Discipline: Technology and Engineering
Subcategory: Chemical/Bimolecular/Process Engineering
Room: Exhibit Hall A
Kaela Evans - Washington University in St. Louis
Co-Author(s): Sopuruchukwu Ezenwa, Purdue University, IN; Rajamani Gounder, Purdue University, IN;
Zeolites are microporous crystalline aluminosilicates that are widely used as catalysts for hydrocarbon upgrading. Alkene oligomerization is affected by zeolite material properties such as crystallite size because longer diffusion paths preferentially increase intracrystalline residence times of bulky intermediates, which lead to secondary reactions that influence product selectivity and deactivation. In recently reported synthetic methods for MFI zeolites, it was observed that increasing the content of catalytically-inert B heteroatoms, while maintaining fixed content of catalytically-active Al heteroatoms, led to decreases in crystallite size. The mechanistic explanation for this effect has not been established. Thus, this research seeks to study the influence of boric acid precursors on the crystallite size of MFI zeolites. We hypothesized that the boric acid concentration in synthesis solution influences the initial and transient pH during crystallization, which further influences the nucleation and crystal growth. To test this hypothesis, a research method was devised to develop relationships between solution composition, pH, and zeolite properties as a function of crystallization time. B-Al-MFI zeolites were synthesized hydrothermally with high boron (Si/B = 2.5) and low boron (Si/B = 100) contents, while maintaining constant amounts of the structure-directing agents (tetrapropylammonium, TPA + and ethylenediamine EDA) used to crystallize MFI. Aliquots of the synthesis mixture were taken periodically to monitor MFI crystallization behavior. During synthesis, the pH of the high and low B-content mixtures remained relatively constant (~9.8 and ~11.3, respectively). X-ray diffraction (XRD) patterns for MFI were observed for both sets of syntheses taken out at 90 hours, however, the micropore volumes were higher (0.10 vs 0.05 cm 3 g – 1 ) for the high B-content syntheses, indicating that higher amounts of amorphous material formed in the low B-content case. Furthermore, complete crystallinity (0.15 cm 3 g -1 ) was achieved for both synthesis at longer synthesis times (100 hours). Scanning electron microscopy (SEM) was used to corroborate these findings regarding crystallinity, and the crystallite sizes were smaller for the higher B-content syntheses, as reported previously. These preliminary results further suggest that pH of the synthesis solution, moderated by the compositions of acidic and basic precursors, may influence the crystallization behavior and final zeolite properties. Future research involves probing the Al, B and structure-directing agent compositions as a function of synthesis time. These efforts can be used to develop a detailed understanding of the influence of synthesis parameters on the crystallite sizes, which can enable predictive catalyst design for hydrocarbon upgrading to chemicals and fuels.
Funder Acknowledgement(s): This poster is based upon work supported primarily by the National Science Foundation under Cooperative Agreement No. EEC-1647722.
Faculty Advisor: Rajamani Gounder, firstname.lastname@example.org
Role: I prepared the calculations, synthesis ,and analyzed XRD, SEM, micropore volume data.