- Email: [email protected]
From biomass to fuels: Hydrotreating of oxygenated compounds
ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y
33 (2008) 3485 – 3488
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
From biomass to fuels: Hydrotreating of oxygenated compounds I. Gandarias, V.L. Barrio, J. Requies, P.L. Arias, J.F. Cambra, M.B. Gu¨emez School of Engineering (UPV/EHU), c/ Alameda Urquijo s/n, 48013 Bilbao, Spain
art i cle info
ab st rac t
Article history:
Biomass is a renewable alternative to fossil raw materials in the production of liquid fuels
Received 20 July 2007
and chemicals. Pyrolyzed biomass derived liquids contain oxygenated molecules that need
Accepted 4 December 2007
to be removed to improve the stability of these liquids. A hydrotreating process, hydro-
Available online 14 March 2008
deoxygenation (HDO), is commonly used for this purpose. Thus, the aim of this work is to
Keywords: Biomass Liquid fuels Hydrodeoxygenation
examine the role of advanced NiMo and NiW catalysts developed for HDS purposes in a HDO reaction. In addition, product distribution and catalyst stability are studied against changes in the feed composition, the solvent, and the catalyst pretreatment. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Ni Mo W
1.
Introduction
Europe currently imports 50% of its total energy needs [1] and transport is in an even more precarious situation as it relies heavily on oil. Thus, the EU has set an objective of 20% substitution of conventional fuels with alternative fuels in the road transport by the year 2020 [2]. Biomass is worth consideration because of its clear advantages: (i) it is easily available for countries having no fossil raw material and its price does not suffer from market fluctuations, (ii) due to its CO2 neutrality and low sulfur and metal content, biomass is environmentally attractive [3]. The bio-oils can be obtained from biomass by high pressure liquefaction or by pyrolysis [4]. The product quality from the high pressure liquefaction is better but, as it gives lower yields at higher cost [5], recent studies are focused on pyrolysis. The characteristics of the bio-oils obtained from the biomass pyrolysis are very different of that obtained from the conventional petroleum. The S-content is negligible while they are rich in oxygenated molecules which are responsible for some deleterious properties of these crudes: high visco-
sity, low volatility, corrosiveness, immiscibility with fossil fuels, thermal instability and tendency to polymerise under exposure to air. Hence, upgrading of bio-oils means removal of oxygen [5]. In the present work the catalytic hydrodeoxygenation (HDO) with commercial NiMo and NiW catalysts is studied using phenol HDO as a model reaction.
2.
Experimental section
Catalysts elemental composition by ICP analysis for the NiMo catalyst was 3.2 of Ni, 16.0 of Mo and 3 of P (wt%) and for the NiW catalyst 4.1 of Ni and 13.9 of W (wt%) both on g-Al2 O3 . The catalyst (0.5 g) was calcined under nitrogen with a flow of 2.5 L/h (NTP) at 673 K under atmospheric pressure. After calcination, the catalyst was pretreated: reducing it with H2 or presulfiding it with 5 vol% of a H2 S=H2 mixture. Pretreatment was continued at 673 K under atmospheric pressure for 4 h with a gas flow rate of 2.5 L/h (NTP) in both cases. The reaction temperature was fixed in the range between 473 and 623 K at 1.5 MPa. The liquid feed containing 1 wt% of phenol in
Corresponding author. Tel.: +34 94 6017282/7282; fax: +34 94 6014179.
E-mail address: [email protected] (V.L. Barrio). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.12.070
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I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y
octane, was fed to the reactor at 10 g/h rate. Simultaneously, a total gas flow rate of 2 L/h (NTP) was fed to the system. In some experiments the gas was pure H2 and in others it was a mixture of H2 =H2 S with 0.1 vol% of H2 S. In the activity tests, the phenol HDO reactions were studied using the catalyst in its prereduced state, in its presulfurized state and in its presulfurized state with a continuous feed of 0.1 vol% of H2 S in the gaseous feed.
3.
