Si-Al

© Copyright 1997 American Chemical Society

DECEMBER 24, 1997 VOLUME 13, NUMBER 26

Letters Fourier Transform Infrared Spectroscopic Comparison of Cu, Co/Si-Al-Zeolite Catalysts Prepared by a Combined Sol-Gel Method Rafael Dı´az,* Julia´n Cruz, and Rocı´o Ocampo Departamento de Ingenierı´a y Ciencias Quı´micas, Universidad Iberoamericana, Prol. Paseo de la Reforma 880, Me´ xico D.F. 01210, Me´ xico Received March 19, 1997. In Final Form: September 29, 1997X Combinations of Si-Al mixed with zeolites have shown a promising future as base supports for acidbase and oxidation reactions. An effective, although expensive, approach for Si-Al support preparation is based on the solid being formed from Si and Al alkoxides in alcohol. A variation of this method based on the mixing of alkoxides and inorganic Si and Al precursors during gelation is presented. Analyses show a crystal network completely dehydroxylated, which may indicate a more effective insertion of metal loadings into the network.

Introduction One of the newest ways of preparing zeolitic catalysts is via mixing silicates, titanates, vanadates, or their mixtures with zeolites during a sol-gel manufacturing process. These solids have shown a very promising future as base supports for acid-base and oxidation reactions. The use of silica-alumina as a base support onto which metals are deposited was previously shown to produce highly efficient catalysts.1-3 Activity in Si-Al is very much related to the preparation method, the precursors used, their ratio, pH, and processing temperature.4,5 This may be observed in reactions such as hydrotreatment of heavy oil,3 dehydration of * To whom correspondence should be addressed: e-mail, [email protected]. X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) Lo´pez,T.; Bosch, P.; Asomoza, M.; Go´mez, R. J. Catal. 1992, 133, 247. (2) Lo´pez, T.; Bosch, P.; Go´mez, R.; Basaldella, E.; Kikot, A.; Pereyra, E. Reac. Kinet. Catal. Lett. 1991, 43 (2), 461. (3) Dı´az, R. M.A. Sc. Thesis, University of Ottawa, Ottawa, Canada, 1988. (4) Lo´pez, T. Reac. Kinet. Catal. Lett. 1992, 47, 21. (5) Dı´az, R.; Lo´pez, T.; Quiroga, C.; Trevin˜o, P. Proc. XIV Iberoamerican Symp. Catal., Soc. Chilena Quı´mica 1994, 1617.

S0743-7463(97)00298-9 CCC: $14.00

2-propanol,6 CO hydrogenation,7 and o-xylene hydrogenation1 to mention a few. Sol-gel based methods have shown greater efficiency for metal catalyst impregnation inside the crystalline network, endowing the catalysts obtained with greater activity.8 Two approaches have been taken to prepare catalysts by the sol-gel method. The first is based on a conventional method using aluminum nitrate and sodium silicate (inorganic precursors)9 and the second is based on formation of Si-Al alkoxides in alcohol (organic precursors).1,2,6,10 The new method mixes both processes at the point where gelation is just starting to occur (as observed in Figure 1). As the inorganic-based method may not necessarily form a gel, but the structure may remain as a sol; the gel structure formed from alkoxides may link to the inorganic Si-Al particles. The new gel network would extend and may keep most of the properties of the pure alkoxide-based network. (6) Lo´pez, T.; Asomoza, M.; Go´mez, R. J. Non-Cryst. Solids 1992, 147&148, 769. (7) Feng, Z.; Postula, W.; Akgerman, A.; Anthony, R. Ind. Eng. Chem. Res. 1995, 34, 78. (8) Asomoza, M.; Lo´pez, T.; Go´mez, R.; Gonza´lez, R. D. Catal. Today 1992, 15, 547. (9) Dı´az, R.; Mann, R.; Sambi, I. Ind. Eng. Chem. Res. 1993, 32, 1354. (10) Lo´pez, T.; Villa, M.; Go´mez, R. J. Phys. Chem. 1991, 95, 1690.

© 1997 American Chemical Society

6862 Langmuir, Vol. 13, No. 26, 1997

Letters

Figure 1. Mixed sol-gel method.

