59th Annual Report on Research 2014

Identifying Pathways of Non-Classical Zeolite Crystallization. In our previous progress report we discussed our work on tailoring zeolite L (LTL type) crystallization using ZGMs. LTL-type zeolite is utilized in catalytic reactions, such as the aromatization of hydrocarbons, and was selected as a candidate for habit modification based on its severe mass transport limitations (i.e., long diffusion path length and small surface area of porous faces). Our studies revealed that inexpensive, readily-available commercial organics (e.g., alcohols) exhibit site-specificity for binding to preferential crystallographic faces of LTL and modifying anisotropic rates of growth. During the past year we examined the mechanism by which zeolite L crystals grow and the effects of ZGMs on the evolution of zeolite crystal habit. The major activities involved the characterization of amorphous primary particles that form (pre-nucleation) and evolve into so-called worm-like particles (WLPs). The latter serve as a basis for zeolite L nucleation, and were therefore tracked at various time intervals over the course of crystal growth in the absence and presence of growth modifiers. The results of this study are being written into a manuscript with the anticipation of submitting this work to a peer-reviewed journal in Fall 2014.

Figure 1. Time-elapsed study of LTL crystallization reveals the transformation from initial worm-like particles (WLPs) to LTL crystals with the nominal cylinder morphology. (A – D) Synthesis in the presence of small colloidal silica (ca. 8-nm diameter) as the starting reagent results in more rapid nucleation. (E – H) Synthesis in the presence of larger colloidal silica (ca. 23-nm diameter) as the starting reagent increases the induction period for LTL nucleation. During the course of LTL crystallization we observed that the population of WLPs is gradually depleted in favor of growing crystals. The exact role of WLPs in crystal growth is still being explored.

New Platforms to Optimize Zeolite Synthesis. We have been working on the development of kinetic phase diagrams for zeolite synthesis in the absence of organic structure-directing agents (OSDAs). We have shown in our previous publications that the construction of OSDA-free ternary diagrams “map” the compositions and conditions of zeolite crystallization that lead to pure phases and permit physicochemical properties to be tuned. In the past year we have worked on the development of small-pore zeolites, which hold promise for catalytic reactions based on their shape selectivity. Efforts have been directed to the synthesis of GIS type zeolite, which has recently been identified in the literature as a promising material for CO₂ sequestration. We have identified compositions that lead to two distinct GIS polymorphs: P1 and P2. This is the first study to conclusively identify these two crystalline forms of GIS, which differ slightly in their pore apertures. We anticipate that these subtle differences in structure may in fact correlate to interesting trends in their selectivity in catalytic reactions. As part of this study we put together a literature survey that was published in Reviews in Chemical Engineering. We recently submitted a manuscript on the preparation of GIS polymorphs.

Figure 2.  Kinetic phase diagram showing Na-P1 and Na-P2 (GIS polymorphs) prepared with growth solutions of molar composition x SiO₂: y Al₂O₃:10 NaOH:173 H₂O. Each sample was prepared at 100°C for 7 days (see Table S1 for details of the synthesis compositions). These samples were prepared at constant Si/Al (1.5 to 9.0) and constant Si/OH molar ratios of (a) 0.37, (b) 0.46, (c) 0.54, (d) 0.75, (e) 0.82, (f) 1.00, and (g) 1.22. The pink area is a multiphase transitional region that separates the pure P1 and P2 phases. Highlighted regions of previously reported syntheses of MER (blue oval) and PHI (yellow triangle) are shown using the data compiled in our recent review of OSDA-free syntheses.

AFM Measurements of Zeolite Surface Growth. In our previous report we described how we had designed a prototype AFM sample cell retrofitted for in situ imaging under conditions amenable to zeolite synthesis. We used this system to characterize the growth of silicalite-1 (MFI type zeolite) at a temperature of 80 ^oC and for timescales of 12 to 24 hours. We imaged the (010) surface and tracked the deposition and subsequent evolution of growth units. This work was a significant effort to first get the system working. This included the following activities: (i) optimization of a drift correlation stabilization algorithm that was designed by Asylum Research specifically for our application; (ii) selection of appropriate materials (e.g., AFM tips, cell liners, etc.); (iii) error analysis of changing AFM tip curvature with continued imaging; and (iv) gathering statistics of surface growth to discover the mechanisms of crystallization. Our results provided the first in situ validation of the silicalite-1 growth mechanism, which is a subject that has been thoroughly examined over the past 25 years without resolution. This work was recently published in Science.

Figure 3. AFM amplitude mode images after (A) 1 hour and (B) 8.5 hours of in situ growth. The corresponding height profiles depict changes along line l_3-4 in panel A.

This award has partially supported the research of one graduate student, one undergraduate student, and one postdoctoral researcher. The results of this period have resulted in two published manuscripts and a third that is currently in review. Moreover, graduate students and the PI have presented this work at conferences and invited seminars, including the American Chemical Society Meetings, the American Institute of Chemical Engineering Annual Meeting, and the North American Catalysis Society Meeting.