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Improved performance of mesoporous zeolite single crystals in catalytic cracking and isomerization of n-hexadecane
Catalysis Communications 5 (2004) 543–546 www.elsevier.com/locate/catcom
Improved performance of mesoporous zeolite single crystals in catalytic cracking and isomerization of n-hexadecane Claus Hviid Christensen
a,b
, Iver Schmidt a, Christina Hviid Christensen
a,*
a
b
Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Lyngby, Denmark Department of Chemistry, Building 206, Technical University of Denmark, DK-2800 Lyngby, Denmark Received 18 April 2004; accepted 9 July 2004 Available online 4 August 2004
Abstract Mesoporous zeolite single-crystal catalysts exhibit the hitherto most pronounced activity enhancement over conventional zeolite catalysts in slurry-phase catalytic cracking and isomerization of n-hexadecane. This improved performance of both pure and Ptloaded mesoporous MFI single crystals is attributed to enhanced mass transport via the extended non-crystallographic intracrystalline mesopores to and from active sites located inside the zeolite micropores. 2004 Elsevier B.V. All rights reserved.
1. Introduction Zeolites are important as solid acid catalysts in many refinery processes where special requirements for shape selectivity exist [1,2]. Among these processes, both cracking and isomerization are of significant societal importance. Cracking is responsible for the production of gasoline from low-value heavier fractions and isomerization is important for production of iso-alkanes or removal of undesired normal or slightly branched alkanes in e.g., diesel [3]. Usually, the desired shape selectivity of zeolites is achieved at the expense of slow mass transport to and from the active sites located inside the zeolite micropores. There are numerous efforts to increase the accessibility of the active sites in zeolites [4–7]. Recently, the benefits of introducing mesopores into zeolites have been demonstrated in several cases as a means of improving the zeolite catalyst performance [8–10]. We have reported the use of mesoporous zeolite single crystals in the catalytic gas-phase alkylation of benzene [11]. The mesoporous zeolite catalyst showed
both higher activity and selectivity to ethylbenzene than the conventional zeolite catalyst. In view of the significantly improved performance of mesoporous zeolites in gas-phase processes, we decided to elucidate the possibilities in slurry-phase reactions where diffusion is much slower and consequently the beneficial effect of the mesopores should be even more pronounced. Here, we report the first comparative investigation of the performance of conventional and mesoporous zeolite single crystals in an acid-catalyzed reaction conducted under slurry-phase conditions. As a test reaction, we have chosen the cracking and isomerization of the longchain alkane, n-hexadecane, since this allows relevant studies of both the pure acidic zeolites and platinum containing zeolite catalysts. Furthermore, this reaction is a useful test reaction in catalytic dewaxing [3]. We find the hitherto most pronounced activity difference between the mesoporous HZSM-5 catalyst and the conventional HZSM-5 catalyst.
2. Experimental *
Corresponding author. E-mail address: [email protected] (C.H. Christensen).
1566-7367/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2004.07.003
Comparable samples of bifunctional platinum zeolite catalysts and zeolite catalysts without platinum were
544
C.H. Christensen et al. / Catalysis Communications 5 (2004) 543–546
tested in a 600 ml stirred Parr batch autoclave with nhexadecane as the reacting specie. The conventional HZSM-5 zeolite was provided from Zeolyst whereas the mesoporous zeolites were synthesized according to literature procedures [11–14] (mesoporous silicalite samples were prepared as the mesoporous ZSM-5 samples without adding any aluminum-source). The acidic properties of the various zeolites were determined by ammonia temperature programmed desorption; for the conventional HZSM-5 zeolite catalyst Si/Al = 85, for the mesoporous HZSM-5 zeolite catalyst Si/Al = 116, and for the mesoporous silicalite-1 Si/Al > 500. The particle size distributions of the platinum particles supported on the zeolite catalysts were determined from transmission electron microscopy. In the n-hexadecane conversion experiments, 2 g of catalyst (when platinum catalysts were used, they contained 1% Pt and were reduced prior to the test) was added to the batch autoclave under inert atmosphere. Then 75 mL of n-hexadecane (provided by Aldrich) was added and the vessel was pressurized to 20 bars with hydrogen (when the catalyst did not contain platinum the autoclave was not pressurized with hydrogen). The drive belt was set to 250 rpm whereafter the reactor was gradually heated to 280 C and kept at that temperature for 16 h. After reaction, the batch autoclave was cooled to room temperature. The resulting reaction mixture was analyzed by an HP-5890 Series II Gas Chromatograph (GC) equipped with a Flame Ionization Detector (FID) and an HP Ultra-2 capillary column (25 m · 0.20 mm ID · 0.33 m). Furthermore, analysis was done with an Agilent 6890A GC equipped with a mass selective detector (Agilent 5973) and an HP Ultra-2 capillary column (50 m · 200 ID · 0.33 lm film thickness).
