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A novel catalytic membrane microreactor for COx free H2 production
Catalysis Communications 12 (2010) 161–164
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
A novel catalytic membrane microreactor for COx free H2 production F.R. García-García, M.A. Rahman, B.F.K. Kingsbury, K. Li ⁎ Department of Chemical Engineering and Chemical Technology, Imperial College London, London SW7 2AZ, UK
a r t i c l e
i n f o
Article history: Received 20 July 2010 Received in revised form 7 September 2010 Accepted 10 September 2010 Available online 18 September 2010 Keywords: Asymmetric Al2O3 hollow fibre Sol–Gel coating Water–Gas shift reaction Catalytic hollow fibre membrane microreactor
a b s t r a c t A novel catalytic hollow fibre membrane microreactor (CHFMMR), consisting of a thin Pd layer and a 30%CuO/CeO2 catalyst supported on an asymmetric Al2O3 hollow fibre has been developed. In the preparation of the CHFMMR, the Pd layer was deposited on outer surface of the hollow fibre substrate by electroless plating first, followed by impregnation of 30%CuO/CeO2 catalyst in the finger pores located in the inner surface of the hollow fibre lumen by sol–gel coating. The catalytic activity of the CHFMMR was tested using water–gas shift (WGS) reaction and compared to that of a catalytic hollow fibre microreactor (CHFMR) and a conventional fixed-bed reactor. The catalytic activity tests were performed at 1 atm and from 200 °C to 500 °C using different flow rates of sweep gas (from 50 to 75 ml/min). The ratio between CO and H2O in the feed was 1 to 0.5 with a space velocity of 80 l g−1 h−1. The conversion obtained in the CHFMMR is 35% higher than that in the conventional fixed-bed reactor. Furthermore, a conversion which was 17% higher than the corresponding thermodynamic equilibrium conversion was achieved in the CHFMMR, at 500 °C with a sweep gas flow rate of 75 ml/min. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, due to the emerging use of H2 as an energy vector in fuel cell and transport applications, there has been increased interest in developing new technologies for a clean and efficient H2 production [1]. In this context, microreactors offer several important advantages over conventional fixed-bed reactors such as a reduction of fluid pressure drops, an increase of catalytic surface to volume ratio and an improvement of heat and mass transfer as well as increased mixing of gases during the reactions [2]. However, currently, design and development of commercially viable microreactor devices for H2 production require low-cost and standardised fabrication techniques using resistant materials that permit the synthesis of appropriate three-dimensional microchannel structures which are stable under reaction conditions [3]. So far, several microreactors for the production of H2 from hydrocarbons have been studied with advantages of high mass and heat transfer fluxes resulting from their larger surface area/volume ratios and shorter transport distances. However, these microreactor systems cannot overcome the equilibrium limitations of catalytic reactions. Membrane reactors [4], in this context, can in principle shift the equilibrium limitation because they remove one of the reaction products from the reaction zone continuously. Therefore, combining a microreactor with a membrane which is selective for H2 (or CO2) would greatly intensify production processes and at the same time capture the CO2 for sequestration.
⁎ Corresponding author. Tel.: +44 207 5945676; fax: +44 207 5945629. E-mail address: [email protected] (K. Li). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.09.015
Traditionally, ceramic supports are employed in the development of catalytic membrane reactors (CMRs), because of their high chemical, thermal and mechanical resistances, along with their small pore sizes and narrow pore size distributions, which made them an attractive alternative to polymeric and stainless steel supports. However, ceramic supports are currently produced as tubes with diameters of several millimetres which have low surface area/volume ratios. This is one of the limitations in the development of CMRs, as it is desirable to maximize the surface area/volume ratio of the membrane module in order to increase the permeation of H2 from the reaction zone. In this work, Al2O3 hollow fibres [5] with a surface area/volume ratio that is considerably higher than commercial Al2O3 tube supports have been employed in the development of a catalytic hollow fibre membrane microreactor (CHFMMR) for COx free H2 production via the WGS reaction [6]. As shown in Fig. 1, the exceptional asymmetric pore structure of the Al2O3 hollow fibres, i.e. a sponge-like outer region and an open porous finger-like inner region, provides a unique support for both the Pd layer and the CuO/CeO2 catalyst [7], which can be deposited and impregnated on the outer surface and inner fingerlike pores of the Al2O3 hollow fibre, respectively. The finger-like pores, after impregnating the catalyst, can be viewed as one microreactor made up of hundreds of microchannels (dp = 10 μm) that are perpendicularly distributed around the lumen of the fibre. This novel concept of the reactor (CHFMMR) which integrates both a catalytic microreactor and a Pd membrane in the same unit offers several advantages over existing H2 production systems. Firstly, the CHFMMR reduces the pressure drop, increases the catalytic surface to volume ratio and improves the heat and mass transfer as well as the mixing of gases during the reaction. Secondly, this reactor
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F.R. García-García et al. / Catalysis Communications 12 (2010) 161–164
a
500µm
b
200µm
c
50µm
d
5µm
Pd Al2O3 HF
Fig. 1. SEM pictures of Al2O3 hollow fibre; a–b) cross section at different magnifications before Pd deposition, c) cross section after Pd deposition and d) top of the Pd membrane surface.