Experimental Section A Si-Al hydrogel is prepared by coprecipitation of two organic precursors, Tetraethoxysilane (TEOS) and aluminum tri-secbutoxide (ATB). The morphology found is very similar to that of silica gels. Bro¨nsted acidity is expected to be predominant in this sol-gel silico-aluminate.4 As already demonstrated one advantage of this preparation method is the ease of access and incorporation of the metal loading into the crystalline network.8 Metal salts are diluted in solution, and exchanged with the hydroxyl groups present on the solid’s surface. Ion exchange does not occur but rather the ions are only “anchored” onto the surface, a common occurrence when inorganic precursors are used. If both of the above methods could be amalgamated in such a way that most properties belonging to the organic precursor based support remained, a catalyst with similar characteristics might be produced at lower cost. The nature of the crystalline network and its performance in a Si-Al-Y zeolite catalyst made from purely inorganic precursor is quite different than that made from alkoxide precursors.11 The adequate moment for this amalgamation would be when both methods are at the gelation stage. Then the two colloidal solutions are mixed, allowing polymerization to continue. After this stage the procedure follows very much the inorganic precursors’ pathway. Two mixtures were prepared, in one case 15% of the solution by weight of the alkoxide-based matrix was mixed with 85% by weight of the inorganic-based matrix. The solid would be termed: ABM-15. The other mixture would contain 30% and 70% respectively; being termed: ABM-30. Figure 1 shows preparation steps in detail. Final thermal treatment was carried out at 1073 K for 12 h. Amorphous Si-Al matrices have been mixed with different types of zeolites, especially Y-zeolite due to its large pore size and its strongly predominant Bro¨nsted acidity. Best results for redox type reactions are found when the zeolite loading is somewhere between 15 and 30% by weight.12,13 Main use has been for HDS, HDN, and HDO; however, new applications are in environmental areas using the ZSM zeolite family.14,15 This argument strengthens the idea of adding a zeolite to the Si-Al matrix, so the base-support can be endowed with specific acid properties. Using CuO supported on Si-Al or ZSM-5 zeolite as de-NOx-CO-HC catalysts has been studied recently.14,15 In our present study CuO, coupled with CoO as promoter in a 3:1 ratio, has been incorporated onto the Si-Al-zeolite matrices by dry impregnation, 3 and 1% by wt, respectively. Zeolites used in this study are Ammoniated Y (Si/Al ) 80), ZSM-5, and β zeolites (11) Dı´az, R.; Lazo, M. F. Proc. Int. Chem. Cong. Pacif. Bas. Soc. Honolulu, USA, Dec. 1995, p ENV-404. (12) Haynes, H. W., Jr.; Parcher, J. F.; Helmer, N. E. Ind. Eng. Chem. Proc. Des. Dev. 1993, 22 (3), 401. (13) Miale, J. N.; Chen, N. Y.; Weisz, P. B. J. Catal. 1966, 6, 276. (14) Steinbach, F.; Brunner, A.; Mu¨ller, H.; Dreschler, A.; Fro¨mming, S.; Strehlau, W.; Stan, U. Proc. 10th Int. Cong. Catal.: Hungary, 1992, p 1299. (15) Iwamoto, M.; Mizuno, N.; Yahiro, H. Proc. 10th Int. Cong. Catal.: Hungary, 1992, p 1285.

Figure 2. FTIR spectra of Si-Al zeolite based bimetallic catalysts made by mixed sol-gel (15% organic precursors, 85% inorganic precursors): (a) Si-Al-Y zeolite support; (b) 3:1 CuO/ CoO on Si-Al-Y zeolite support; (c) Si-Al-β zeolite support; (d) 3:1 CuO/CoO on Si-Al-β zeolite support; (e) Si-Al-ZSM-5 zeolite support; (f) 3:1 CuO/CoO on Si-Al-ZSM-5 zeolite support. from PQ Corp. The final base-support consisted of 75% by wt of amorphous Si-Al and 25% by wt of zeolite.

Results and Discussion Spectroscopic analysis was carried out by Fourier transform infrared (FTIR), other analyses made were differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). FTIR spectra show many similarities with samples made with the solely alkoxide-based sol-gel method. A more detailed comparison between both preparation methods for a CuO/CoO catalyst has already been presented.11 Figure 2 shows those catalysts made using ABS-15 and Figure 3 shows those made using ABS-30. It is important to notice that the spectra of the two types of catalysts resemble more closely those of catalysts made by the alkoxide-based sol-gel method. For the ABS-15 (Figure 2) the expected OH stretching band at about 3450 cm-1 from silanols, water, and ethanol occluded in the gel does not appear, with the exception of one case. The deformation mode of water at about 1638 cm-1 is barely noticeable and only in the unloaded supports. At about 1050 cm-1 the -Si-O-Si- stretching vibration is found. This band is quite wide and thus it also masks the -Al-O- and -Al-O-Si- stretching vibrations. Other typical vibrations are also found to be more similar to the alkoxide-based method, including the -Si-O-Si- and Si-O- deformation modes usually found