3. Results and discussion Based on X-ray powder diffraction studies, all zeolites used were concluded to be completely crystalline and to contain exclusively MFI-structured material. The average crystal size of both the conventional and mesoporous zeolites can be seen from the scanning electron microscope (SEM) images in Fig. 1. It was shown by selected-area electron diffraction conducted in a transmission electron microscope (TEM) that the later material indeed consisted of mesoporous zeolite single crystals rather than just agglomerates of randomly oriented nanocrystals [13]. This is also in accordance with the well-developed morphologies revealed by the SEM investigations. According to the TEM investigations, the particle size distributions of Pt on the conventional and mesoporous bifunctional catalysts were close to identical with average platinum particle sizes of about 3 nm as shown in Fig. 2.
Fig. 1. SEM images of conventional (top) and mesoporous (bottom) HZSM-5 zeolite catalysts used for n-hexadecane cracking and isomerization.
No appropriate internal standard could be found for the n-hexadecane slurry-phase reaction. In order to determine the degree of cracking and isomerization observed in the catalytic reaction, the various products in the resulting product mixture, were lumped into cracking products (Cn<16), isomerization products (isoC16H34), unreacted n-hexadecane (n-C16H34) and Cn>16 (this fraction is almost negligible; it is formed by alkene oligomerization and only with the pure zeolite catalysts). Representative examples of GC-chromatograms can be viewed in Figs. 3 and 4. The high selectivity towards C4–C6 hydrocarbons together with the lack of symmetry of the product peaks, which was generally observed in the GC-chromatograms, is caused by a significant contribution from secondary cracking reactions. In Table 1, the catalyst performance in terms of conversion and C16 isomerization degree of n-hexadecane is listed. In agreement with literature, the conventional HZSM-5 catalyst in general favours cracked products over isomerized n-hexadecane [15]. It is clearly observed that the introduction of noncrystallographic intracrystalline mesoporosity [16,17] in the HZSM-5 zeolite catalyst enhances the n-hexadecane conversion remarkably. The mesoporous HZSM-5 zeolite (Table 1, Entry 2) shows a three times higher conversion of n-hexadecane compared to the conventional sample (Table 1, Entry 1). The catalysts can, in terms of activity, be listed as follows: mesoporous Pt/HZSM5 > conventional Pt/HZSM-5 > mesoporous HZSM-
C.H. Christensen et al. / Catalysis Communications 5 (2004) 543–546
545
0.5
Pt/Conventional H-ZSM-5
0.45
Pt/Mesoporous H-ZSM-5
0.4
Frequency
0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 1
3
5
7
9
11
13
15
17
19
21
23
25
27
Mean Diameter (nm) Fig. 2. Particle size distribution of Pt particles on Pt/conventional and Pt/mesoporous HZSM-5 zeolite catalysts used for n-hexadecane cracking and isomerization.
Fig. 3. GC-chromatograms of product mixtures from n-hexadecane conversion catalyzed by conventional and mesoporous HZSM-5 zeolite catalysts.
Fig. 4. GC-chromatograms of product mixtures from n-hexadecane conversion catalyzed by Pt/conventional and Pt/mesoporous HZSM-5 zeolite catalysts.