works at significantly lower temperature and/or using lesser amounts of catalyst than conventional reactors. Finally, this reactor combines the processes of generating and separating H2 in a single step by the integration of a H2 selective Pd membrane [8]. 2. Experimental section In this study, Al2O3 hollow fibres, prepared by a dry-wet spinning technique followed by sintering at 1450 °C, have been used either as a support for 30%CuO/CeO2 catalyst in a catalytic hollow fibre microreactor (CHFMR) or as a support for both a Pd membrane and 30% CuO/CeO2 catalyst in a CHFMMR. During the fabrication of the CHFMMR, in order to avoid dissolution of the 30%CuO/CeO2 catalyst into the ammonia plating solution, the Pd layer was deposited first on the outer layer of the Al2O3 hollow fibre using an electroless plating technique [9]. Prior to the activation process, Al2O3 hollow fibre were cleaned using deionised water and activated subsequently by sequential dipping in SnCl2–HCl (1 g/l, pH:2) and PdCl2–HCl (1 g/l, pH:2) solutions. The activation process was repeated six times, after which the surface colour of Al2O3 hollow fibre changed from white to dark brown. The activated substrates were then plated with Pd by electroless plating technique until obtain 6 μm thickness Pd membranes. Finally the Pd membranes were dried overnight at 120 °C. The Pd layer was characterized by SEM, and He and H2 permeabilities. During the fabrication of both CHFMR and CHFMMR, the impregnation of the 30%CuO/CeO2 catalyst into the finger-like channels located in the Al2O3 hollow fibre lumen was carried out using a sol–gel technique [10]. First, the homogeneous sol–gel solution was injected into the lumen of Al2O3 hollow fibre substrates using a glass pipette. This process was repeated several times until finger-like voids were considered to be totally filled with the sol–gel solution. Secondly, the impregnated Al2O3 hollow fibres substrates were dried at 60 °C for 24 h. Finally, the xerogel formed during the drying process was eliminated by oxidation at 400 °C in a 25 ml/min
air flow for 1 h. It is important to note that under this oxidation conditions the Pd membrane was not damaged; even more, it is well known that a partial surface oxidation of the Pd layer can activate it improving its H2 permeability [11,12]. The amount of catalyst deposited inside the finger-like region was determined by measuring the weight of the Al2O3 hollow fibre substrates before and after the impregnation and calcinations steps and was found to be around 18 mg in all the cases. The deposited catalyst was characterized by ICP-MS, specific surface area and Hg porosimetry. In order to study the usefulness of the finger-like structure for the WGS reaction, the Al2O3 hollow fibres impregnated with 30%CuO/CeO2 catalyst were ground to a powder (i.e. to eliminate the finger-like region) for use in a fixed-bed reactor for the purpose of comparison. The catalytic activity tests for the fixed-bed reactor, the CHFMR and the CHFMMR were performed at atmospheric pressure between 300 °C and 500 °C. In all cases, the ratio between CO/H2O in the feed was 1 to 0.5 with a space velocity of 80 l g−1 h−1.
3. Results and discussion The structure and morphology of the Al2O3 hollow fibres employed in this work can be observed in the SEM images of Fig. 1a–b. An asymmetric pore structure is observed: a sponge-like structure in the outer region (approximately 20% of its thickness) and a porous fingerlike structure in the inner region (remaining 80% of its thickness). The differences between the average pore sizes of the sponge-like structure (dp = 0.1–0.2 μm) and the finger-like structure (dp = 10 μm), as shown in Fig. 2, allow for use of the alumina hollow fibres as a single support for both the Pd membrane and 30%CuO/CeO2 catalyst for the development of the CHFMMR. Moreover, Fig. 1b clearly shows that the finger-like region is made up of hundreds of microchannels, with dp= 10 μm and a length of 400 μm. The relationship between the thickness of the fingerlike and the sponge like regions imparts the Al2O3 hollow fibres with sufficient mechanical strength while retaining a high permeability.
