Catalytic materials provide the Davis group with a way to influence macroscopic physical behavior by studying and controlling atomic-scale properties.

Zeolites are ideal catalytic materials for this research because they have nanostructure on the same scale as the molecules that react within their pores. Their nanostructure is a function of the conditions under whch they are created, and because of the molecular scale of the catalytic surfaces, their macroscopic properties are thus a function of those same conditions.

Zeolites are microporous crystalline oxides with a high surface to volume ratio. Strictly speaking, they are aluminosilicates of form Mx*(Si1-x+Alx)O2*yH2O, but the Davis group is also interested in many zeolite-like materials that are also microporous and have important catalytic properties, such as aluminophosphates, zincosilicates, and titanosilicates.

Though the catalytic properties of zeolites are of central interest to the Davis group, they are primarily used (by mass) as adsorbents and ion-exchange materials. They were first used for organic synthesis in the 1960s for catalytic cracking of petroleum. ZSM-5, first synthesized in 1967, was applied to numerous refining and petrochemical processes with great success. Here are some simple examples to illustrate why zeolites' nanostructure affects a bulk property like catalysis:

How are zeolites made?

Zeolites are actually natural products. Not all naturally occuring zeolites have been recreated in the laboratory, and conversely, many zeolites have been synthesized that do not occur in nature.

Making zeolites is a delicate art. Typically, the experimenter will combine non-molecular building blocks like alumina and silica with some alkali metal hydroxide and water, then incubate the mixture in an enclosed environment at 50-200 C for a certain period of time. The equivalent to a flattened souffle for zeolite synthesis is an amorphous mess. But if all goes well, x-ray diffraction experiments will reveal a crystalline structure. Organic chemicals called Structure Directing Agents (SDAs) are often added to steer the self-assembly of the zeolite in a desired direction.

Zeolite synthesis is not similar to synthetic organic chemistry. The zeolite is assembled through a series of spontaneous molecular recognition events that cannot be understood in the same dot-and-arrow schema that synthetic organic schemes are constructed from. In fact, this process is not very well understood at all.

The final structure is a product of synthetic conditions and the post-synthetic treatment. One of the most interesting factors the experimenter controls is the hydrophobicity and structure of the SDA. Though this relationship is not facile, several lines of research in the Davis group have turned up explanations and successful predictions of final nanostructure as a function of SDA choice.

View scanning electron micrographs of zeolites synthesized by the Davis group

You can read more about zeolites and other ordered porous materials in recent reviews:

Corma A; Davis, ME; (2004) ChemPhysChem 5 304-313.

Davis, ME; (2002) Nature 417 813-821.

Before the synthesis of VPI-5, the largest zeolites had pore sizes below 10 angstroms. The pores of the zeolites contained 12 oxygen atoms or fewer. In 1987, the Davis group synthesized the aluminophosphate VPI-5, an 18-ring microporous material with a 13 angstrom pore diameter. This was the first of a series of extra large pore zeolite-like materials.

In 1997 at the California Institute of Technology, the Davis group synthesized CIT-5, an extra-large pore 14-ring zeolite with a pore diameter of just over 10 angstroms--the second extra-large pore zeolite discovered. This represented an important development in the field because CIT-5 has the high thermal stability of a high-silica zeolite, but large pore size like VPI-5 and its successors. And unlike UTD-1, the first extra-large pore zeolite, CIT-5 does not require an organometallic SDA and the lengthy post-synthetic workup that follows. CIT-5 is structure directed by N(16)-methylsparteinium with lithium cations:

The following nanostructure results:

Why are extra-large pore zeolites important?

The range of reactions over which a zeolite can apply shape selective pressure is a function of the zeolites pore size and structure. Zeolites with pore sizes between ten and twenty angstroms would impact fine chemicals, pharmecuticals, and life sciences.

Davis, ME; Saldarriaga, C; Montes, C; Garces, J; Crowder, C; (1988) Nature 331, 698.

Freyhardt, CC; Tsapatsis, M; Lobo, RF; Balkus, KJ; Davis, ME; (1996) Nature 381, 295.

Wagner, P; Yoshikawa, M; Lovallo, M; Tsuji, K; Taspatsis, M; Davis, ME; (1997) Chem. Commun. 2179.

