaction. After the reduction, the catalyst was cooled down to 473 K in a flow of N2. The glycerol aqueous solution was introduced into the catalyst with a flow of 0.06 mL min–1, and then the catalyst was heated up to the desired reaction temperature. Ar gas was used as a carrier with a flow of 30 mL min–1. The condensed products were trapped in an ice bath condenser. CO, CO2, CH4, and Ar in the outlet gas were analyzed with a gas chromatography (Shimadzu GC-8A) equipped with a thermal conductivity detector (TCD) and a packed column of activated charcoal. H2 in the outlet gas was analyzed with an off-line gas chromatography (Shimadzu GC232) equipped with a TCD and a packed column of molecular sieve 5A.

3. Results and Discussion

3. 1. Investigation of Reaction Conditions for Conversion of Glycerol to Methane

3. 1. 1. Effect of Glycerol Concentration in the Feed

The conversion of glycerol to methane was carried out over the Ni_SiO2/Al2O3 catalyst with 20 wt% Ni and 20 wt% SiO2 loadings. First, the effect of the glycerol concentration in the feed on the catalytic per-formances was examined at 673 K under atmospheric pressure (Fig. 1). When the 50 wt% glycerol aqueous solution was used as a feed, CH4 and CO2 were mainly produced as a carbon-based compound with no CO pro-duced, indicating that CO resulting from the decompo-sition of glycerol was readily converted to CH4 and CO2 through the methanation and the WGS reaction, respec-tively.

The total amounts of CO, CO2 and CH4 proportionally

312


increased with the increase in the glycerol concentra-tion. In addition, even in the absence of the Ni cata-lyst, glycerol was converted to H2, CO, CO2 and CH4 with space-time yields (STYs) of 0.06, 0.01, 0.02 and 0 mol kg-cat.–1 h–1, respectively, at 6 h after the reaction started (50 wt% glycerol aqueous solution, 673 K and 0.3 MPa). Therefore, the conversion of glycerol was not the rate-limiting step.

The increase in the glycerol concentration in the feed drastically increased the STYs of H2 and CO. By con-trast, the STY of CH4 decreased, although higher STYs of H2 are favorable for the methanation of CO. The STY of CO2 was improved by increasing the glycerol concentration from 50 to 60 wt%, while was reduced with a further increase from 60 to 70 wt%. According to the previous findings31)～33), high ratios of steam/glycerol are favorable for the production of H2 due to the promotion of the WGS reaction and the suppression of the methanation. Nevertheless, the methane forma-tion was enhanced using high ratios of steam/glycerol in this study. The decomposition of glycerol over a metal surface results in the formation of CO on the metal surface34). The adsorbed CO can be removed from the metal surface through the WGS reaction to form CO2. Meanwhile, Lim et al. have proposed that a large amount of CO on a metal surface prevent H2O from adsorbing

on the metal surface to inhibit the WGS reaction35). Therefore, the covering of the metal surface with CO also suppresses the adsorption of H2

35)～38). Thus, it is suggested that increasing the glycerol concentration enhanced the CO generation, leading to improving the STY of CO2 through the WGS reaction. However, a further increase in the glycerol concentration resulted in the saturation of the Ni surface with a large amount of CO in addition to decreasing the steam amount, leading to the decline of the WGS reaction as well as the metha-nation of CO and CO2.3. 1. 2. Effect of Pressure in the Reaction System

The effect of the pressure on the catalytic perfor-mances was investigated with the 50 wt% glycerol aqueous solution, which was the optimum glycerol con-centration for the formation of CH4 at 673 K (Fig. 2). The STY of CH4 was increased by raising the pressure from atmospheric pressure to 0.3 MPa, while that remained virtually unchanged with a further increase from 0.3 to 0.5 MPa. With the increase in the pres-sure, the STY of H2 was sharply decreased due to the consumption in the methanation, and the STY of CO2 was slightly decreased. No formation of CO was ob-served during the reaction, independent of the pressure. High pressures are favorable for the methanation of CO and CO2 on the basis of the thermodynamic equilibrium.

313


Reaction conditions: cat., 0.3 g; reaction temperature, 673 K; pressure, atmospheric pressure.

Fig. 1● Reforming of Glycerol over 20 wt%Ni-20 wt%SiO2/γ-Al2O3 with Different Glycerol Concentrations of (a) 50 wt%, (b) 60 wt%, and (c) 70 wt% in the Feed

Reaction conditions: cat., 0.3 g; reaction temperature, 673 K; WHSV, 6.7 h–1.