F.R. García-García et al. / Catalysis Communications 12 (2010) 161–164
a
b
60
0.6 2
1
0 0.01
0.1
1
10
100
1000
Poro size (µm) Fig. 2. Hg porosimetry of Al2O3 hollow fibre before (−) and after (—) catalyst impregnation.
SEM pictures of the Pd membrane deposited on the outer layer of the Al2O3 hollow fibre by an electroless plating deposition technique is shown in Fig. 1c–d. Fig. 1c shows that the thickness of the deposited Pd layer is 6 μm, in agreement with the Pd layer thickness of 5.7 μm calculated by a gravimetric method. Furthermore, the Pd layer deposited is homogeneous, uniform and free of defects, as observed in Fig. 1d. The permeation of the Pd membrane was tested using pure H2 and Ar gases, before being used in the CHFMMR, in order to determine its H2 permeability and H2/Ar selectivity. It can be observed that H2 permeation through the Pd membrane follows Sievert's law and that the H2 permeances of the Pd membrane at 450 °C are 10.7 l m−2 s−1 atm−1/2. However Ar did not permeate through the Pd membrane under the temperature (T = 450 °C) and pressure conditions (ΔP from 0 to 3 atm) studied. This indicates that the Pd layer deposited on the Al2O3 hollow fibre is free of defects with a very high H2/Ar selectivity. Finally, it is important to note that since the pore size of the sponger layer of the alumina hollow fibre is quite uniform with narrow distribution (dp = 0.1–0.2 μm), it is not necessary to prior deposit an intermediate layer to prevent Pd intrusion. According to the icp-MS analysis, the CuO/CeO2 ratio is 3:7, which is in agreement with the amount of CuO calculated theoretically. Fig. 2 shows Hg porosimetry results for the Al2O3 hollow fibres before and after impregnation with 30%CuO/CeO2 catalyst. Before catalyst impregnation, two different pore size distributions can be distinguished, corresponding to the sponge-like structure (dp = 0.1– 0.2 μm) and the finger-like structure (dp = 10 μm). Catalyst impregnation results in a slight decrease in the Al2O3 hollow fibre pore size, especially for the sponge-like structure after catalyst impregnation. This result can be explained since during the catalyst impregnation process the sol–gel solution penetrates the Al2O3 hollow fibre microporous structure, forming a xerogel after the drying process. After calcination of the xerogel, the catalyst particles formed in this process occupy some of the pore volume of the support. In addition, the presence of this mixed oxide of Cu and Ce increases by three times the specific surface area of the Al2O3 hollow fibre (from 2.2 to 7.1 m2/g after impregnation). This behavior can be explained because the mixed oxides deposited by the sol–gel technique usually show a much higher surface area than if deposited by other conventional techniques. Two different parameters, temperatures and sweep gas flow rates, have been investigated in order to optimize the performance of the CHFMMR for the WGS reaction. Fig. 3a compares the conversion obtained for the WGS reaction as a function of the reaction temperature, in a fixed-bed reactor, the catalytic hollow fibre microreactor (CHFMR) and the catalytic hollow fibre membrane microreactor (CHFMMR), using different sweep gas flow rates. The performance of the fixed-bed reactor was used as a reference for comparison with the results obtained for the CHFMR and the CHFMMR. Results show that the maximum conversion obtained in the fixed-bed reactor
Flux H2 (ml/min)
50
Conversion (%)
log differential Intrusion (u.a.)
3
163
40 30 20
0.5
0.4
0.3
10 0.2 0 300
400
500
Temperature (°C)
300
400
500
Temperature (°C)
Fig. 3. a) Conversion vs. temperature in WGS reaction: thermodynamic equilibrium (….), fixed-bed reactor (□), CHFMR (o) and CHFMMR using different flow rates of the sweep gas: 50 ml/min (■), 65 ml/min (●) and 70 ml/min (▲). b) H2 permeation through the Pd membrane in the CHFMMR during WGS reaction as a function of temperature and the rate flow of sweep gas: 50 ml/min (■), 65 ml/min (●) and 70 ml/min (▲).