5 > conventional HZSM-5 mesoporous Pt/silicalite1mesoporous silicalite-1. Mesoporous silicalite-1, with and without platinum, which contains no acidic sites,
exhibit very low activity. This agrees well with the observation that n-hexadecane conversion is an acid catalyzed reaction, i.e., it depends directly upon the acidity of the
546
C.H. Christensen et al. / Catalysis Communications 5 (2004) 543–546
Table 1 Catalyst performance in n-hexadecane conversion
4. Conclusion
Catalyst
Wt% of Pta (%)
Conversionb (%)
C16 isomerization degreec (%)
Conventional HZSM-5 Mesoporous HZSM-5 Mesoporous silicalite-1 Conventional HZSM-5 Mesoporous HZSM-5 Mesoporous silicalite-1
– – – 1.0 1.0 1.0
17 52 4 71 84 6
1.4 6.6 0.3 11.3 45.2 2.1
a
Platinum was introduced by incipient wetness impregnation using a aqueous Pt(NH3)4(HCO3)2-solution. b Determined as (Cn<16 + iso-C16H34 + Cn>16)/(Cn<16 + iso-C16H34 + n-C16H34 (unreacted) + Cn>16). c Determined as iso-C16H34/(iso-C16H34 + n-C16H34 (unreacted)).
In summary, we have reported the largest activity difference between the conventional and mesoporous HZSM-5 zeolite material observed so far. Our newly developed material, the mesoporous zeolite single crystals, combines in each individual crystal the crystallographic intracrystalline micropore system typical of zeolites with the mesopores typical of mesoporous materials. Shortening of the micropore diffusion path has proven to dramatically enhance the mass transport within the zeolite crystal both in liquid phase as well as in gas phase reaction. Acknowledgements
catalyst [15]. Thus, the conventional zeolite is expected to be a more active cracking catalysts compared to the mesoporous zeolite, due to the larger number of acid sites in the conventional zeolite. However, in the catalytic reaction of n-hexadecane, the mesoporous HZSM-5 zeolite is found to exhibit a far superior activity regarding both cracking as well as isomerization. The mesoporous zeolite consists of much larger crystals than the conventional zeolite. In fact, the nominal crystal size of the conventional sample is almost four times smaller than that of the mesoporous sample. Generally, the presence of large zeolite crystals causes mass transport limitations, particularly at high conversions and with relatively large reactant and product molecules [5]. This is due to the lower external surface area compared to that of small zeolite crystals. In our experiments there is obviously a pronounced effect of the presence of mesopores. When the zeolite catalysts are impregnated with platinum and the test reaction is conducted in the presence of hydrogen, the conversion is increased as expected. No difference in the activity ranking relative to that of the pure zeolite catalysts is observed, so also under these conditions there is a significant beneficial effect of the mesoporosity. Alkenes were not observed in the product mixtures from these experiments since they were easily converted to alkanes at the high hydrogen pressure. In general, very small amounts of C14 and C15 were found in the GC-chromatograms of the product mixtures. This illustrates that only insignificant amounts of methane and ethane were formed directly during the reaction time and demonstrates that the product distributions found are reliable, since negligible amounts of product are lost to the gas phase.
