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The synthesis of mesoporous materials with semicrystalline microporous walls
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
137
The synthesis of mesoporous materials with semicrystalline microporous walls Sung II Cho ^, Yong Ku Kwon^, Sang-Eon Park'^ and Geon-Joong Kim^ ^Department of Chemical Engineering, Inha University, Incheon 402-751, Korea. ^Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea. ^Korea Research Institute of Chemical Technology, Taejon 305-600, Korea. The route to synthesize the mesoporous materials having microporous semicrystalline frameworks has been developed in this study; the successful incorporation of the microporous ZSM-5 within the framework walls of the ordered mesoporous structure significantly improves the hydrothermal stability and acidity of MCM-41 and related mesoporous materials. 1. INTRODUCTION Recent research efforts have been paid to the synthesis of microporous[l] or mesoporous materials[2-4] with large pore sizes and uniform pore distribution[5]. However, typical mesoporous materials have the amorphous frameworks that are insufficient to withstand the severe process conditions, e.g. steam regeneration of deactivated catalysis[l]. Because of these limitations in use, future applications in the conversion of large molecules would depend on improving their characteristics. Obviously, significant advances of the physicochemical properties of these mesoporous materials can be expected when the crystalline zeolites such as ZSM-5 or Y-zeolite are incorporated into the silica frameworks. In this paper, we report the synthesis of highly ordered mesoporous silica and aluminosilicate having semicrystalline, microporous zeolite frameworks. Our approach is mainly based on the sequential synthesis of MCM-41 and MFI-type zeolite by two step treatments using tetrapropylammonium bromide (TPABr) and cetyltrimethylammonium bromide, Ci6Hi3(CH3)3NBr (C16TMAE), surfactant as a structure-directing agent. In addition, the catalytic properties of these materials were examined in the alkylation of diisopropylbenzene with isopropanol as a test reaction. 2. EXPERIMENTAL For the MFI synthesis, silicon oxide (SiOz), aluminum oxide (AI2O3), sodium oxide (NazO), TPABr and H2O was used at 50 °C and then the aqueous solution of CieTMAB surfactant was added to the resulting precursor MFI nuclei solution with stirring at the same temperature and the mixed solution was aged at 100°C for 1 day. And the pH was adjusted to approximately 11 by the addition of hydrochloric acid (HCl) with vigorous stirring. Then, the mixture was heated again at 100 °C for 1 day. 1,3,5-Trimethylbenzene (TMB) was added to the mixed solution that was heated to reflux for additional 1 day. And, the pH value of the final mixture
138
was adjusted to 12 by addition of tetrapropyl ammonium hydroxide (TPAOH) solution. Finally, methanol with the same volume as TMB was added to the reactant mixture. The final mixture was transferred to a Teflon-coated autoclave for further reaction at 100 - 170°C. XRD patterns were recorded on a Rigaku Rotaflex diffractometer using CuKa radiation. The morphology of the samples was examined by TEM (Phillips CM-220) and SEM (Hitachi S-4200). N2 adsorption and desorption isotherms were determined on a Micrometrics ASAP 2000 sorptometer at -196 °C. NH3-TPD curves were performed in the range of 150 - 500 °C 3. RESULTS AND DISCUSSION Typical X-ray diffractograms of the silica-surfactant assembly with the addition of TMB display a series of intense and sharp peak intensities in the small and wide angle region, indicating the formation (IHU-1) of both MCM-41 mesophase and crystalline ZSM-5 microphase as shown in Figure 1. The intense (100) peak for the silica-surfactant assembly before the heat treatment at 170 °C (Figure lA) slightly moves to the lower angle region during crystallization (Figure IB, C and D), indicating the expansion of the hexagonal packing distance between the mesopores during the second crystallization to induce the microporosity of ZSM-5. The fact that higher order peaks near 26 = 3 - 7° are absent after the second crystallization (Figure ID) indicates that the structural perfection of the hexagonal mesophase is reduced due to the crystallization of the MFI-zeolite nuclei, leading to the disordered channel formation of IHU-1.The XRD pattern in Figure IE shows simultaneous formation of the disordered mesoporous and ZSM-5 structure. As a result, the physical mixture of ZSM-5 crystal and collapsed MCM-41 was obtained as a product without addition of TMB and methanol. All of the mesoporous structures were collapsed and pure ZSM-5 crystals were formed through the transformation from meso-phase with the prolonged reaction time (Figure IF). Transmission electron microscopy (TEM) micrograph of IHU-1 shows a partially disordered pore structure with thick framework walls (Figure 2). The repeated addition of hydrochloric acid leads to MCM-41 with thicker framework (IHU1) walls, within which a high density of nanocrystallites can be nucleated and grown.