is 23% lower than that obtained in the CHFMR under the same reaction conditions. An increase in the catalytic activity can be explained by the presence of the microchannels in the finger-like region where the catalyst is dispersed, which improves the catalytic surface to volume ratio and the heat and mass transfer as well as the mixing of gases during the reaction. On the other hand, the conversion obtained in the CHFMMR is about 20% higher than the corresponding value for the CHFMR. Moreover, the higher the sweep gas flow rate, the higher the conversion. H2 produced during the reaction is selectively removed from the reaction medium in the CHFMMR and so, according to the “Le Chatelier” principle, the equilibrium is shifted towards the product side, thus increasing the conversion, which explains its superior performance. It is interesting to note that at 500 °C and at a sweep gas flow of 70 ml/min the conversion increases from 43% in a CHFMR to 60% in a CHFMMR, corresponding to a 17% increase with respect to the thermodynamic equilibrium conversion. Fig. 3b shows the H2 flow rate through the Pd membrane during the reaction at different temperatures and sweep gas flow rates. Both a higher sweep gas flow rate and a higher temperature increased the H2 permeability through the Pd membrane indicating that Sievert's law governs H2 transport in this system. The efficiency of the Pd membrane in the CHFMMR during the WGS reaction was calculated at different temperatures and sweep gas flow rates based upon the H2 recovery index; this can be defined as the H2 fraction permeated through the Pd membrane with respect to the total amount of H2 produced during the reaction. It can be observed that the H2 recovery index remains constant with the temperature variation but increases with the sweep gas flow rate from 68% to 76%. This behavior indicates that higher sweep gas flow rates involve larger pressure differences across the Pd membrane, thus increasing the H2 permeability through the Pd membrane. Finally, it is important to highlight that the Pd membrane shows a high stability, even after exposure to temperatures of up to 500 °C for long periods of time during the WGS reaction. In addition, all the CHFMMRs prepared and tested in this study have shown a high level of reproducibility. 4. Conclusions In this study, we have demonstrated that the finger-like structure of the Al2O3 hollow fibres prepared in this work can be considered as a set of hundreds of microchannels of a microreactor. This leads to an increase in the catalytic performance for the WGS reaction when 30%CuO/CeO2
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catalyst is dispersed in these micro-tubular channels, due to an improvement in heat and mass transfer and in the mixing of gases during the reaction. Moreover, the Al2O3 hollow fibres can be employed as a unique support for both the 30%CuO/CeO2 catalyst and a Pd membrane in the development of a CHFMMR, due to their asymmetric porous structures. The catalytic activity results obtained for the fixed-bed reactor, the CHFMR and the CHFMMR showed that the conversion in the CHFMMR is the highest. Furthermore, at 500 °C and at a sweep gas flow rate of 75 ml/min, the conversion increased from 43% in a CHMFR to 60% in the CHFMMR, which is 17% higher than the corresponding thermodynamic equilibrium conversion. In addition, high purity H2 has been produced in the CHFMMR during the WGS reaction. This study shows that H2 permeability through the Pd membrane increases with temperature and sweep gas flow rate under operating conditions studied. The CHFMMR offers exclusive advantages over existing H2 production systems and can be considered as an extremely promising system to produce COx free H2 via the WGS reaction. Our preliminary work demonstrates that the fabrication techniques and process understanding generated from this study are generic and may be applied to
a wide range of heterogeneously catalysed gas phase reactions which are related to H2 production. Acknowledgments The authors gratefully acknowledge the research funding provided by EPSRC in the United Kingdom (grant no. EP/G01244X/1). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
J.N. Armor, Appl. Catal., A 176 (1999) 159–176. G. Kolb, V. Hessel, Chem. Eng. J. 98 (2004) 1–38. J.D. Holladay, Y. Wang, E. Jones, Chem. Rev. 104 (2004) 4767–4790. Membrane catalytic reactors, Top. Catal. 29 (2004) 1. B.F.K. Kingsbury, K. Li, J. Membr. Sci. 328 (2009) 134. D.S. Newsome, Catal. Rev. Sci. Eng. 21 (1980) 275–318. L. Li, Y. Zhan, Q. Zheng, Y. Zheng, C. Chen, Y. She, X. Lin, K. Wei, Catal. Lett. 130 (2009) 532. Catalysis in membrane reactors, Catal. Today 104 (2005) 101. P.P. Mardilovich, Y. She, Y.H. Ma, M.H. Rei, AIChE J. 44 (1998) 310. M. Kakihana, J Sol–gel Sci Technol. 6 (1996) 55. S.A. Koffler, J. Hudson, G.S. Ansell, Trans. AIME 245 (1969) 1735–1740. L. Rubin, Engelhard Technical Bull. 7 (1966) 55.