We are indebted to Lars Ku¨rstein for analyzing the product mixtures and to Anna Carlsson for conducting the TEM investigations. References [1] P.A. Jacobs, J.A. Martens, Stud. Surf. Sci. Catal. 58 (1991) 445. [2] A. Corma, J. Catal. 216 (2003) 298. [3] I.E. Maxwell, J.K. Minderhoud, W.H.J. Stork, J.A.R. van Veen, in: G. Ertl, H. Kno¨zinger, J. Weitkamp (Eds.), Handbook of Heterogenous Catalysis, vol. 4, Wiley, New York, 1997, p. 2017 (Chapter 3.13). [4] Y. Liu, T.J. Pinnavaia, J. Am. Chem. Soc. 125 (2003) 2376. [5] J. Houzvicka, I. Schmidt, C.J.H. Jacobsen, Stud. Surf. Sci. Catal. 135 (2001) 158. [6] A. Corma, M.J. Diaz-Cabanas, J. Martinez-Triguero, F. Rey, J. Rius, Nature 396 (1998) 353. [7] A. Karlsson, M. Sto¨cker, R. Schmidt, Micropor. Mesopor. Mat. 27 (1999) 181. [8] M. Tromp, J.A. van Bokhoven, M.T. Garriga Oostenbrink, J.H. Bitter, K.P. deJong, D.C. Koningsberger, J. Catal. 190 (2000) 209. [9] S. van Donk, A. Broersma, O.L.J. Gijzeman, J.A. van Bokhoven, J.H. Bitter, K.P. de Jong, J. Catal. 204 (2001) 272. [10] S. van Donk, A.H. Janssen, J.H. Bitter, K.P. de Jong, Catal. Rev. 45 (2003) 297. [11] C.H. Christensen, K. Johannsen, I. Schmidt, C.H. Christensen, J. Am. Chem. Soc. 125 (2003) 13370. [12] C. J. H. Jacobsen, C. Madsen, J. Houzvicka, I. Schmidt, A. Carlsson, US Patent 6,565,826. [13] C.J.H. Jacobsen, C. Madsen, J. Houzvicka, I. Schmidt, A. Carlsson, J. Am. Chem. Soc. 122 (2000) 7116. [14] C.J.H. Jacobsen, C. Madsen, J. Houzvicka, A. Carlsson, I. Schmidt, Stud. Surf. Sci. Catal. 135 (2001) 167. [15] K.C. Park, S.K. Ihm, Appl. Catal. A 203 (2000) 201. [16] A. Boisen, I. Schmidt, A. Carlsson, S. Dahl, M. Brorson, C.J.H. Jacobsen, Chem. Commun. (2003) 958. [17] A.H. Janssen, I. Schmidt, C.J.H. Jacobsen, A.J. Koster, K.P. de Jong, Micropor. Mesopor. 65 (2003) 59.
Improved performance of mesoporous zeolite single crystals in catalytic cracking and isomerization of n-hexadecane Claus Hviid Christensen
a,b
, Iver Schmidt a, Christina Hviid Christensen
a,*
a
b
Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Lyngby, Denmark Department of Chemistry, Building 206, Technical University of Denmark, DK-2800 Lyngby, Denmark Received 18 April 2004; accepted 9 July 2004 Available online 4 August 2004
Abstract Mesoporous zeolite single-crystal catalysts exhibit the hitherto most pronounced activity enhancement over conventional zeolite catalysts in slurry-phase catalytic cracking and isomerization of n-hexadecane. This improved performance of both pure and Ptloaded mesoporous MFI single crystals is attributed to enhanced mass transport via the extended non-crystallographic intracrystalline mesopores to and from active sites located inside the zeolite micropores. 2004 Elsevier B.V. All rights reserved.
1. Introduction Zeolites are important as solid acid catalysts in many refinery processes where special requirements for shape selectivity exist [1,2]. Among these processes, both cracking and isomerization are of significant societal importance. Cracking is responsible for the production of gasoline from low-value heavier fractions and isomerization is important for production of iso-alkanes or removal of undesired normal or slightly branched alkanes in e.g., diesel [3]. Usually, the desired shape selectivity of zeolites is achieved at the expense of slow mass transport to and from the active sites located inside the zeolite micropores. There are numerous efforts to increase the accessibility of the active sites in zeolites [4–7]. Recently, the benefits of introducing mesopores into zeolites have been demonstrated in several cases as a means of improving the zeolite catalyst performance [8–10]. We have reported the use of mesoporous zeolite single crystals in the catalytic gas-phase alkylation of benzene [11]. The mesoporous zeolite catalyst showed
both higher activity and selectivity to ethylbenzene than the conventional zeolite catalyst. In view of the significantly improved performance of mesoporous zeolites in gas-phase processes, we decided to elucidate the possibilities in slurry-phase reactions where diffusion is much slower and consequently the beneficial effect of the mesopores should be even more pronounced. Here, we report the first comparative investigation of the performance of conventional and mesoporous zeolite single crystals in an acid-catalyzed reaction conducted under slurry-phase conditions. As a test reaction, we have chosen the cracking and isomerization of the longchain alkane, n-hexadecane, since this allows relevant studies of both the pure acidic zeolites and platinum containing zeolite catalysts. Furthermore, this reaction is a useful test reaction in catalytic dewaxing [3]. We find the hitherto most pronounced activity difference between the mesoporous HZSM-5 catalyst and the conventional HZSM-5 catalyst.