Results
3.1. Temperature and activation type effect on the conversion and selectivity
Conversion (%)
In Fig. 1, the conversion obtained is presented as a function of the reaction temperature and activation type for the NiMo (Fig. 1a) and NiW (Fig. 1b). A temperature increase implied a conversion increase for both catalysts. Moreover, the NiMo catalyst was more active than the NiW catalyst for all the temperatures tested in its reduced or sulfided state. The activation type had the opposite effect on the catalysts. The NiW catalyst was more active when it was sulfided while the NiMo catalyst was more active when it was reduced. Therefore, for the NiMo catalyst, it seemed that the MoS2 active phase formation was not favored. This phase is postulated as the active sites precursor according with the control remote theory of Delmon and Grange [6]. This could be attributed to the NiAl2 O4 , MoO3 or Al2 ðMoO4 Þ3 phase formation during the catalyst preparation, species which are not desirable because they do not favor the MoS2 formation [7]. Nevertheless, the non-sulfided species formed were very active in HDO reactions. The main reaction products obtained from the HDO reaction were mainly benzene, cyclohexane, cyclohexene and methylcyclopentane. In previous phenol HDO works the presence of methyl-cyclopentane was not observed [8]. In our experiments, methyl-cyclopentane was detected. The quantity of methylcyclopentane measured for the NiW catalyst was higher than for the NiMo catalyst. Using the sulfided NiW catalyst the methyl-cyclopentane selectivity was around 50%. European regulations are restricting aromatics content in fuels. By 2010 the EU will limit aromatics content to 14%. As a consequence, for bio-oils refining it is not only very important to obtain high conversions for the HDO process, it is also
crucial to minimize the aromatics formation. In our case, as the reaction temperature increased benzene formation increased, due to a higher contribution of the HG route. As it was commented before, for higher temperatures the conversion also increased, thus it seems that it would be convenient to find an optimal balance between conversion and aromatics content. For the NiW catalyst the presulfurization favored the HIDHG via and in consequence the reduction of the aromatics formation (benzene) but only at high temperatures (623 K). If both catalysts were compared, the NiMo catalyst presented a higher hydrogenation activity, specially, at low temperatures. At the highest temperature both presented a similar behavior.
3.2.
Catalyst stability
Long activity tests were performed in order to study the catalyst stability in its reduced or sulfided state. For the NiMo catalyst prereduced, at 523, 573 and 623 K, complete conversion was observed for the time on stream measured. For longer times on stream ð1100 minÞ deactivation was measured and in literature it is reported [9] that coke seems to be the responsible of the deactivation. In the case of the presulfided NiMo catalyst, at low temperatures the activity increased with time on stream. Nevertheless, at high temperatures the activity decreased. This behavior could be explained if the previous results were taken into account. The NiMo catalyst was more active in the reduced state. Thus, at low temperatures when the reaction was taking place as there were no sulfur donor species, this catalyst was desulfided and in consequence the activity increased. At high temperatures (573–623 K), this positive effect of the continuous desulfidation was compensated with the activity loss caused by the coke formation which is more important at higher temperatures. In the case of the NiW catalyst, in the prereduced sample, the activity strongly decreased and, in the presulfided sample, the activity remained constant.
3.3. Effect of H2S addition as sulfur donor specie during the hydrotreatment In these experiments, the influence of the H2 S addition on the catalyst stability and on the selectivity was studied for the
100
100
80
80
60
60
40
40
20
20
0
200°C
250°C 300°C Temperature
33 (2008) 3485 – 3488
350°C H2S/H2
0 H2
250°C
300°C Temperature
350°C
Fig. 1 – Phenol conversion as a temperature function for the NiMo (a) and NiW (b) catalyst pretreated with H2 and H2 S=H2 .
ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y
NiMo
Conversion (%)
60
40
523 K
20 473 K 0
0
100
200 Time (min.)
300
400
Fig. 2 – Conversion of the NiMo catalyst: reaction with H2 ðE; mÞ and with H2 S=H2 (’; ).