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
A novel catalytic membrane microreactor for COx free H2 production F.R. García-García, M.A. Rahman, B.F.K. Kingsbury, K. Li ⁎ Department of Chemical Engineering and Chemical Technology, Imperial College London, London SW7 2AZ, UK
a r t i c l e
i n f o
Article history: Received 20 July 2010 Received in revised form 7 September 2010 Accepted 10 September 2010 Available online 18 September 2010 Keywords: Asymmetric Al2O3 hollow fibre Sol–Gel coating Water–Gas shift reaction Catalytic hollow fibre membrane microreactor
a b s t r a c t A novel catalytic hollow fibre membrane microreactor (CHFMMR), consisting of a thin Pd layer and a 30%CuO/CeO2 catalyst supported on an asymmetric Al2O3 hollow fibre has been developed. In the preparation of the CHFMMR, the Pd layer was deposited on outer surface of the hollow fibre substrate by electroless plating first, followed by impregnation of 30%CuO/CeO2 catalyst in the finger pores located in the inner surface of the hollow fibre lumen by sol–gel coating. The catalytic activity of the CHFMMR was tested using water–gas shift (WGS) reaction and compared to that of a catalytic hollow fibre microreactor (CHFMR) and a conventional fixed-bed reactor. The catalytic activity tests were performed at 1 atm and from 200 °C to 500 °C using different flow rates of sweep gas (from 50 to 75 ml/min). The ratio between CO and H2O in the feed was 1 to 0.5 with a space velocity of 80 l g−1 h−1. The conversion obtained in the CHFMMR is 35% higher than that in the conventional fixed-bed reactor. Furthermore, a conversion which was 17% higher than the corresponding thermodynamic equilibrium conversion was achieved in the CHFMMR, at 500 °C with a sweep gas flow rate of 75 ml/min. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, due to the emerging use of H2 as an energy vector in fuel cell and transport applications, there has been increased interest in developing new technologies for a clean and efficient H2 production [1]. In this context, microreactors offer several important advantages over conventional fixed-bed reactors such as a reduction of fluid pressure drops, an increase of catalytic surface to volume ratio and an improvement of heat and mass transfer as well as increased mixing of gases during the reactions [2]. However, currently, design and development of commercially viable microreactor devices for H2 production require low-cost and standardised fabrication techniques using resistant materials that permit the synthesis of appropriate three-dimensional microchannel structures which are stable under reaction conditions [3]. So far, several microreactors for the production of H2 from hydrocarbons have been studied with advantages of high mass and heat transfer fluxes resulting from their larger surface area/volume ratios and shorter transport distances. However, these microreactor systems cannot overcome the equilibrium limitations of catalytic reactions. Membrane reactors [4], in this context, can in principle shift the equilibrium limitation because they remove one of the reaction products from the reaction zone continuously. Therefore, combining a microreactor with a membrane which is selective for H2 (or CO2) would greatly intensify production processes and at the same time capture the CO2 for sequestration.
⁎ Corresponding author. Tel.: +44 207 5945676; fax: +44 207 5945629. E-mail address: [email protected] (K. Li). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.09.015
Traditionally, ceramic supports are employed in the development of catalytic membrane reactors (CMRs), because of their high chemical, thermal and mechanical resistances, along with their small pore sizes and narrow pore size distributions, which made them an attractive alternative to polymeric and stainless steel supports. However, ceramic supports are currently produced as tubes with diameters of several millimetres which have low surface area/volume ratios. This is one of the limitations in the development of CMRs, as it is desirable to maximize the surface area/volume ratio of the membrane module in order to increase the permeation of H2 from the reaction zone. In this work, Al2O3 hollow fibres [5] with a surface area/volume ratio that is considerably higher than commercial Al2O3 tube supports have been employed in the development of a catalytic hollow fibre membrane microreactor (CHFMMR) for COx free H2 production via the WGS reaction [6]. As shown in Fig. 1, the exceptional asymmetric pore structure of the Al2O3 hollow fibres, i.e. a sponge-like outer region and an open porous finger-like inner region, provides a unique support for both the Pd layer and the CuO/CeO2 catalyst [7], which can be deposited and impregnated on the outer surface and inner fingerlike pores of the Al2O3 hollow fibre, respectively. The finger-like pores, after impregnating the catalyst, can be viewed as one microreactor made up of hundreds of microchannels (dp = 10 μm) that are perpendicularly distributed around the lumen of the fibre. This novel concept of the reactor (CHFMMR) which integrates both a catalytic microreactor and a Pd membrane in the same unit offers several advantages over existing H2 production systems. Firstly, the CHFMMR reduces the pressure drop, increases the catalytic surface to volume ratio and improves the heat and mass transfer as well as the mixing of gases during the reaction. Secondly, this reactor
162
F.R. García-García et al. / Catalysis Communications 12 (2010) 161–164
a
500µm
b
200µm
c
50µm
d
5µm
Pd Al2O3 HF
Fig. 1. SEM pictures of Al2O3 hollow fibre; a–b) cross section at different magnifications before Pd deposition, c) cross section after Pd deposition and d) top of the Pd membrane surface.