Nery JG ; Hwang SJ; Davis, ME; (2002) Micro. Meso. Mat. 52 19-28.

Ogino, I ; Davis, ME; (2004) Micro. Meso. Mat. 67 67-78.

Current methods for zeolite synthesis use organic molecules as their SDAs. Once the synthesis is complete a high temperature combustion step is required to remove these SDAs from the zeolite framework. This process both destroys the SDA, a high cost material, and the energy release and the water produced in the combustion can damage the inorganic structure.

In 2003, the Davis group reported a methodology for the synthesis of zeolite ZSM-5 using a SDA with an attached cyclic ketal group that can be fragmented inside the zeolite. The fragmentation allows the parts of the SDA to be removed without a high combustion step in the treatment. Also, once removed the fragments can be recombined to form the original SDA, hence the “recyclable SDA”. The concept of the methodology is shown here.

In the diagram above the methodology follows 5 basic steps:

• Assemble an SDA from at least two components using covalent bonds or non-covalent interactions that will survive the zeolite formation conditions (e.g. high pH).
• Form the zeolite, thus trapping the SDA inside the framework.
• Cleave the SDA components in ways as to not destroy the framework. (e.g. a low pH wash)
• Remove all the fragments.
• Reassemble the SDA.

As noted the “recyclable SDA” methodology provides a way to produce zeolite with out destroying the high cost SDA. Also removing the combustion step is beneficial for forming various zeolite products such as molecular sieve membranes that can be damaged by the mechanical stresses of thermal expansion or low dielectric components that need air to fill the microporous space to achieve the desired properties.

The Davis group is currently using the methodology to prepare larger SDAs and testing it with the synthesis of other zeolites.

Lee H ; Zones SI; Davis, ME; (2003) Nature, 425 385-387.

A main goal of catalysis is high selectivity. Based on observations from soluble and zeolite-based catalysts, high selectivity can be correlated to structural uniformity. In order to investigate the structure-property relationship inorganic-organic hybrid materials can be used. The inorganic material can be used to provide a support with surface area and porosity to which the organic functional groups are attached as active catalytic sites. Some of the areas of interest included studying the effects of cooperation of multiple organic functional groups and the use of multiple organic group types to perform multistep reactions.

The Davis group reported in 2003 on the synthesis and characterization of organic-fuctionalized, mesoporous silicas. In the work, the mesoporous solid SBA-15 was used onto which thiol and sulfonate esters were organized. These organic groups where then converted to sulfonic acids to form the catalyst for the condensation reaction between acetone and phenol to produce bisphenol A. The production of this catalyst required first developing methodologies for creating silica containing the organic functional groups that are desired and none that are not. This is accomplished using reversible blocking reagents and proper choice of precursors to protect the groups and produce the desired ordered sites. The precursors where then reacted with SBA-15 to produce the inorganic-organic hybrid material which was then modified to give the active sites need for the reaction.

The mesoporous catalyst with organized acid sites was used to perform the condensation reaction between phenol and acetone to give biphenol A. The organized acid sites produced a reaction rate that was higher than observed from homogeneous catalysts run at the same conditions thus revealing that the solid catalyst with the organized sites shows some type of cooperativity. As a control, solid catalysts without the organization gave reaction rates lower than those observed from the homogeneous catalysts and is the normal result. Work is ongoing to understand the origin of the enhanced rate of reaction and to extend these findings to other reactions and solid materials.

The role of thiol in the condensation of phenol and acetone to bisphenol A under sulfonic acid catalysis is being investigated as a followup to the dual sulfonic acid site positioning work.   We have examined the role of thiol in the homogeneous catalysis of bisphenol A thoroughly using a wide variety of sulfonic acid/thiol combinations.  We have identified the importance of thiol in this reaction and have proposed a mechanistic explanation for the involvement of thiol.  Incredible rate enhancements have been observed when thiol and sulfonic acid have been used in combination, and a bifunctional SBA-15 mesoporous material with thiol and sulfonic acid sites has been synthesized and proven to be a remarkable catalyst.   Current efforts are towards carefully positioning neighboring thiol and sulfonic acid sites in a controlled ratio in order to further investigate this effect.

Dufaud, V; Davis, ME (2003) ; JACS 125 9403-9413.