NiW
95
Conversion (%)
90 85 80 75 70 65 60
0
100
200 Time (min.)
300
400
Fig. 3 – Conversion of the NiW catalyst: temp. 623 K, reaction with H2 ðEÞ and H2 S=H2 ð’Þ.
phenol HDO. In Fig. 2, the activity for the NiMo catalyst was presented at 473 and 523 K feeding H2 or H2 S=H2 to the system. This catalyst was more active when hydrogen was fed to the system, thus H2 S=H2 addition did not improved the catalyst activity. As it was commented, the active centers desulfurization enabled higher HDO reaction rates. Moreover, this catalyst seemed to be activated with time on stream due to the absence of species able to sulfide the catalyst. For the NiW catalyst, the H2 S=H2 addition during the hydrotreating was also studied at 523, 573 and 623 K (Fig. 3). At the lowest temperatures (523 and 573 K), the H2 S=H2 addition did not seem to noticeably affect the HDO activity. And on the contrary, at the highest temperature (623 K), it promoted the HDO reaction increasing the phenol conversion (see Fig. 3). These results corroborated that the NiW catalyst was more active in the sulfided state. Therefore, at high temperatures after the H2 S adsorption, if the active centers sulfurization was enhanced the phenol HDO reaction was promoted. In all the cases and for all the temperatures, the NiW catalyst deactivated with time on stream. Even at 623 K, when the activity is promoted a continuous deactivation was
33 (2008) 3485 – 3488
3487
observed. This could be attributed to coke deactivation or to continuous catalyst desulfidation. Regarding the H2 S=H2 addition effect on the selectivity different behaviors were observed. For the NiMo catalyst, the H2 S=H2 addition inhibited the HDO reaction through both vias, although the HG was less affected. Thus, the aromatics content in the products was higher. This is in contrast with the results presented in the literature using a CoMo=Al2 O3 catalyst for the phenol HDO [10,11]. A possible hypothesis deals with the use of a commercial catalyst with a significant P content. For the NiW catalyst, the effect of the H2 S=H2 addition on the selectivity was to promote the HID-HG route for all the temperatures. As a consequence product mixture aromatic content decreased.
4.
Conclusions
The NiMo catalyst presented a higher activity than the NiW catalyst for the phenol HDO reactions in all the temperature range studied and the product aromatics content originated for the NiMo catalyst was lower. Indeed the NiMo catalyst was less active in the sulfided state. This issue suggested that the formation of the MoS2 active phase was not enabled, perhaps due to the presence of P. The NiW catalyst presented a higher isomerization activity if compared with the NiMo catalyst. As a consequence, higher conversions to methyl-cyclopentane were achieved. In the case of the presulfided NiW catalyst, it was around a 50% of the total conversion. A reaction temperature increase implied a phenol conversion increase, but with a higher aromatics content. This was a negative consequence taking into account the aromatics restrictive European regulations. The effect of the H2 S=H2 addition during the hydrotreating was different for each catalyst. For the NiMo catalyst the H2 S acted as an inhibitor whereas for the NiW catalyst it had not influence at low temperatures and it was a promoter at high temperatures. Thus, this effect showed the great influence of the reaction temperature on the sulfurization level for each catalyst. For all the cases, the H2 S=H2 addition did not improve the catalyst stability and changed the process selectivity which could be attributed to the presence of two types of active sites: one for the HG route and other for the HID-HG route. The influence of the H2 S addition on the selectivity could be used to control the aromatic content in the resulting product.
Acknowledgments This work was supported by the Basque Government and the University of the Basque Country. R E F E R E N C E S
[1] hhttp://ec.europa.eu/energy/res/publications/doc/ 2004_brochure_biofuels_en.pdfi. [2] EC (European Commission). European Transport policy for 2010: time to decide; 2001. Available at: hhttp://europa.eu.inti. [3] Demirbas A. Prog Energy Combust Sci 2006;33(1):1.
ARTICLE IN PRESS 3488
I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y
[4] Fukuda H, Kondo A, Noda H. J Biosci Bioeng 2001;92(5):405. [5] Furimsky E. Appl Catal A 2000;199(2):147. [6] Delmon B, Grange P. In: Farinha Portela M, editor. Proceedings of the 2nd conference on industrial catalysis. Lisbon, 1988. p. 83. [7] Grimblot J. Catal Today 1998;41(1–3):111.
33 (2008) 3485 – 3488
[8] Viljava T.-R. PhD Dissertation, Helsinki University of Technology, Espoo; 2001. [9] Gevert BS, Otterstedt J-E. Biomass 1987;13(2):105. [10] Laurent E, Delmon B. Ind Eng Chem Res 1993;32:2516. [11] Gevert BS, Otterstedt J-E, Massoth FE. Appl Catal 1987; 31(1):119.