2. Experimental *
Corresponding author. E-mail address: [email protected] (C.H. Christensen).
1566-7367/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2004.07.003
Comparable samples of bifunctional platinum zeolite catalysts and zeolite catalysts without platinum were
544
C.H. Christensen et al. / Catalysis Communications 5 (2004) 543–546
tested in a 600 ml stirred Parr batch autoclave with nhexadecane as the reacting specie. The conventional HZSM-5 zeolite was provided from Zeolyst whereas the mesoporous zeolites were synthesized according to literature procedures [11–14] (mesoporous silicalite samples were prepared as the mesoporous ZSM-5 samples without adding any aluminum-source). The acidic properties of the various zeolites were determined by ammonia temperature programmed desorption; for the conventional HZSM-5 zeolite catalyst Si/Al = 85, for the mesoporous HZSM-5 zeolite catalyst Si/Al = 116, and for the mesoporous silicalite-1 Si/Al > 500. The particle size distributions of the platinum particles supported on the zeolite catalysts were determined from transmission electron microscopy. In the n-hexadecane conversion experiments, 2 g of catalyst (when platinum catalysts were used, they contained 1% Pt and were reduced prior to the test) was added to the batch autoclave under inert atmosphere. Then 75 mL of n-hexadecane (provided by Aldrich) was added and the vessel was pressurized to 20 bars with hydrogen (when the catalyst did not contain platinum the autoclave was not pressurized with hydrogen). The drive belt was set to 250 rpm whereafter the reactor was gradually heated to 280 C and kept at that temperature for 16 h. After reaction, the batch autoclave was cooled to room temperature. The resulting reaction mixture was analyzed by an HP-5890 Series II Gas Chromatograph (GC) equipped with a Flame Ionization Detector (FID) and an HP Ultra-2 capillary column (25 m · 0.20 mm ID · 0.33 m). Furthermore, analysis was done with an Agilent 6890A GC equipped with a mass selective detector (Agilent 5973) and an HP Ultra-2 capillary column (50 m · 200 ID · 0.33 lm film thickness).
3. Results and discussion Based on X-ray powder diffraction studies, all zeolites used were concluded to be completely crystalline and to contain exclusively MFI-structured material. The average crystal size of both the conventional and mesoporous zeolites can be seen from the scanning electron microscope (SEM) images in Fig. 1. It was shown by selected-area electron diffraction conducted in a transmission electron microscope (TEM) that the later material indeed consisted of mesoporous zeolite single crystals rather than just agglomerates of randomly oriented nanocrystals [13]. This is also in accordance with the well-developed morphologies revealed by the SEM investigations. According to the TEM investigations, the particle size distributions of Pt on the conventional and mesoporous bifunctional catalysts were close to identical with average platinum particle sizes of about 3 nm as shown in Fig. 2.
Fig. 1. SEM images of conventional (top) and mesoporous (bottom) HZSM-5 zeolite catalysts used for n-hexadecane cracking and isomerization.