works at significantly lower temperature and/or using lesser amounts of catalyst than conventional reactors. Finally, this reactor combines the processes of generating and separating H2 in a single step by the integration of a H2 selective Pd membrane [8]. 2. Experimental section In this study, Al2O3 hollow fibres, prepared by a dry-wet spinning technique followed by sintering at 1450 °C, have been used either as a support for 30%CuO/CeO2 catalyst in a catalytic hollow fibre microreactor (CHFMR) or as a support for both a Pd membrane and 30% CuO/CeO2 catalyst in a CHFMMR. During the fabrication of the CHFMMR, in order to avoid dissolution of the 30%CuO/CeO2 catalyst into the ammonia plating solution, the Pd layer was deposited first on the outer layer of the Al2O3 hollow fibre using an electroless plating technique [9]. Prior to the activation process, Al2O3 hollow fibre were cleaned using deionised water and activated subsequently by sequential dipping in SnCl2–HCl (1 g/l, pH:2) and PdCl2–HCl (1 g/l, pH:2) solutions. The activation process was repeated six times, after which the surface colour of Al2O3 hollow fibre changed from white to dark brown. The activated substrates were then plated with Pd by electroless plating technique until obtain 6 μm thickness Pd membranes. Finally the Pd membranes were dried overnight at 120 °C. The Pd layer was characterized by SEM, and He and H2 permeabilities. During the fabrication of both CHFMR and CHFMMR, the impregnation of the 30%CuO/CeO2 catalyst into the finger-like channels located in the Al2O3 hollow fibre lumen was carried out using a sol–gel technique [10]. First, the homogeneous sol–gel solution was injected into the lumen of Al2O3 hollow fibre substrates using a glass pipette. This process was repeated several times until finger-like voids were considered to be totally filled with the sol–gel solution. Secondly, the impregnated Al2O3 hollow fibres substrates were dried at 60 °C for 24 h. Finally, the xerogel formed during the drying process was eliminated by oxidation at 400 °C in a 25 ml/min
air flow for 1 h. It is important to note that under this oxidation conditions the Pd membrane was not damaged; even more, it is well known that a partial surface oxidation of the Pd layer can activate it improving its H2 permeability [11,12]. The amount of catalyst deposited inside the finger-like region was determined by measuring the weight of the Al2O3 hollow fibre substrates before and after the impregnation and calcinations steps and was found to be around 18 mg in all the cases. The deposited catalyst was characterized by ICP-MS, specific surface area and Hg porosimetry. In order to study the usefulness of the finger-like structure for the WGS reaction, the Al2O3 hollow fibres impregnated with 30%CuO/CeO2 catalyst were ground to a powder (i.e. to eliminate the finger-like region) for use in a fixed-bed reactor for the purpose of comparison. The catalytic activity tests for the fixed-bed reactor, the CHFMR and the CHFMMR were performed at atmospheric pressure between 300 °C and 500 °C. In all cases, the ratio between CO/H2O in the feed was 1 to 0.5 with a space velocity of 80 l g−1 h−1.
3. Results and discussion The structure and morphology of the Al2O3 hollow fibres employed in this work can be observed in the SEM images of Fig. 1a–b. An asymmetric pore structure is observed: a sponge-like structure in the outer region (approximately 20% of its thickness) and a porous fingerlike structure in the inner region (remaining 80% of its thickness). The differences between the average pore sizes of the sponge-like structure (dp = 0.1–0.2 μm) and the finger-like structure (dp = 10 μm), as shown in Fig. 2, allow for use of the alumina hollow fibres as a single support for both the Pd membrane and 30%CuO/CeO2 catalyst for the development of the CHFMMR. Moreover, Fig. 1b clearly shows that the finger-like region is made up of hundreds of microchannels, with dp= 10 μm and a length of 400 μm. The relationship between the thickness of the fingerlike and the sponge like regions imparts the Al2O3 hollow fibres with sufficient mechanical strength while retaining a high permeability.