33 (2008) 3485 – 3488
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
From biomass to fuels: Hydrotreating of oxygenated compounds I. Gandarias, V.L. Barrio, J. Requies, P.L. Arias, J.F. Cambra, M.B. Gu¨emez School of Engineering (UPV/EHU), c/ Alameda Urquijo s/n, 48013 Bilbao, Spain
art i cle info
ab st rac t
Article history:
Biomass is a renewable alternative to fossil raw materials in the production of liquid fuels
Received 20 July 2007
and chemicals. Pyrolyzed biomass derived liquids contain oxygenated molecules that need
Accepted 4 December 2007
to be removed to improve the stability of these liquids. A hydrotreating process, hydro-
Available online 14 March 2008
deoxygenation (HDO), is commonly used for this purpose. Thus, the aim of this work is to
Keywords: Biomass Liquid fuels Hydrodeoxygenation
examine the role of advanced NiMo and NiW catalysts developed for HDS purposes in a HDO reaction. In addition, product distribution and catalyst stability are studied against changes in the feed composition, the solvent, and the catalyst pretreatment. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Ni Mo W
1.
Introduction
Europe currently imports 50% of its total energy needs [1] and transport is in an even more precarious situation as it relies heavily on oil. Thus, the EU has set an objective of 20% substitution of conventional fuels with alternative fuels in the road transport by the year 2020 [2]. Biomass is worth consideration because of its clear advantages: (i) it is easily available for countries having no fossil raw material and its price does not suffer from market fluctuations, (ii) due to its CO2 neutrality and low sulfur and metal content, biomass is environmentally attractive [3]. The bio-oils can be obtained from biomass by high pressure liquefaction or by pyrolysis [4]. The product quality from the high pressure liquefaction is better but, as it gives lower yields at higher cost [5], recent studies are focused on pyrolysis. The characteristics of the bio-oils obtained from the biomass pyrolysis are very different of that obtained from the conventional petroleum. The S-content is negligible while they are rich in oxygenated molecules which are responsible for some deleterious properties of these crudes: high visco-
sity, low volatility, corrosiveness, immiscibility with fossil fuels, thermal instability and tendency to polymerise under exposure to air. Hence, upgrading of bio-oils means removal of oxygen [5]. In the present work the catalytic hydrodeoxygenation (HDO) with commercial NiMo and NiW catalysts is studied using phenol HDO as a model reaction.
2.
Experimental section
Catalysts elemental composition by ICP analysis for the NiMo catalyst was 3.2 of Ni, 16.0 of Mo and 3 of P (wt%) and for the NiW catalyst 4.1 of Ni and 13.9 of W (wt%) both on g-Al2 O3 . The catalyst (0.5 g) was calcined under nitrogen with a flow of 2.5 L/h (NTP) at 673 K under atmospheric pressure. After calcination, the catalyst was pretreated: reducing it with H2 or presulfiding it with 5 vol% of a H2 S=H2 mixture. Pretreatment was continued at 673 K under atmospheric pressure for 4 h with a gas flow rate of 2.5 L/h (NTP) in both cases. The reaction temperature was fixed in the range between 473 and 623 K at 1.5 MPa. The liquid feed containing 1 wt% of phenol in
Corresponding author. Tel.: +34 94 6017282/7282; fax: +34 94 6014179.
E-mail address: [email protected] (V.L. Barrio). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.12.070
ARTICLE IN PRESS 3486
I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y
octane, was fed to the reactor at 10 g/h rate. Simultaneously, a total gas flow rate of 2 L/h (NTP) was fed to the system. In some experiments the gas was pure H2 and in others it was a mixture of H2 =H2 S with 0.1 vol% of H2 S. In the activity tests, the phenol HDO reactions were studied using the catalyst in its prereduced state, in its presulfurized state and in its presulfurized state with a continuous feed of 0.1 vol% of H2 S in the gaseous feed.
3.