Fig. 2● Reforming of Glycerol over 20 wt%Ni-20 wt%SiO2/γ-Al2O3 under Different Pressures of (a) Atmospheric Pressure, (b) 0.3 MPa, and (c) 0.5 MPa

Therefore, the formation of CH4 can be enhanced by increasing the pressure from atmospheric pressure, whereas the formation of CH4 is insensitive to changes in pressures under pressurized conditions.3. 1. 3. Effect of Reaction Temperature

Since the reaction temperature strongly influences the glycerol decomposition, the WGS reaction, and the methanation, the effect of the temperature on the cata-lytic performances was investigated with the 50 wt% glycerol aqueous solution. At 593 K under atmospheric pressure, CO and H2 were predominantly produced during the reaction (Fig. 3). Although the formation of CH4 and CO2 was observed during the initial period of the react ion, the STYs of both CH4 and CO2 decreased to below 10 mol kg-cat.–1 h–1 at 5 h after the reaction started. With the increase in the temperature, the STY of CH4 was gradually improved together with the increase in the STY of CO2. The STYs of CO and H2 were increased with an increase in the temperature from 593 to 623 K; however, those were drastically de-creased by further increasing to 673 K, and no forma-tion of CO was observed. It is suggested that both the WGS reaction and the methanation occurred slowly, and that the methanation would be more favorable than the WGS reaction at the low temperatures, which is in agreement with the previous report39). At the higher temperature of 673 K, both the methanation and the WGS reaction readily proceeded, resulting in the com-plete consumption of CO.

The effect of the temperature on the catalytic perfor-mances was also investigated under the pressure of 0.3 MPa (Fig. 4), since the CH4 formation is more favorable under pressurized conditions as shown in Fig. 2. Although CO and H2 were mainly produced at a low temperature of 593 K, no formation of CO was observed at the temperatures above 623 K. Both the CH4 and CO2 STYs drastically increased with the increase in the temperature from 593 to 623 K. The further increase from 623 to 673 K resulted in the in-crease in the STY of CH4 accompanied with a decrease

in the STYs of CO2 and H2. Interestingly, under the pressurized conditions, the improvement in the STYs of CH4 and CO2 together with the complete consumption of CO was observed at the low temperature of 623 K, which was 50 K lower than the temperature for the complete consumption of CO under atmospheric pres-sure.

Considering our findings above, it is concluded that in the conversion of glycerol over the Ni_SiO2/Al2O3 catalyst, CH4 is efficiently formed in the feed with 50 wt% glycerol aqueous solution at 673 K under the pressure of 0.3 MPa.3. 2. Conversion of Glycerol over Ni–SiO2/Al2O3

Catalysts with Different Compositions3. 2. 1. Effect of Ni Loading

The nature as well as the amount of Ni particles strongly influences the catalytic performances in the steam-reforming of glycerol40)～42). Figure 5 shows the results of the conversion of glycerol over Ni-20 wt%SiO2/Al2O3 with three different Ni loadings of 10, 15, and 20 wt% at 673 K under the pressure of 0.3 MPa. With the increase in the Ni loading, the in-crease in the STY of CH4 was observed together with the decrease in the STYs of H2 and CO. The STY of CO2 was increased by increasing the Ni loading from 10 to 15 wt%, while that was decreased by further in-creasing to 20 wt%. These results above indicate that Ni particles promoted the methanation as well as the WGS reaction.3. 2. 2. Effect of SiO2 Loading

In order to investigate the effect of the SiO2 loading on the catalytic performances as well as deactivation, the conversion of glycerol was carried out by varying the SiO2 loading of 20 wt%Ni_SiO2/Al2O3 under the optimized conditions (Fig. 6). No apparent declines in the formation of CH4 and CO2 were observed over all the catalysts during the reaction. The conventional Ni/Al2O3 catalyst exhibited a gradual increase in the STYs of H2 (42 mol kg–1 h–1 at 1 h to 53 mol kg–1 h–1 at 5 h) and CO (0.8 mol kg–1 h–1 at 1 h to 1.2 mol kg–1 h–1 at

314


Reaction conditions: cat., 0.3 g; pressure, atmospheric pressure; WHSV, 6.7 h–1.

Fig. 3● Reforming of Glycerol over 20 wt%Ni-20 wt%SiO2/γ-Al2O3 at Different Reaction Temperatures of (a) 593 K, (b) 623 K, and (c) 673 K

5 h). By contrast, all the Ni_SiO2/Al2O3 catalysts showed little variation in the STYs of the products during the reaction, suggesting that silica on the cata-lyst remained unchanged during the reaction. In addi-tion, the Ni_SiO2/Al2O3 catalysts exhibited similar STYs of the products, and a higher STY of CH4 than Ni/Al2O3. The molar ratios of the outlet gas compositions of the Ni_SiO2/Al2O3 catalysts were close to CH4/CO2/CO/H2＝1/0.87/0.006/0.35, while the equilibrium com-

position under the reaction conditions in Fig. 6 was es-timated as CH4/CO2/CO/H2＝1/0.92/0.01/0.51. Thus, it is indicated that the CH4 concentration in the products was increased by the silica-modification.

