Enabling New Reaction Pathways with Tailored Molecular Sieve Catalysts
Molecular sieves, such as zeolites and their analogues, are quintessential catalysts and support materials for a diverse range of applications due to their unique size exclusion properties that, when coupled with catalytically active sites, can facilitate innumerable chemical transformations with enhanced selectivity. The tunability of both the physicochemical properties and the macroscopic functionality of molecular sieve catalysts enables them, in principle, to be tailored to specific reactions by controlling the confining geometry and local chemical environment surrounding an active site.
Our goal is to develop precisely controlled methods for creating new molecular sieve catalysts with specific crystal structures and predictable functions that can be tuned to open highly selective pathways in challenging gas- and liquid-phase systems, such as alkane dehydrogenation and aldol condensation reactions pertinent to transforming shale- and biomass-derived feedstocks into valuable chemicals and fuels. Achieving these objectives requires new approaches to:
- Identify relevant chemical descriptors that govern the catalytic behavior of these microporous solids
- Understand mechanisms that regulate self-assembly processes during synthesis of molecular sieves
- Develop protocols to chemically control molecular sieve formation with structural specificity
Evaluating and Tuning Molecular Sieve Performance via Catalytic Functionalization of Formaldehyde
The formose reaction is a complex sequence of autocatalytic condensation, isomerization, and oligomerization steps found in prebiotic chemistry to produce sugars from formaldehyde (CH2O) via C-C bond coupling as shown below.
This liquid-phase, aldol-based reaction network is an ideal system for evaluating and tuning the physicochemical properties of molecular sieve catalysts due to the plethora of unstable oxygenates formed via unselective base-catalyzed homogeneous routes as well as the sensitivity of alkanal C, H, and O functionalities to changes in catalyst composition and surrounding environments. The formose reaction may also have commercial applicability as a synthetic route for integrating natural gas and renewable feedstocks, such as biomass-derived glycolaldehyde (GLA), to produce a diverse range of useful oxygenated intermediates as shown in the scheme below (blue path). Furthermore, enabling selective production of simple sugars from CH2O would open new research areas in molecular sieve catalysis, such as chiral assembly of small molecules and synthesis of edible sugars (e.g., D-glucose) from CO2 and H2 during extended missions in space and on Mars (red path).
In this work, we examine how systematic control over molecular sieve framework topology, active site electronic and geometric structure, and solvating properties of intra-crystalline void spaces preferentially opens new pathways for C-C and C-O bond formation from CH2O in a formose-based synthesis approach. These insights will lead to improved reaction selectivity for oxygenated intermediates and simple carbohydrates from CH2O as well as new concepts in molecular sieve design, such as methods to facilitate chiral assembly of small molecules.
Synthesis of Microporous Molecular Sieves by Design
The design and synthesis of tailor-made molecular sieves with prescribed structures and improved function requires a fundamental understanding of how various solution conditions influence the self-assembly of inorganic-organic aggregates during nucleation and growth processes and the degree to which these conditions can be controlled to form the desired product. Since these materials are metastable phases relative to quartz, the hydrothermal synthesis of molecular sieves is a kinetically controlled process, implying that structures of interest may be crystallized under conditions where the energy barrier for transformation to a more thermodynamically stable, undesired structure is high. Recent advancements in computational screening techniques combined with the development of de novo design algorithms have led to the prediction of hundreds of thousands of thermodynamically viable molecular sieve frameworks. However, even though a diverse range of hypothetical structures have been proposed, a controlled synthesis approach to selectively produce a new target framework of interest has not been demonstrated.
We investigate the structure-direction and self-assembly processes during synthesis of molecular sieves by using a combination of spectroscopic techniques and kinetic modeling to determine how control over solution conditions regulates structural specificity. These findings, along with a priori prediction of new, energetically favorable organic structure directing agents (OSDAs), will be used to selectively produce target molecular sieves; the beginning elements of such a methodology have been successfully demonstrated in academia by the Davis group at Caltech in combination with the Deem group at Rice University. We aim to build on these early successes and further develop this approach.
Tandem Catalytic Pathways for Light Hydrocarbon Upgrading
One strategy for opening new pathways and improving product yields from non-selective reactions beyond what can be achieved in a single-step process is to use a tandem catalytic approach that combines two or more mechanistically distinct catalytic cycles within a single reactor. The tandem catalysis methodology enables cooperative interaction and thermodynamic leveraging between individual reaction sequences to afford an overall process that is both efficient and energetically favorable. A key advantage to this approach is that the individual catalyst functions for a given reaction can be optimized separately and then coupled together to achieve the desired, sequential transformations. However, a major obstacle in the development of such systems is that the individual mechanistic cycles are either incompatible or operate most efficiently in vastly different kinetic regimes.
To investigate the potential of this approach for tuning light hydrocarbon transformations, we use the CO2-assisted dehydrogenation of propane (PDH) as a test reaction to examine how endothermic, structure-insensitive alkane dehydrogenation and exothermic, structure-sensitive CO2 methanation pathways can be combined to enhance alkene yields and improve catalyst stability. The combination of alkane dehydrogenation and CO2 methanation is an attractive system to examine thermodynamic leveraging via tandem catalytic pathways since both reactions are non-interfering and can be performed under similar operating environments. Furthermore, the ability to exploit surface structural features that preferentially control C-H or C-O bond activation during these two functionally distinct, yet simultaneously occurring catalytic cycles, promises not only improved kinetic performance for alkane dehydrogenation, but also new strategies that can be applied to other systems of interest.