Results
3.1. Temperature and activation type effect on the conversion and selectivity
Conversion (%)
In Fig. 1, the conversion obtained is presented as a function of the reaction temperature and activation type for the NiMo (Fig. 1a) and NiW (Fig. 1b). A temperature increase implied a conversion increase for both catalysts. Moreover, the NiMo catalyst was more active than the NiW catalyst for all the temperatures tested in its reduced or sulfided state. The activation type had the opposite effect on the catalysts. The NiW catalyst was more active when it was sulfided while the NiMo catalyst was more active when it was reduced. Therefore, for the NiMo catalyst, it seemed that the MoS2 active phase formation was not favored. This phase is postulated as the active sites precursor according with the control remote theory of Delmon and Grange [6]. This could be attributed to the NiAl2 O4 , MoO3 or Al2 ðMoO4 Þ3 phase formation during the catalyst preparation, species which are not desirable because they do not favor the MoS2 formation [7]. Nevertheless, the non-sulfided species formed were very active in HDO reactions. The main reaction products obtained from the HDO reaction were mainly benzene, cyclohexane, cyclohexene and methylcyclopentane. In previous phenol HDO works the presence of methyl-cyclopentane was not observed [8]. In our experiments, methyl-cyclopentane was detected. The quantity of methylcyclopentane measured for the NiW catalyst was higher than for the NiMo catalyst. Using the sulfided NiW catalyst the methyl-cyclopentane selectivity was around 50%. European regulations are restricting aromatics content in fuels. By 2010 the EU will limit aromatics content to 14%. As a consequence, for bio-oils refining it is not only very important to obtain high conversions for the HDO process, it is also
crucial to minimize the aromatics formation. In our case, as the reaction temperature increased benzene formation increased, due to a higher contribution of the HG route. As it was commented before, for higher temperatures the conversion also increased, thus it seems that it would be convenient to find an optimal balance between conversion and aromatics content. For the NiW catalyst the presulfurization favored the HIDHG via and in consequence the reduction of the aromatics formation (benzene) but only at high temperatures (623 K). If both catalysts were compared, the NiMo catalyst presented a higher hydrogenation activity, specially, at low temperatures. At the highest temperature both presented a similar behavior.
3.2.
Catalyst stability
Long activity tests were performed in order to study the catalyst stability in its reduced or sulfided state. For the NiMo catalyst prereduced, at 523, 573 and 623 K, complete conversion was observed for the time on stream measured. For longer times on stream ð1100 minÞ deactivation was measured and in literature it is reported [9] that coke seems to be the responsible of the deactivation. In the case of the presulfided NiMo catalyst, at low temperatures the activity increased with time on stream. Nevertheless, at high temperatures the activity decreased. This behavior could be explained if the previous results were taken into account. The NiMo catalyst was more active in the reduced state. Thus, at low temperatures when the reaction was taking place as there were no sulfur donor species, this catalyst was desulfided and in consequence the activity increased. At high temperatures (573–623 K), this positive effect of the continuous desulfidation was compensated with the activity loss caused by the coke formation which is more important at higher temperatures. In the case of the NiW catalyst, in the prereduced sample, the activity strongly decreased and, in the presulfided sample, the activity remained constant.
3.3. Effect of H2S addition as sulfur donor specie during the hydrotreatment In these experiments, the influence of the H2 S addition on the catalyst stability and on the selectivity was studied for the
100
100
80
80
60
60
40
40
20
20
0
200°C
250°C 300°C Temperature
33 (2008) 3485 – 3488
350°C H2S/H2
0 H2
250°C
300°C Temperature
350°C
Fig. 1 – Phenol conversion as a temperature function for the NiMo (a) and NiW (b) catalyst pretreated with H2 and H2 S=H2 .
ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y
NiMo
Conversion (%)
60
40
523 K
20 473 K 0
0
100
200 Time (min.)
300
400
Fig. 2 – Conversion of the NiMo catalyst: reaction with H2 ðE; mÞ and with H2 S=H2 (’; ).