The catalytic properties can be affected by changes in the natures of the Ni particles during the reaction. Changes in the morphology of the Ni particles were evaluated by TEM observations. Figure 7 shows TEM images of fresh Ni catalysts after the reduction

315


Reaction conditions: cat., 0.3 g; pressure, 0.3 MPa; WHSV, 6.7 h–1.

Fig. 4● Reforming of Glycerol over 20 wt%Ni-20 wt%SiO2/γ-Al2O3 at Different Reaction Temperatures of (a) 593 K, (b) 623 K, (c) 673 K, and (d) 723 K

Reaction conditions: cat., 0.3 g; reaction temperature, 673 K, pressure, 0.3 MPa; WHSV, 6.7 h–1.

Fig. 5● Reforming of Glycerol over Ni-20 wt%SiO2/γ-Al2O3 with Different Ni Loadings of (a) 10 wt%, (b) 15 wt%, and (c) 20 wt%

316


Reaction conditions: cat., 0.3 g; reaction temperature, 673 K, pressure, 0.3 MPa; WHSV, 6.7 h–1.

Fig. 6● Reforming of Glycerol over 20 wt%Ni_SiO2/γ-Al2O3 with Different SiO2 Loadings of (a) 0 wt%, (b) 10 wt%, (c) 20 wt%, and (d) 30 wt%

Fig. 7● TEM Images of (A) Fresh and (B) Spent 20 wt%Ni_SiO2/γ-Al2O3 with Different SiO2 Loadings of (a) 0 wt%, (b) 10 wt%, (c) 20 wt%, and (d) 30 wt%

and spent Ni catalysts after 340 min of the reaction. The TEM images revealed that changes in the size of the Ni particles were dependent on the SiO2 amount. The Ni particles of Ni/Al2O3 were obviously enlarged after the reaction, indicating that the sintering of the Ni particles took place even at the lower temperature of 673 K compared with conventional temperatures for the steam-reforming of glycerol. Slight increases in the size of the Ni particles were observed on the spent Ni-10 wt%SiO2/Al2O3. The further loading of SiO2 re-sulted in unchanged sizes of the Ni particles even after the reaction. It is indicated that the SiO2 loading sup-pressed the sintering of the Ni particles during the reac-tion due to the modification of the Ni particles with SiO2.

The coke deposition on supported Ni catalysts causes serious deactivation in the steam-reforming of glycer-ol34),40),43)～45). Carbonaceous species deposited on the catalyst were evaluated. The TEM measurement revealed that filamentous carbonaceous species were deposited on the catalysts during the reaction, irrespective of the SiO2 loading (Supplementary information, Fig. 8). The coke deposition on the catalyst was also detected by XRD measurement (Fig. 9). The fresh catalysts exhibited peaks attributed to Ni metal at 44.5, 51.8, and 76.5º 46) together with peaks to γ-Al2O3. After the reaction for 340 min, a peak ascribed to amor-phous carbon was observed at 25.8º; in addition, peaks to nickel carbide newly appeared at 39.2, 41.5, 58.5, 71.2, and 78.0º. The peaks attributed to both amor-phous carbon and nickel carbide were weakened and broadened on Ni-30 wt%SiO2/Al2O3. In all the XRD patterns any peaks ascribed to other Ni compounds such as NiO were not observed regardless of the SiO2 load-ing, indicating that Ni species remained stable under the present reaction conditions, although Ni species can be oxidized by steam at low temperatures47).

The amounts of coke deposited on the catalysts were evaluated by TG-DTA analyses (Fig. 10). The spent catalysts were heated in a flow of air. All the spent catalysts exhibited a large exothermic peak at about 753 K with a shoulder peak and a small peak in a range