Letters

Langmuir, Vol. 13, No. 26, 1997 6863 Table 1. TGA Analysis sample

weight loss (%)

sample

weight loss (%)

ABS-15/Y ABS-15/β ABS-15/ZSM-5 ABS-15/Y/Cu-Co ABS-15/β/Cu-Co ABS-15/ZSM-5/Cu-Co

1.66 1.31 1.21 0.69 0.88 a

ABS-30/Y ABS-30/β ABS-30/ZSM-5 ABS-30/Y/Cu-Co ABS-30/β/Cu-Co ABS-30/ZSM-5/Cu-Co

1.29 1.38 1.52 1.28 1.69 a

a

Not detectable.

turing. Table 1 shows weight loss after TGA analysis for the studied samples. An interesting feature in this study is the fact that the two types of solid surfaces are virtually dehydroxylated, which is an effect not achieved with either of the methods alone. The hydroxyl ions on the surface have been almost completely substituted. Also no water was detected inside the crystal network. On the basis of the TGA analysis no single zeolite seems to perform better than the other. Differences due to the zeolite used only show on the DSC analysis. An exothermic peak shown at 593-603 K for those catalysts using β and ZSM-5 zeolites shifts to higher temperatures (about 640 K) in the ABS-15 matrix alone, whereas the ABS-15/Y based catalysts show the inverse behavior, the peak shows at 630 K and shifts down to 598 K. In the case of ABS-30 based catalysts a more dramatic case occurs. It seems that in all cases the crystal network rearranges completely at about 743 K, showing a closer behavior to single zeolite spectra.

Figure 3. FTIR spectra of Si-Al zeolite based bimetallic catalysts made by mixed sol-gel (30% organic precursors, 70% inorganic precursors): (a) Si-Al-Y zeolite support; (b) 3:1 CuO/ CoO on Si-Al-Y zeolite support; (c) Si-Al-β zeolite support; (d) 3:1 CuO/CoO on Si-Al-β zeolite support; (e) Si-Al-ZSM-5 zeolite support; (f) 3:1 CuO/CoO on Si-Al-ZSM-5 zeolite support.

at around 793 and 457 cm-1. No features due to the metal are observed. Cu-O should appear at about 562 cm-1 and Co-O at about 666 cm-1. Some shifting to lower energy regions may have occurred. Samples using ABS30 show somewhat similar bands. Dehydroxylation does not occur to the same extent for the two types of catalysts. A small difference in calcining temperature, 1073 K for catalysts shown in Figure 2 and 1023 K for catalysts shown in Figure 3, may not completely explain this difference. TGA analysis indicates that very little weight loss occurs in all samples (about 2% wt). This is compared with a 14-18% by weight loss from the inorganic-based catalyst and a 3-5% by weight loss of alkoxide-based catalyst.11 NH4OH desorption is clearly observed with DSC analysis. Some exothermic peaks are observed possibly due to chemisorption, oxidative degradation, or network restruc-

Conclusions Catalysts based on ABS-15 seem to keep most of the properties of alkoxide-based catalysts. Virtually complete dehydroxylation seems to be the most noticeable feature. It may imply an almost total substitution of surface hydroxyl ions. This seems to confirm that the gel structure formed from alkoxides links to the inorganic Si-Al particles, extending the crystalline network. ABS-30 catalysts still keep a rather small number of hydroxyl ions, allowing further ion substitution by other Si-Al chains or metal ions. If metal ions are substituted, some very active complexes may be formed, as in the case of Cu-Co complexes useful in environmental catalysis.15,16 Such complexes are more effective and long-lasting when embedded into the network as is the case of alkoxidebased catalysts.6 Acknowledgment. This work was supported by CONACyT (0811P-A-9506), TWAS (95-168 RG/CHE/LA) and UIA (DCI-402/97) grants. Supporting Information Available: DSC and TGA thermograms of zeolites (12 pages). Ordering information is given on any current masthead page. LA970298F (16) Panayotov, D.; Khristova, M.; Mehandjiev, D. J. Catal. 1995, 156, 219. (17) Dı´az, R.; Cruz, J. Proc. 11th Int. Cong. Catal. USA, 1996, Po407.