2th*ta(d«ar**t)
Fig. 1. XRD data of the IHU-1, followed by additional heat treatment at 170 °C for: (A) 0 hours: (B) 3 hours; (C) 7 hours; (D) 24 hours
Fig. 2. Tem image of the IHU-1 at lOOt for 3days with repeated pH adjustment to 11 and additional heat treatment at 170°C for 3 hours
139
These data are coincident with the XRD results of MCM-41 containing ZSM-5 nuclei and IHU-1 with semicrystalline walls (Figure l).These data also confirm the conversion of the zeolite precursors into the ZSM-5 nano-crystals in the frameworks The calcined IHU-1 had a N2 BET surface of 920 m^/g and a pore volume of l.Ocm^/g. Aluminum containing IHU-1 (IHU-2) was prepared to test its catalytic activity and selectivity for the alkylation of diisopropylbenzene(DIPB) with isopropanol. For comparison, H-type Al MCM-41 was also prepared and tested as a catalyst. The amount of aluminium in catalysts was almost the same. Catalytic activities determined in the alkylation of DIPB with isopropanol over various catalysts are given in Figure 3. In the alkylation of DIPB, the conversion of DIPB over the H-type Al MCM-41 was lower than that over the H-type IHU-2 because of the abundance of acidic sites on the surface of IHU-2. The conversions of DIPB at 400 °C were 18% over IHU-2 and 10% over H-type Al MCM-41, respectively. Triisopropylbenzene (TIPB) was found mainly in the product mixture at 200 - 300 °C, whereas the cracking of DIPB was proceed at 400 - 600°C, resulting in the production of benzene and mono-isopropylbenzene(MIPB). The main products obtained over IHU-2 were benzene and MIPB over 300 °C. The high selectivity to benzene and MIPB over IHU-2 suggests that IHU-2 has stronger acid sites than Al containing H-type MCM-41 catalyst (Figure 4 and Figure 5). HZSM-5 was inactive for this reaction. Microporous ZSM-5 cannot adsorb TIPB in the pore because of its small size, whereas MCM-41 with the larger pore size converts it into various hydrocarbons. As a result, the incorporation of aluminum into the mesoporous walls (IHU-2) greatly enhances the acidity and reactivity of IHU-1 to be utilized as one of the most promising catalysts. The catalytic activity of the separate agglomerates of H-type ZSM-5 and H-type Al MCM-41 was similar to that of Al MCM-41, confirming that the reaction of TMB molecules occurred only within the mesopores of MCM-41/MFI composite. The acidic nature of H-type AlMCM-41 MOVWI -a/AH3oanu) (mole ratio of Si02/Al203=35) and H-type IHU-2 (mole ratio of Si02/Al203=39) was measured by temperature-programmed desorption (TPD) of ammonia. They have very strong acidic site distribution, similar to that present in the H-type ZSM-5 zeolite. The acidity of MCM-41 was found to be comparable to that of amorphous silicaalumina, but weaker than that of ZSM-5. ZSM-5 offers unique catalytic activities 400 500 and selectivity alteration for certain Teniperature('C) reactions compared to amorphous silicaFig. 3. The catalytic activities of IHU-2 and alumina because of porosity, crystallinity HMCM-41 in the alkylation of DIPB with and strong acidity. It is evident that Hisopropanol type IHU-2 has strong acidic site distribution, similar to that present in the H-type ZSM-5 zeolite.