of 473-573 K. A slight increase in the sample weight accompanying an exothermic peak was observed at about 603 K, and then the weight loss started at about 623 K. For comparison, the fresh catalysts were also heated in a flow of air for TG-DTA analyses (Fig. 11). All the fresh catalysts showed a slight increase in the sample weight accompanying an exothermic peak in the range of 473-573 K. Therefore, the TG-DTA profiles of the spent catalysts indicated that the slight increase in the sample weight with the exothermic peak at the lower temperature was derived from the oxidation of Ni metal to NiO. On the other hand, the exothermic peak with the weight loss was derived from the combustion of coke. Weight loss below 773 K is ascribed to the combustion of filamentous or encapsulated carbon spe-cies40). The combustion of nickel carbide occurs at low temperatures in the range of 523-723 K, while the prominent peak derived from carbonaceous species appears at high temperatures in the range of 673-873 K48),49). Therefore, the weight loss with the larg-est exothermic peak was attributed to the combustion of filamentous carbon species, which are observed in the TEM images, and the shoulder peak at around 623-673 K to the combustion of nickel carbide. The shoulder peak was negligible on Ni-30 wt%SiO2/Al2O3, being consistent with the XRD finding (Fig. 9). The total weight loss in the range of 473-1173 K decreased with the increase in the SiO2 loading; the weight losses of the spent catalysts with the SiO2 loading of 0, 10, 20, and 30 wt% were 39.6, 13.1, 13.4, and 8.0 %, respec-tively. These results indicate that the co-loading of Ni particles with SiO2 is effective in the suppression of the coke deposition on the catalyst in the conversion of glycerol to methane.

The coke deposition can be suppressed on the cata-lyst with silica-modified Ni nanoparticles due to the absence of enough spaces for the carbon growth on the surface of Ni27)～30). Thus, it is assumed that the modi-fication with silica partly covered the Ni particles to decrease spaces for the carbon growth in the glycerol conversion as well, resulting in the decrease in the amount of deposited coke in comparison with conven-

317


(a) 0 wt%, (b) 10 wt%, (c) 20 wt%, and (d) 30 wt%.

Fig. 8● TEM Images of Spent 20 wt%Ni_SiO2/γ-Al2O3 with Different SiO2 Loadings

tional Ni/Al2O3.Furthermore, it was found that the Ni_SiO2/Al2O3

catalyst was able to effectively convert glycerol ob-tained in the bio-diesel production although methanol and/or alkaline species were present in the glycerol solution. Therefore, this catalyst can be applied to the synthesis of CH4 from glycerol in industrial processes.

4. Conclusions

Co-loading of Ni particles with SiO2 on γ-Al2O3 has been achieved by the combination of the sol-gel and impregnation methods to preserve Ni particles by modi-fying with SiO2. The synthesized Ni_SiO2/Al2O3 cata-lysts were applied to the direct synthesis of methane from glycerol in the presence of water without the supply of hydrogen. Higher reaction temperatures, pressures, and water contents in the glycerol aqueous solution resulted in positive effects on both the methanation and the WGS reaction, reaching to the maximum STY of CH4 of 122 mol kg–1 h–1 over 20 wt%Ni-20 wt%SiO2/Al2O3 using the 50 wt% glycerol aqueous solution as a feed at 673 K under the pressure of 0.3 MPa.

The modification of Ni particles with SiO2 retained the size of Ni particles although the sintering of Ni particles was apparently observed on Ni/γ-Al2O3. Furthermore, the coke deposition was significantly sup-pressed by the SiO2 loading. Ni-30 wt%SiO2/Al2O3 exhibited one-third of the coke deposition as Ni/γ-Al2O3. SiO2 on the catalyst would maintain the dis-tances between Ni particles as a buffer, and for sup-pressing the deposition of carbonaceous intermediates on the catalyst by partially modifying the surface of acidic γ-Al2O3 as well as Ni particles. Therefore, the modification of metal species with SiO2 can improve the stability of the catalyst to extend the catalyst life in the formation of methane from glycerol produced during the manufacture of bio-diesel.

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要　　　旨

シリカ修飾ニッケル触媒を用いたグリセリンからメタンへの直接変換

今井　裕之，山脇　愛美，黎　　暁紅

北九州市立大学国際環境工学部，808-0135 北九州市若松区ひびきの1番1号

バイオディーゼル製造で廃棄されるグリセリンをエネルギー資源として活用するため，グリセリンからの水素生成と同時に炭素成分のメタン化を行った。ニッケル源とシリカ源の混合ゲルをアルミナに担持することで，シリカをニッケル粒子間のバッファーとした Ni_SiO2/Al2O3触媒を合成した。Ni_SiO2/

Al2O3触媒において，水素の外部供給なしに，グリセリンからメタンの選択的合成を達成した。反応条件の検討により，高い

温度，圧力，水／グリセリン比においてメタン生成が有利になることを見出した。また，シリカ修飾は，反応中におけるニッケル粒子のシンタリング抑制および触媒上への重炭素質（コーク）蓄積の抑制に効果を示した。特に，コーク生成・蓄積はシリカの担持量に影響を受け，シリカ量の増加とともに蓄積コーク量は減少し，ニッケルの炭化によるニッケルカーバイドの生成が抑制された。

direct synthesis of methane from glycerol by using silica

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