(340g) Optimizing Reaction Conditions and Zeolite Properties for Different Catalytic Applications

My research interests involve investigating, understanding and optimizing the materials properties and reaction conditions for catalytic applications. In this regard, during my Ph.D., I have focused on understanding the crucial impact the reaction conditions can have on alkylation reactions and designing zeolite materials with improved physio-chemical properties for commercially relevant reactions such as methanol-to-hydrocarbon (MTH) reaction, hydrocarbon cracking reactions and dehydrogenation reactions.

Introduction: Zeolites

Zeolites are crystalline nanoporous aluminosilicates materials consisting of tetrahedral SiO₄ and AlO₄^- units, connected together by sharing the oxygen atoms to form three-dimensional network of channels (figure a). The unique properties of zeolites such as exceptional hydrothermal stability, tunable acidity, well defined nano-sized pores, make them suitable for wide variety of applications including catalysis, ion-exchange, gas adsorption, and drug delivery. The optimization of the physio-chemical properties of zeolites have made many commercial processes possible, however; improving these properties is not a trivial task.¹ It is essential to understand the underlying mechanisms of zeolite crystallization and the impact of different parameters involved on these properties, in order to be able to rationally design the zeolites with desired properties for different applications.

The performance of zeolites as catalysts strongly depends on reaction conditions as well. Zeolites performances as catalysts can vary dramatically depending on the reaction conditions.² The optimization of zeolite properties and finding the right reaction conditions are absolutely essential to make any chemical process commercially feasible. Here, I will briefly share few of my attempts to optimize these factors to improve the zeolites performance as catalysts:

Spatiotemporal coke coupling enhances para-xylene selectivity in highly stable MCM-22 catalysts under high pressure condition

Para-xylene is one of the most important aromatic compounds used in the synthesis of various fine chemicals. Toluene alkylation with methanol (TAM) catalyzed by zeolites is an emerging and commercially attractive route to produce p-xylene; however, this process often suffers from low catalyst stability and requires the use of diluents (hydrogen and/or water), low space velocity, and high toluene-to-methanol ratios, which collectively results in low p-xylene yield and have cost implications as well. So to overcome these problems, I switched the reaction condition from conventional atmospheric condition to high partial pressures of reactants (4.2 MPa total pressure without any diluents). Under new conditions, MCM-22 shows exceptional catalyst lifetime with the highest p-xylene yield reported to date. The increase in operating pressure suppresses the side-reactions such as methanol-to-hydrocarbons and multiple alkylation of aromatic rings, resulting in significant catalyst lifetime improvement. To understand the catalytic behavior of MCM-22 catalyst, we have deconvoluted the structure-function relationship for different topological features (supercages, sinusoidal channels, and external surface pockets, figure b), as they can have profound impact on the catalytic performance. By using catalytic testing, density functional theory calculations and molecular dynamic simulations, we were able to figure out that active sites in external surface pockets of MCM-22 are unselective and their deactivation is necessary to achieve high p-xylene selectivity. We also show that the nature of coke species in supercages greatly influences catalyst performance through a unique pathway that is referred to as spatiotemporal coke coupling. I have also synthesized and tested many other medium pore zeolite (10-member ring pore size) such as ZSM-23, TNU-10, ZSM-5, IM-5 and TNU-9. Our findings reveal that ZSM-23 and TNU-10 deactivate rapidly due to high diffusion limitations and a propensity to form coke within large pores. Alternative structures, ZSM-5, IM-5 and TNU-9, show moderately higher catalyst stability, but experience significant coke build-up in channel intersections. While MCM-22 have sinusoidal channels without any large spaces for coke build-up, makes it an exceptional catalyst for the TAM reaction.