No appropriate internal standard could be found for the n-hexadecane slurry-phase reaction. In order to determine the degree of cracking and isomerization observed in the catalytic reaction, the various products in the resulting product mixture, were lumped into cracking products (Cn<16), isomerization products (isoC16H34), unreacted n-hexadecane (n-C16H34) and Cn>16 (this fraction is almost negligible; it is formed by alkene oligomerization and only with the pure zeolite catalysts). Representative examples of GC-chromatograms can be viewed in Figs. 3 and 4. The high selectivity towards C4–C6 hydrocarbons together with the lack of symmetry of the product peaks, which was generally observed in the GC-chromatograms, is caused by a significant contribution from secondary cracking reactions. In Table 1, the catalyst performance in terms of conversion and C16 isomerization degree of n-hexadecane is listed. In agreement with literature, the conventional HZSM-5 catalyst in general favours cracked products over isomerized n-hexadecane [15]. It is clearly observed that the introduction of noncrystallographic intracrystalline mesoporosity [16,17] in the HZSM-5 zeolite catalyst enhances the n-hexadecane conversion remarkably. The mesoporous HZSM-5 zeolite (Table 1, Entry 2) shows a three times higher conversion of n-hexadecane compared to the conventional sample (Table 1, Entry 1). The catalysts can, in terms of activity, be listed as follows: mesoporous Pt/HZSM5 > conventional Pt/HZSM-5 > mesoporous HZSM-
C.H. Christensen et al. / Catalysis Communications 5 (2004) 543–546
545
0.5
Pt/Conventional H-ZSM-5
0.45
Pt/Mesoporous H-ZSM-5
0.4
Frequency
0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 1
3
5
7
9
11
13
15
17
19
21
23
25
27
Mean Diameter (nm) Fig. 2. Particle size distribution of Pt particles on Pt/conventional and Pt/mesoporous HZSM-5 zeolite catalysts used for n-hexadecane cracking and isomerization.
Fig. 3. GC-chromatograms of product mixtures from n-hexadecane conversion catalyzed by conventional and mesoporous HZSM-5 zeolite catalysts.
Fig. 4. GC-chromatograms of product mixtures from n-hexadecane conversion catalyzed by Pt/conventional and Pt/mesoporous HZSM-5 zeolite catalysts.
5 > conventional HZSM-5 mesoporous Pt/silicalite1mesoporous silicalite-1. Mesoporous silicalite-1, with and without platinum, which contains no acidic sites,
exhibit very low activity. This agrees well with the observation that n-hexadecane conversion is an acid catalyzed reaction, i.e., it depends directly upon the acidity of the
546
C.H. Christensen et al. / Catalysis Communications 5 (2004) 543–546
Table 1 Catalyst performance in n-hexadecane conversion
4. Conclusion
Catalyst
Wt% of Pta (%)
Conversionb (%)
C16 isomerization degreec (%)
Conventional HZSM-5 Mesoporous HZSM-5 Mesoporous silicalite-1 Conventional HZSM-5 Mesoporous HZSM-5 Mesoporous silicalite-1
– – – 1.0 1.0 1.0
17 52 4 71 84 6
1.4 6.6 0.3 11.3 45.2 2.1
a
Platinum was introduced by incipient wetness impregnation using a aqueous Pt(NH3)4(HCO3)2-solution. b Determined as (Cn<16 + iso-C16H34 + Cn>16)/(Cn<16 + iso-C16H34 + n-C16H34 (unreacted) + Cn>16). c Determined as iso-C16H34/(iso-C16H34 + n-C16H34 (unreacted)).