F.R. García-García et al. / Catalysis Communications 12 (2010) 161–164
a
b
60
0.6 2
1
0 0.01
0.1
1
10
100
1000
Poro size (µm) Fig. 2. Hg porosimetry of Al2O3 hollow fibre before (−) and after (—) catalyst impregnation.
SEM pictures of the Pd membrane deposited on the outer layer of the Al2O3 hollow fibre by an electroless plating deposition technique is shown in Fig. 1c–d. Fig. 1c shows that the thickness of the deposited Pd layer is 6 μm, in agreement with the Pd layer thickness of 5.7 μm calculated by a gravimetric method. Furthermore, the Pd layer deposited is homogeneous, uniform and free of defects, as observed in Fig. 1d. The permeation of the Pd membrane was tested using pure H2 and Ar gases, before being used in the CHFMMR, in order to determine its H2 permeability and H2/Ar selectivity. It can be observed that H2 permeation through the Pd membrane follows Sievert's law and that the H2 permeances of the Pd membrane at 450 °C are 10.7 l m−2 s−1 atm−1/2. However Ar did not permeate through the Pd membrane under the temperature (T = 450 °C) and pressure conditions (ΔP from 0 to 3 atm) studied. This indicates that the Pd layer deposited on the Al2O3 hollow fibre is free of defects with a very high H2/Ar selectivity. Finally, it is important to note that since the pore size of the sponger layer of the alumina hollow fibre is quite uniform with narrow distribution (dp = 0.1–0.2 μm), it is not necessary to prior deposit an intermediate layer to prevent Pd intrusion. According to the icp-MS analysis, the CuO/CeO2 ratio is 3:7, which is in agreement with the amount of CuO calculated theoretically. Fig. 2 shows Hg porosimetry results for the Al2O3 hollow fibres before and after impregnation with 30%CuO/CeO2 catalyst. Before catalyst impregnation, two different pore size distributions can be distinguished, corresponding to the sponge-like structure (dp = 0.1– 0.2 μm) and the finger-like structure (dp = 10 μm). Catalyst impregnation results in a slight decrease in the Al2O3 hollow fibre pore size, especially for the sponge-like structure after catalyst impregnation. This result can be explained since during the catalyst impregnation process the sol–gel solution penetrates the Al2O3 hollow fibre microporous structure, forming a xerogel after the drying process. After calcination of the xerogel, the catalyst particles formed in this process occupy some of the pore volume of the support. In addition, the presence of this mixed oxide of Cu and Ce increases by three times the specific surface area of the Al2O3 hollow fibre (from 2.2 to 7.1 m2/g after impregnation). This behavior can be explained because the mixed oxides deposited by the sol–gel technique usually show a much higher surface area than if deposited by other conventional techniques. Two different parameters, temperatures and sweep gas flow rates, have been investigated in order to optimize the performance of the CHFMMR for the WGS reaction. Fig. 3a compares the conversion obtained for the WGS reaction as a function of the reaction temperature, in a fixed-bed reactor, the catalytic hollow fibre microreactor (CHFMR) and the catalytic hollow fibre membrane microreactor (CHFMMR), using different sweep gas flow rates. The performance of the fixed-bed reactor was used as a reference for comparison with the results obtained for the CHFMR and the CHFMMR. Results show that the maximum conversion obtained in the fixed-bed reactor
Flux H2 (ml/min)
50
Conversion (%)
log differential Intrusion (u.a.)
3
163
40 30 20
0.5
0.4
0.3
10 0.2 0 300
400
500
Temperature (°C)
300
400
500
Temperature (°C)
Fig. 3. a) Conversion vs. temperature in WGS reaction: thermodynamic equilibrium (….), fixed-bed reactor (□), CHFMR (o) and CHFMMR using different flow rates of the sweep gas: 50 ml/min (■), 65 ml/min (●) and 70 ml/min (▲). b) H2 permeation through the Pd membrane in the CHFMMR during WGS reaction as a function of temperature and the rate flow of sweep gas: 50 ml/min (■), 65 ml/min (●) and 70 ml/min (▲).