I have also found an interesting problem regarding TAM reaction in literature. For this reaction, the overall selectivity of p-xylene is a function of xylene isomerization and intrinsic selectivity from alkylation reaction, however; the individual contributions of these two factors are unknown. Researchers have looked into the intrinsic selectivity using density function theory over ZSM-5, but are always limited to only one crystallographically unique tetrahedral site (T-site) even though ZSM-5 has 12 of them.³ From my experimental work, I knew that looking at a single T-site is not enough to characterize the whole ZSM-5 so I used density functional theory calculation and micro-kinetic modelling to study the reaction mechanisms and kinetic contributions of these mechanisms to overall intrinsic p-xylene selectivity and alkylation rates over four different T-sites present in topologically different locations. This enables us to better characterize the kinetic behavior of ZSM-5 for TAM reaction showing large heterogeneity that exist in a single zeolite framework.

In this study, I get the chance to learn about zeolite synthesis, characterization, catalytic testing, coke-analysis and perform density functional theory calculations and recognize how we can use these tools to better understand the catalytic behavior of zeolites which can help me to identify the optimum catalysts and conditions for different catalytic applications.

Unexpected Roles of Heteroatoms in Zeolite Synthesis

The incorporation of heteroatoms has opened up various new and exciting avenues for the typical (alumino)silicate-based zeolite chemistry. Their role in zeolite synthesis ranges from adjusting their acidity by isomorphous substitution of atoms like B, P, Fe, Sn and Ti, to the structure direction during their synthesis. Heteroatoms in zeolites have also led to the development of exciting bi-functional catalysts where heteroatoms typically acts as primary catalysts while zeolites provide Brønsted acid sites and shape selectivity. Here, I present two cases where incorporation of heteroatoms in zeolite synthesis leads to significant improvements in their physio-chemical properties in unexpected ways.

The use of germanium have been limited to the synthesis of germanosilicates which in turn are used in assembly, disassembly, organization and reassembly process to synthesize new zeolite frameworks. Here, I was able to develop a facile and generalizable strategy to synthesize nano-sized zeolites using germanium oxide. Germanium oxide acts as zeolite growth modifier and reduces the size (<100 nm) and/or changes the morphology of MEL, MOR and MFI zeolites (figure c, d and e, respectively). This leads to the significant improvement in their diffusion properties and hence, their performance in different catalytic applications like methanol-to-hydrocarbon (MTH) and cumene cracking reactions. The synthesis of nano-sized zeolite is not trivial and often involves the complicated synthesis procedures, use of expensive organics and/or results in low yields. This strategy of using GeO₂ as growth modifier provides solution to all these problems and provide a simple and practical way to synthesize nano-sized zeolites.

The synthesis of high silica Faujasite (FAU) has been a challenge via organic-free route. HOU-3 remains the FAU with highest silicon-to-aluminum ratio (SAR = 3), synthesized with this route. I was able to figure out a way to synthesize high silica FAU with SAR=3.5, just by the addition of zinc oxide in synthesis gel. This leads to increase in the hydrothermal stability of FAU and provide extra Lewis acid sites due to the presence of small clusters of ZnO in the material. The ZnO-assisted high silica FAU acts as bi-functional catalyst which showed high activity and selectivity towards glycerol carbonate formation from glycerol carbonylation with urea.

Overall, these two examples show how incorporation of heteroatoms can lead to significant improvements in zeolite properties and hopefully will encourage researchers to further explore this aspect of zeolite synthesis. While working on these projects, I get to know the complexities associated with the zeolite synthesis and learn how different parameters involved such as silica source, alumina source, temperature, growth modifiers etc., can impact the zeolite synthesis and was able to use that knowledge to rationally design zeolite with improved properties.

References

1. Vermeiren, K., & Gilson, J. P. (2009). Impact of zeolites on the petroleum and petrochemical industry. Topics in Catalysis, 52, 1131-1161.
2. Parmar, D., Grabow, L.C., Rimer, J. D., et al. (Submitted). Spatiotemporal coke coupling enhances para-xylene selectivity in highly stable MCM-22 catalysts.
3. Wen, Z., Zhu, X., et al. (2016). Methylation of toluene with methanol over HZSM-5: A periodic density functional theory investigation. Chinese Journal of Catalysis, 37, 1882-1890.