In summary, we have reported the largest activity difference between the conventional and mesoporous HZSM-5 zeolite material observed so far. Our newly developed material, the mesoporous zeolite single crystals, combines in each individual crystal the crystallographic intracrystalline micropore system typical of zeolites with the mesopores typical of mesoporous materials. Shortening of the micropore diffusion path has proven to dramatically enhance the mass transport within the zeolite crystal both in liquid phase as well as in gas phase reaction. Acknowledgements
catalyst [15]. Thus, the conventional zeolite is expected to be a more active cracking catalysts compared to the mesoporous zeolite, due to the larger number of acid sites in the conventional zeolite. However, in the catalytic reaction of n-hexadecane, the mesoporous HZSM-5 zeolite is found to exhibit a far superior activity regarding both cracking as well as isomerization. The mesoporous zeolite consists of much larger crystals than the conventional zeolite. In fact, the nominal crystal size of the conventional sample is almost four times smaller than that of the mesoporous sample. Generally, the presence of large zeolite crystals causes mass transport limitations, particularly at high conversions and with relatively large reactant and product molecules [5]. This is due to the lower external surface area compared to that of small zeolite crystals. In our experiments there is obviously a pronounced effect of the presence of mesopores. When the zeolite catalysts are impregnated with platinum and the test reaction is conducted in the presence of hydrogen, the conversion is increased as expected. No difference in the activity ranking relative to that of the pure zeolite catalysts is observed, so also under these conditions there is a significant beneficial effect of the mesoporosity. Alkenes were not observed in the product mixtures from these experiments since they were easily converted to alkanes at the high hydrogen pressure. In general, very small amounts of C14 and C15 were found in the GC-chromatograms of the product mixtures. This illustrates that only insignificant amounts of methane and ethane were formed directly during the reaction time and demonstrates that the product distributions found are reliable, since negligible amounts of product are lost to the gas phase.
We are indebted to Lars Ku¨rstein for analyzing the product mixtures and to Anna Carlsson for conducting the TEM investigations. References [1] P.A. Jacobs, J.A. Martens, Stud. Surf. Sci. Catal. 58 (1991) 445. [2] A. Corma, J. Catal. 216 (2003) 298. [3] I.E. Maxwell, J.K. Minderhoud, W.H.J. Stork, J.A.R. van Veen, in: G. Ertl, H. Kno¨zinger, J. Weitkamp (Eds.), Handbook of Heterogenous Catalysis, vol. 4, Wiley, New York, 1997, p. 2017 (Chapter 3.13). [4] Y. Liu, T.J. Pinnavaia, J. Am. Chem. Soc. 125 (2003) 2376. [5] J. Houzvicka, I. Schmidt, C.J.H. Jacobsen, Stud. Surf. Sci. Catal. 135 (2001) 158. [6] A. Corma, M.J. Diaz-Cabanas, J. Martinez-Triguero, F. Rey, J. Rius, Nature 396 (1998) 353. [7] A. Karlsson, M. Sto¨cker, R. Schmidt, Micropor. Mesopor. Mat. 27 (1999) 181. [8] M. Tromp, J.A. van Bokhoven, M.T. Garriga Oostenbrink, J.H. Bitter, K.P. deJong, D.C. Koningsberger, J. Catal. 190 (2000) 209. [9] S. van Donk, A. Broersma, O.L.J. Gijzeman, J.A. van Bokhoven, J.H. Bitter, K.P. de Jong, J. Catal. 204 (2001) 272. [10] S. van Donk, A.H. Janssen, J.H. Bitter, K.P. de Jong, Catal. Rev. 45 (2003) 297. [11] C.H. Christensen, K. Johannsen, I. Schmidt, C.H. Christensen, J. Am. Chem. Soc. 125 (2003) 13370. [12] C. J. H. Jacobsen, C. Madsen, J. Houzvicka, I. Schmidt, A. Carlsson, US Patent 6,565,826. [13] C.J.H. Jacobsen, C. Madsen, J. Houzvicka, I. Schmidt, A. Carlsson, J. Am. Chem. Soc. 122 (2000) 7116. [14] C.J.H. Jacobsen, C. Madsen, J. Houzvicka, A. Carlsson, I. Schmidt, Stud. Surf. Sci. Catal. 135 (2001) 167. [15] K.C. Park, S.K. Ihm, Appl. Catal. A 203 (2000) 201. [16] A. Boisen, I. Schmidt, A. Carlsson, S. Dahl, M. Brorson, C.J.H. Jacobsen, Chem. Commun. (2003) 958. [17] A.H. Janssen, I. Schmidt, C.J.H. Jacobsen, A.J. Koster, K.P. de Jong, Micropor. Mesopor. 65 (2003) 59.