140 100-
80-
geo- A 1 ^ 40-
% \
(/5 —0—bUHHK
—•—dinne
\
20-
—
*
0— I — I — I — I — I — 1 — I — I — ^ — I — I — I — | —
300
3BD
400
490
5D0
5QD
eOO
'foiperatm(T)
Fig. 4. The selectivity versus reaction temperature over IHU-2
•
^_____::zz=—•
— 1 — ' — 1 — ' — 1 — • — 1 — 1 — I — ' — 1 — 1 — r
3D0
3BD
400
493
aD
5BD
eOO
liii|xraim(t)
Fig. 5. The selectivity versus reaction temperature over MCM-41
When the MFI microporous structure is incorporated into the walls of MCM-41, a significant increase in peak area around 350 *^C is observed compared to H-type Al MCM-41, indicating that the strong acid sites are introduced within the micropores of ZSM-5 walls. Consequently, the H-type IHU-2 has strong acid sites within the framework walls that catalyze large aromatic molecules to produce various hydrocarbons in the catalytic reaction mentioned above. 4. CONCLUSION The successful incorporation of the microporous ZSM-5 within the framework walls of the ordered mesoporous structure significantly improves the hydrothermal stability and acidity of MCM-41 and related mesoporous materials. The excellent catalytic properties of a series of the IHU expand the area for the application of porous materials and can be used in a number of commercial processes in the future. REFERENCES 1. A. Corma, Chem. Rev. 97 (1997) 2373. 2. J. S. Beck, M. C. Vartuli, Current Opinion in Solid State and Mater. Sci. 1 (1996) 76. 3. D. Zhao, Q. Huo, J. Feng, B. F Chmelka, G. D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. 4. D. M. Antonelli, Y. Ying, Angew. Chem. Int. Ed. 35 (1996) 426. 5. Q. Huo, D. I. Margolese, G. D. Stucky, Chem. Mater. 8 (1996) 1147.
137
The synthesis of mesoporous materials with semicrystalline microporous walls Sung II Cho ^, Yong Ku Kwon^, Sang-Eon Park'^ and Geon-Joong Kim^ ^Department of Chemical Engineering, Inha University, Incheon 402-751, Korea. ^Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea. ^Korea Research Institute of Chemical Technology, Taejon 305-600, Korea. The route to synthesize the mesoporous materials having microporous semicrystalline frameworks has been developed in this study; the successful incorporation of the microporous ZSM-5 within the framework walls of the ordered mesoporous structure significantly improves the hydrothermal stability and acidity of MCM-41 and related mesoporous materials. 1. INTRODUCTION Recent research efforts have been paid to the synthesis of microporous[l] or mesoporous materials[2-4] with large pore sizes and uniform pore distribution[5]. However, typical mesoporous materials have the amorphous frameworks that are insufficient to withstand the severe process conditions, e.g. steam regeneration of deactivated catalysis[l]. Because of these limitations in use, future applications in the conversion of large molecules would depend on improving their characteristics. Obviously, significant advances of the physicochemical properties of these mesoporous materials can be expected when the crystalline zeolites such as ZSM-5 or Y-zeolite are incorporated into the silica frameworks. In this paper, we report the synthesis of highly ordered mesoporous silica and aluminosilicate having semicrystalline, microporous zeolite frameworks. Our approach is mainly based on the sequential synthesis of MCM-41 and MFI-type zeolite by two step treatments using tetrapropylammonium bromide (TPABr) and cetyltrimethylammonium bromide, Ci6Hi3(CH3)3NBr (C16TMAE), surfactant as a structure-directing agent. In addition, the catalytic properties of these materials were examined in the alkylation of diisopropylbenzene with isopropanol as a test reaction. 2. EXPERIMENTAL For the MFI synthesis, silicon oxide (SiOz), aluminum oxide (AI2O3), sodium oxide (NazO), TPABr and H2O was used at 50 °C and then the aqueous solution of CieTMAB surfactant was added to the resulting precursor MFI nuclei solution with stirring at the same temperature and the mixed solution was aged at 100°C for 1 day. And the pH was adjusted to approximately 11 by the addition of hydrochloric acid (HCl) with vigorous stirring. Then, the mixture was heated again at 100 °C for 1 day. 1,3,5-Trimethylbenzene (TMB) was added to the mixed solution that was heated to reflux for additional 1 day. And, the pH value of the final mixture