NiW
95
Conversion (%)
90 85 80 75 70 65 60
0
100
200 Time (min.)
300
400
Fig. 3 – Conversion of the NiW catalyst: temp. 623 K, reaction with H2 ðEÞ and H2 S=H2 ð’Þ.
phenol HDO. In Fig. 2, the activity for the NiMo catalyst was presented at 473 and 523 K feeding H2 or H2 S=H2 to the system. This catalyst was more active when hydrogen was fed to the system, thus H2 S=H2 addition did not improved the catalyst activity. As it was commented, the active centers desulfurization enabled higher HDO reaction rates. Moreover, this catalyst seemed to be activated with time on stream due to the absence of species able to sulfide the catalyst. For the NiW catalyst, the H2 S=H2 addition during the hydrotreating was also studied at 523, 573 and 623 K (Fig. 3). At the lowest temperatures (523 and 573 K), the H2 S=H2 addition did not seem to noticeably affect the HDO activity. And on the contrary, at the highest temperature (623 K), it promoted the HDO reaction increasing the phenol conversion (see Fig. 3). These results corroborated that the NiW catalyst was more active in the sulfided state. Therefore, at high temperatures after the H2 S adsorption, if the active centers sulfurization was enhanced the phenol HDO reaction was promoted. In all the cases and for all the temperatures, the NiW catalyst deactivated with time on stream. Even at 623 K, when the activity is promoted a continuous deactivation was
33 (2008) 3485 – 3488
3487
observed. This could be attributed to coke deactivation or to continuous catalyst desulfidation. Regarding the H2 S=H2 addition effect on the selectivity different behaviors were observed. For the NiMo catalyst, the H2 S=H2 addition inhibited the HDO reaction through both vias, although the HG was less affected. Thus, the aromatics content in the products was higher. This is in contrast with the results presented in the literature using a CoMo=Al2 O3 catalyst for the phenol HDO [10,11]. A possible hypothesis deals with the use of a commercial catalyst with a significant P content. For the NiW catalyst, the effect of the H2 S=H2 addition on the selectivity was to promote the HID-HG route for all the temperatures. As a consequence product mixture aromatic content decreased.
4.
Conclusions
The NiMo catalyst presented a higher activity than the NiW catalyst for the phenol HDO reactions in all the temperature range studied and the product aromatics content originated for the NiMo catalyst was lower. Indeed the NiMo catalyst was less active in the sulfided state. This issue suggested that the formation of the MoS2 active phase was not enabled, perhaps due to the presence of P. The NiW catalyst presented a higher isomerization activity if compared with the NiMo catalyst. As a consequence, higher conversions to methyl-cyclopentane were achieved. In the case of the presulfided NiW catalyst, it was around a 50% of the total conversion. A reaction temperature increase implied a phenol conversion increase, but with a higher aromatics content. This was a negative consequence taking into account the aromatics restrictive European regulations. The effect of the H2 S=H2 addition during the hydrotreating was different for each catalyst. For the NiMo catalyst the H2 S acted as an inhibitor whereas for the NiW catalyst it had not influence at low temperatures and it was a promoter at high temperatures. Thus, this effect showed the great influence of the reaction temperature on the sulfurization level for each catalyst. For all the cases, the H2 S=H2 addition did not improve the catalyst stability and changed the process selectivity which could be attributed to the presence of two types of active sites: one for the HG route and other for the HID-HG route. The influence of the H2 S addition on the selectivity could be used to control the aromatic content in the resulting product.
Acknowledgments This work was supported by the Basque Government and the University of the Basque Country. R E F E R E N C E S
[1] hhttp://ec.europa.eu/energy/res/publications/doc/ 2004_brochure_biofuels_en.pdfi. [2] EC (European Commission). European Transport policy for 2010: time to decide; 2001. Available at: hhttp://europa.eu.inti. [3] Demirbas A. Prog Energy Combust Sci 2006;33(1):1.
ARTICLE IN PRESS 3488
I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y
[4] Fukuda H, Kondo A, Noda H. J Biosci Bioeng 2001;92(5):405. [5] Furimsky E. Appl Catal A 2000;199(2):147. [6] Delmon B, Grange P. In: Farinha Portela M, editor. Proceedings of the 2nd conference on industrial catalysis. Lisbon, 1988. p. 83. [7] Grimblot J. Catal Today 1998;41(1–3):111.
33 (2008) 3485 – 3488
[8] Viljava T.-R. PhD Dissertation, Helsinki University of Technology, Espoo; 2001. [9] Gevert BS, Otterstedt J-E. Biomass 1987;13(2):105. [10] Laurent E, Delmon B. Ind Eng Chem Res 1993;32:2516. [11] Gevert BS, Otterstedt J-E, Massoth FE. Appl Catal 1987; 31(1):119.