Synthesis, characterization and catalytic activityofhierarchical zeolite, ZH-5
Rajesh Kumar Parsapura and Parasuraman Selvam*a,b
aNCCR and Department of Chemistry, Indian Institute of Technology-Madras, Chennai 600 036, India
bNew Industry Creation Hatchery Centre, Tohoku University, Sendai 980 8579, Japan
E-mail:
Keywords: Hierarchical zeolites, mesoporous ZSM-5, solid acid catalysts, glycerol dehydration, bio-acrolein
1. Introduction
The tremendous success of the zeolites as the prominent heterogeneous catalysts is due to their unique physicochemical properties,viz., high surface area, strong acidity, shape selectivity, ion-exchange and high thermal stability. Among the various zeolites, the structures with MFI (ZSM-5) topologies are of paramount importance in catalysis.Nevertheless, the presence of micropores in these zeolites not only restricts the bulkier molecules to exploit the internal surface but also retards the molecular diffusion of the reactants and in-turn deactivate the catalyst. Therefore, attempts have been made to synthesize extra-large pore zeolites and ordered mesoporous materials (OMM)by using various templates [1]. However, the practical applications of such materials are limited owing to their moderate acidities andlow stabilities. On the other hand, in recent years, hierarchical zeolites (ZH)with ordered mesopores and crystalline zeolitic walls with mircropores are gaining considerable interest due to their ability to overcome the difficulties encountered by both microporous and mesoporous materials. However, synthesis of such structures is extremely challenging, and only few have succeeded in synthesizing so called hierarchical zeolites [2]. In this study, we present a successfulsynthesis of hierarchical zeolite, designated as ZH-5, with MFI-typemicropores and MCM-type mesopores, and their use in selective dehydration of glycerol.
2. Experimental
A dual templating strategy has been implied in which dimethyloctadecyl[3(trimethoxysilyl)propyl]ammonium chloride (DOAC)is used as the mesoporogen and TPABr is used as the micropore structure directing agent. In a typical synthesis, a prescribed amount of sodium aluminate was added to the NaOH solution under stirring followed by the addition of TPABr. Then a mixture of TEOS and DOAC are added under vigorous stirring and the solution was further stirred for 2 h.The obtained solution with the composition 1Al2O3: 10TPABr: 10Na2O: 38SiO2: 4ODAC was hydrothermally treated at 150ºC for 48 h. The resulting solidwas filtered, washed and dried followed by calcination at 550ºC in air. The calcined and proton exchanged sample is designated as ZH-5.
3. Results and discussion
Figure 1 depicts the XRD pattern of hierarchical ZSM-5 which shows characteristic reflections in the low-angle (inset) typical of 2D-hexagonal structure (MCM-41) while the high-angle pattern is distinctive of orthorhombic (MFI) crystal symmetry.
Figure 1. XRD patterns of ZH-5.
The formation of the hierarchical structure is further supported by the presence of type-I and type-IV isotherms in N2 sorption measurements (not shown here). The structural properties of ZH-5 are shown in table 1. Figure 2 depicts the NH3-TPD profile of ZH-5 in comparison with microporous ZSM-5. The presence of broad peak around 670 K indicates the presence of strong Brønsted acid sites.However, the less acidity compared to its microporous counterpart can be attributed to the interrupted frameworksby the mesopores.
Figure 2. NH3-TPD profiles of (a) ZSM-5; (b) ZH-5.
Figure 3adepicts the 27Al MAS NMR spectra of ZH-5 which shows a single peak at 55 ppm corresponding to aluminium in the tetrahedral coordination(55 ppm) and the absence of a peak at ‘0 ppm’indicates the absence of extra-framework aluminium species. Figure 3a shows the 29Si MAS NMR spectra of ZH-5 which shows a broad peak above ‘-110 ppm’ corresponding to Q4 sites of the zeolite. Whereas, the broad satellite at -99 and -105 ppm could be attributed to the Q3 silanol groups and 3Si(1Al) type coordination respectively.
Figure 3. 27Al (a) and 29Si (b) MAS-NMR of ZH-5.
Table 1. Textural properties of various zeolites.
Catalyst / SSAa (m2g1)______/ DBJHb(nm)
______/ Vpc(cm3g1)
______
Micro / Meso / Meso / Micro / Meso
ZSM-5 / 230 / 103 / --- / 0.09 / 0.05
ZH-5 / 143 / 403 / 3.0 / 0.07 / 0.50
aSpecific surface area, bPore size, cPore volume.
Figure 4 presents the TEM images of ZH-5 which shows the presence of mesopores amidst the matrix of zeolitic micropores. The orthorhombic crystal symmetry of the ZH-5 is further reflected in the SAED pattern (inset).
Figure 4. TEM images of ZH-5. Inset: SAED pattern.
Figure 5 shows the catalytic activity of ZH-5 in selective dehydration of glycerol. The enhanced diffusion by mesopores is clearly reflected in the improved catalytic performance and enhanced lifetime up to 15 h even at high concentrations (25 wt%) of glycerol which is hitherto not reported.
Figure 5. Catalytic activity of ZH-5
4. Conclusion
In this study, we have demonstrated a successful synthesis of hierarchically zeolite, ZH-5. Further, we have also shown that the catalyst exhibit excellent activity and enhanced lifetime for the selective dehydration of glycerol to acrolein. Further work is in progress.
Acknowledgments
The authors thank DST, New Delhi for funding NCCR. One of the authors, RKP thanks CSIR for the award of SRF.
References
[1]P. Selvam, S. K. Bhatia, C. G. Sonwane, Ind. Eng. Chem. Res.,2001, 40, 3237.
[2] M. Choi, H. S. Cho, R. Srivastava, C. Venkatesan, D. -H. Choi, R. Ryoo, Nat. Mater., 2006, 5, 718.