138
was adjusted to 12 by addition of tetrapropyl ammonium hydroxide (TPAOH) solution. Finally, methanol with the same volume as TMB was added to the reactant mixture. The final mixture was transferred to a Teflon-coated autoclave for further reaction at 100 - 170°C. XRD patterns were recorded on a Rigaku Rotaflex diffractometer using CuKa radiation. The morphology of the samples was examined by TEM (Phillips CM-220) and SEM (Hitachi S-4200). N2 adsorption and desorption isotherms were determined on a Micrometrics ASAP 2000 sorptometer at -196 °C. NH3-TPD curves were performed in the range of 150 - 500 °C 3. RESULTS AND DISCUSSION Typical X-ray diffractograms of the silica-surfactant assembly with the addition of TMB display a series of intense and sharp peak intensities in the small and wide angle region, indicating the formation (IHU-1) of both MCM-41 mesophase and crystalline ZSM-5 microphase as shown in Figure 1. The intense (100) peak for the silica-surfactant assembly before the heat treatment at 170 °C (Figure lA) slightly moves to the lower angle region during crystallization (Figure IB, C and D), indicating the expansion of the hexagonal packing distance between the mesopores during the second crystallization to induce the microporosity of ZSM-5. The fact that higher order peaks near 26 = 3 - 7° are absent after the second crystallization (Figure ID) indicates that the structural perfection of the hexagonal mesophase is reduced due to the crystallization of the MFI-zeolite nuclei, leading to the disordered channel formation of IHU-1.The XRD pattern in Figure IE shows simultaneous formation of the disordered mesoporous and ZSM-5 structure. As a result, the physical mixture of ZSM-5 crystal and collapsed MCM-41 was obtained as a product without addition of TMB and methanol. All of the mesoporous structures were collapsed and pure ZSM-5 crystals were formed through the transformation from meso-phase with the prolonged reaction time (Figure IF). Transmission electron microscopy (TEM) micrograph of IHU-1 shows a partially disordered pore structure with thick framework walls (Figure 2). The repeated addition of hydrochloric acid leads to MCM-41 with thicker framework (IHU1) walls, within which a high density of nanocrystallites can be nucleated and grown.
2th*ta(d«ar**t)
Fig. 1. XRD data of the IHU-1, followed by additional heat treatment at 170 °C for: (A) 0 hours: (B) 3 hours; (C) 7 hours; (D) 24 hours
Fig. 2. Tem image of the IHU-1 at lOOt for 3days with repeated pH adjustment to 11 and additional heat treatment at 170°C for 3 hours
139
These data are coincident with the XRD results of MCM-41 containing ZSM-5 nuclei and IHU-1 with semicrystalline walls (Figure l).These data also confirm the conversion of the zeolite precursors into the ZSM-5 nano-crystals in the frameworks The calcined IHU-1 had a N2 BET surface of 920 m^/g and a pore volume of l.Ocm^/g. Aluminum containing IHU-1 (IHU-2) was prepared to test its catalytic activity and selectivity for the alkylation of diisopropylbenzene(DIPB) with isopropanol. For comparison, H-type Al MCM-41 was also prepared and tested as a catalyst. The amount of aluminium in catalysts was almost the same. Catalytic activities determined in the alkylation of DIPB with isopropanol over various catalysts are given in Figure 3. In the alkylation