is 23% lower than that obtained in the CHFMR under the same reaction conditions. An increase in the catalytic activity can be explained by the presence of the microchannels in the finger-like region where the catalyst is dispersed, which improves the catalytic surface to volume ratio and the heat and mass transfer as well as the mixing of gases during the reaction. On the other hand, the conversion obtained in the CHFMMR is about 20% higher than the corresponding value for the CHFMR. Moreover, the higher the sweep gas flow rate, the higher the conversion. H2 produced during the reaction is selectively removed from the reaction medium in the CHFMMR and so, according to the “Le Chatelier” principle, the equilibrium is shifted towards the product side, thus increasing the conversion, which explains its superior performance. It is interesting to note that at 500 °C and at a sweep gas flow of 70 ml/min the conversion increases from 43% in a CHFMR to 60% in a CHFMMR, corresponding to a 17% increase with respect to the thermodynamic equilibrium conversion. Fig. 3b shows the H2 flow rate through the Pd membrane during the reaction at different temperatures and sweep gas flow rates. Both a higher sweep gas flow rate and a higher temperature increased the H2 permeability through the Pd membrane indicating that Sievert's law governs H2 transport in this system. The efficiency of the Pd membrane in the CHFMMR during the WGS reaction was calculated at different temperatures and sweep gas flow rates based upon the H2 recovery index; this can be defined as the H2 fraction permeated through the Pd membrane with respect to the total amount of H2 produced during the reaction. It can be observed that the H2 recovery index remains constant with the temperature variation but increases with the sweep gas flow rate from 68% to 76%. This behavior indicates that higher sweep gas flow rates involve larger pressure differences across the Pd membrane, thus increasing the H2 permeability through the Pd membrane. Finally, it is important to highlight that the Pd membrane shows a high stability, even after exposure to temperatures of up to 500 °C for long periods of time during the WGS reaction. In addition, all the CHFMMRs prepared and tested in this study have shown a high level of reproducibility. 4. Conclusions In this study, we have demonstrated that the finger-like structure of the Al2O3 hollow fibres prepared in this work can be considered as a set of hundreds of microchannels of a microreactor. This leads to an increase in the catalytic performance for the WGS reaction when 30%CuO/CeO2
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catalyst is dispersed in these micro-tubular channels, due to an improvement in heat and mass transfer and in the mixing of gases during the reaction. Moreover, the Al2O3 hollow fibres can be employed as a unique support for both the 30%CuO/CeO2 catalyst and a Pd membrane in the development of a CHFMMR, due to their asymmetric porous structures. The catalytic activity results obtained for the fixed-bed reactor, the CHFMR and the CHFMMR showed that the conversion in the CHFMMR is the highest. Furthermore, at 500 °C and at a sweep gas flow rate of 75 ml/min, the conversion increased from 43% in a CHMFR to 60% in the CHFMMR, which is 17% higher than the corresponding thermodynamic equilibrium conversion. In addition, high purity H2 has been produced in the CHFMMR during the WGS reaction. This study shows that H2 permeability through the Pd membrane increases with temperature and sweep gas flow rate under operating conditions studied. The CHFMMR offers exclusive advantages over existing H2 production systems and can be considered as an extremely promising system to produce COx free H2 via the WGS reaction. Our preliminary work demonstrates that the fabrication techniques and process understanding generated from this study are generic and may be applied to
a wide range of heterogeneously catalysed gas phase reactions which are related to H2 production. Acknowledgments The authors gratefully acknowledge the research funding provided by EPSRC in the United Kingdom (grant no. EP/G01244X/1). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
J.N. Armor, Appl. Catal., A 176 (1999) 159–176. G. Kolb, V. Hessel, Chem. Eng. J. 98 (2004) 1–38. J.D. Holladay, Y. Wang, E. Jones, Chem. Rev. 104 (2004) 4767–4790. Membrane catalytic reactors, Top. Catal. 29 (2004) 1. B.F.K. Kingsbury, K. Li, J. Membr. Sci. 328 (2009) 134. D.S. Newsome, Catal. Rev. Sci. Eng. 21 (1980) 275–318. L. Li, Y. Zhan, Q. Zheng, Y. Zheng, C. Chen, Y. She, X. Lin, K. Wei, Catal. Lett. 130 (2009) 532. Catalysis in membrane reactors, Catal. Today 104 (2005) 101. P.P. Mardilovich, Y. She, Y.H. Ma, M.H. Rei, AIChE J. 44 (1998) 310. M. Kakihana, J Sol–gel Sci Technol. 6 (1996) 55. S.A. Koffler, J. Hudson, G.S. Ansell, Trans. AIME 245 (1969) 1735–1740. L. Rubin, Engelhard Technical Bull. 7 (1966) 55.