Industrial Blvd., Sugar Land, TX 77478-2589. Introduction. The catalytic properties of zeolites are often determined by the presence of the framework ...
Anirban Ghosh,1 Gopalakrishnan Juttu,2 Scott Mitchell2 and Daniel F. Shantz1 1
Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122 2 SABIC Technology Center, SABIC Americas, Inc., 1600 Industrial Blvd., Sugar Land, TX 77478-2589 Introduction The catalytic properties of zeolites are often determined by the presence of the framework heteroatoms such as aluminum.1 While aluminum containing zeolites have been extensively investigated, there are relatively few works in the literature which have investigated Ge,Al containing zeolites.2,3 Germanium-containing zeolites have recently garnered great attention in the literature due to the work by Corma’s lab demonstrating several low framework density phases made in the presence of germanium.2 Most literature reports of germanium containing phases have reported fluoride-based syntheses. This, along with the interesting catalytic properties of Ge,Al-ZSM-5 motivate the work performed here, a comparison of these phases made in fluoride and alkali media. Experimental All germanium-containing zeolites were synthesized in the presence of acetic acid to control the pH of the gel in the range of 8 to 11 to maximize the amount of germanium incorporated into the zeolite framework.4 For reference purposes a Silicalite-1 sample was synthesized from a mixture of composition SiO2: 0.27NaOH: 0.46TPAOH: 15H2O. Syntheses were performed to make Ge-silicalite-1, Alsilicalite-1 (ZSM-5), and Ge,Al-silicalite-1 (Ge-ZSM-5). The Ge-ZSM-5 samples were synthesized from mixtures of composition xSiO2: yGeO2: zNaAlO2: 0.27NaOH: 0.46TPAOH: 15H2O, where x + y + z = 1; x/y = 10, 25 and 50; and x/z = 50 and 100 for each x/y. The Ge-Silicalite-1 samples were synthesized from mixtures of composition xSiO2: yGeO2: 0.27NaOH: 0.46TPAOH: 15H2O, where x + y = 1; and x/y = 10, 25 and 50. The Al-Silicalite samples were synthesized from mixtures of composition xSiO2: zNaAlO2: 0.27NaOH: 0.46TPAOH: 15H2O, where x + z = 1; and x/z = 50 and 100. Powder X-ray diffraction (PXRD) measurements were performed using a Bruker-AXS D8 Vario powder diffractometer with Cu Kα radiation over a 2θ range of 5–90°. Scanning electron microscopy (SEM) was performed on a JEOL JSM-6400 microscope operating at 0.2–40 keV. X-ray fluorescence (XRF) measurements were performed on a Rigaku ZSX100e instrument with a Rh target. The sample was fused with lithium tetraborate before elemental analysis. Infrared spectroscopy was performed on a Nexus 670 FT-IR Spectrometer from Thermo Nicolet. 29Si MAS and 27Al MAS NMR were performed on a Bruker Avance 400 spectrometer
operating at 79.49 and 104.26 MHz. X-ray photoelectron spectra (XPS) were recorded on a Kratos Axis Ultra Imaging X-ray photoelectron spectrometer (Manchester, U.K.) using a monochromatic Al Kα source (400 W) in a UHV environment (ca. 5 × 10–9 Torr). During acquisition, surfaces were kept from charging by the application of low energy electrons. Micromeritics ASAP 2010 micropore system using approximately 0.05 g of sample. The samples were degassed under vacuum at 100 °C for 2 h, and then at 300 °C overnight before the analysis. Results and Discussion X-ray diffraction results show that crystalline zeolite phases are formed in all syntheses when the gel pH is kept between 8 and 11. It is observed that consistent with previous literature that germanium inclusion increases with decreasing pH. SEM images indicate that the crystals are typically quite large, greater than 5 micrometers a side in all cases. The crystal size is observed to increase both with decreasing solution pH and with increasing germanium content. Nitrogen adsorption measurements on calcined samples give micropore volumes between 0.10-0.14 cm3/g, consistent with previous literature work.2 XRF measurements, some of which are summarized in Figure 1, indicate several points. First, the data shows that a gel composition of Si/Al = 50 it is difficult to achieve high level of germanium inclusion except at the lower pH values. Second, the XRF data also shows that the aluminum in nearly all cases is completely occluded in the materials. 100 0 wt% AcOH 12.5 wt% AcOH 25 wt% AcOH
80 Si/Ge, mixture
SYNTHESIS, CHARACTERIZATION, AND CATALYTIC EVALUATION OF GE,AL-ZSM-5
60
40
20
0 0
100
200 300 Si/Ge, product
400
500
Figure 1. Summary of XRF results for samples made with a Si/Al ration in the gel of 50. Infrared spectroscopy for the samples as well as 29Si NMR are consistent with the previous literature.3 27Al MAS NMR indicates the absence of octahedral aluminum, and this will be discussed in the context of catalytic activity. X-ray photoelectron spectroscopy (XPS) indicates that there is significant surface enrichment of germanium and depletion of aluminum as compared to the bulk compositions obtained from the XRF results (Table 1). Based on XPS there is
Prep. Pap.-Am. Chem. Soc., Div. Pet. Chem. 2008 , 53 (1), 178 Proceedings Published 2008 by the American Chemical Society
essentially no aluminum on the crystal surface. These results indicate germanium and aluminum gradients are present in these materials; current work is determining if this can be altered by adjusting the pH during synthesis. Table 1. Comparison of XRF and XPS Data. Sample
Element
Ge,Al-silicalite-1 (Si/Ge = 10) (Si/Al = 45) Ge-silicalite-1 (Si/Ge = 10)
Weight (%) XPS
XRF
Si
20.4
39.3
Na
6.2
1.7
Ge
24.3
8.9
Si
24.3
38.8
Na
4.1
2.0
Ge
22.9
9.3
Current and ongoing work is studying the acidity of the synthesized materials via 13C NMR of acetone adsorbed on protonic forms of the zeolite. TPD, TPR and propane aromatization over these materials are also ongoing and will be reported. Conclusions. The current demonstrates the feasibility of synthesizing Ge,Al-ZSM-5 samples via non-fluoride mediated synthesis. The effect of synthesis conditions on heteroatom loading, composition gradients within crystals (and their potential modification) are also reported. Determining how acidity and reactivity vary with crystal composition is ongoing and will also be reported. Financial support from Sabic Americas is gratefully acknowledged.. References (1) Cundy, C. S.; Cox, P. A. Chem. Rev. 2003, 193, 663-701. (2) Corma, A.; Davis, M. E. ChemPhysChem 2004, 5, 304-313. (3) van de Water, L. G. A.; van der Waal, J. C.; Jansen, J. C.; Maschmeyer, T. J. Catal. 2004, 223, 170-178. (4) Juttu, G.; Khanmemedova, A. K.; Mitchell, S. F. U.S. Patent 7,209,650 B1.
Prep. Pap.-Am. Chem. Soc., Div. Pet. Chem. 2008 , 53 (1), 179 Proceedings Published 2008 by the American Chemical Society