of DIPB, the conversion of DIPB over the H-type Al MCM-41 was lower than that over the H-type IHU-2 because of the abundance of acidic sites on the surface of IHU-2. The conversions of DIPB at 400 °C were 18% over IHU-2 and 10% over H-type Al MCM-41, respectively. Triisopropylbenzene (TIPB) was found mainly in the product mixture at 200 - 300 °C, whereas the cracking of DIPB was proceed at 400 - 600°C, resulting in the production of benzene and mono-isopropylbenzene(MIPB). The main products obtained over IHU-2 were benzene and MIPB over 300 °C. The high selectivity to benzene and MIPB over IHU-2 suggests that IHU-2 has stronger acid sites than Al containing H-type MCM-41 catalyst (Figure 4 and Figure 5). HZSM-5 was inactive for this reaction. Microporous ZSM-5 cannot adsorb TIPB in the pore because of its small size, whereas MCM-41 with the larger pore size converts it into various hydrocarbons. As a result, the incorporation of aluminum into the mesoporous walls (IHU-2) greatly enhances the acidity and reactivity of IHU-1 to be utilized as one of the most promising catalysts. The catalytic activity of the separate agglomerates of H-type ZSM-5 and H-type Al MCM-41 was similar to that of Al MCM-41, confirming that the reaction of TMB molecules occurred only within the mesopores of MCM-41/MFI composite. The acidic nature of H-type AlMCM-41 MOVWI -a/AH3oanu) (mole ratio of Si02/Al203=35) and H-type IHU-2 (mole ratio of Si02/Al203=39) was measured by temperature-programmed desorption (TPD) of ammonia. They have very strong acidic site distribution, similar to that present in the H-type ZSM-5 zeolite. The acidity of MCM-41 was found to be comparable to that of amorphous silicaalumina, but weaker than that of ZSM-5. ZSM-5 offers unique catalytic activities 400 500 and selectivity alteration for certain Teniperature('C) reactions compared to amorphous silicaFig. 3. The catalytic activities of IHU-2 and alumina because of porosity, crystallinity HMCM-41 in the alkylation of DIPB with and strong acidity. It is evident that Hisopropanol type IHU-2 has strong acidic site distribution, similar to that present in the H-type ZSM-5 zeolite.
140 100-
80-
geo- A 1 ^ 40-
% \
(/5 —0—bUHHK
—•—dinne
\
20-
—
*
0— I — I — I — I — I — 1 — I — I — ^ — I — I — I — | —
300
3BD
400
490
5D0
5QD
eOO
'foiperatm(T)
Fig. 4. The selectivity versus reaction temperature over IHU-2
•
^_____::zz=—•
— 1 — ' — 1 — ' — 1 — • — 1 — 1 — I — ' — 1 — 1 — r
3D0
3BD
400
493
aD
5BD
eOO
liii|xraim(t)
Fig. 5. The selectivity versus reaction temperature over MCM-41
When the MFI microporous structure is incorporated into the walls of MCM-41, a significant increase in peak area around 350 *^C is observed compared to H-type Al MCM-41, indicating that the strong acid sites are introduced within the micropores of ZSM-5 walls. Consequently, the H-type IHU-2 has strong acid sites within the framework walls that catalyze large aromatic molecules to produce various hydrocarbons in the catalytic reaction mentioned above. 4. CONCLUSION The successful incorporation of the microporous ZSM-5 within the framework walls of the ordered mesoporous structure significantly improves the hydrothermal stability and acidity of MCM-41 and related mesoporous materials. The excellent catalytic properties of a series of the IHU expand the area for the application of porous materials and can be used in a number of commercial processes in the future. REFERENCES 1. A. Corma, Chem. Rev. 97 (1997) 2373. 2. J. S. Beck, M. C. Vartuli, Current Opinion in Solid State and Mater. Sci. 1 (1996) 76. 3. D. Zhao, Q. Huo, J. Feng, B. F Chmelka, G. D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. 4. D. M. Antonelli, Y. Ying, Angew. Chem. Int. Ed. 35 (1996) 426. 5. Q. Huo, D. I. Margolese, G. D. Stucky, Chem. Mater. 8 (1996) 1147.