"Previous attempts at synthesizing materials like PSU-1 involved specially designed templates making the process expensive," says Dr. Sridhar Komarneni, professor of clay mineralogy. "The processes also require stringent conditions for the synthesis to work." Komarneni, working with Dr. Bharat L. Newalkar, postdoctoral fellow in Penn State's Materials Research Institute; Uday T. Turaga, graduate student in the fuel science program and geoenvironmental engineering; and Dr. Hiroaki Katsuki of Saga Ceramics Research Laboratory, Japan, used a hybrid mechanism to synthesize the same product.
"We believe that this approach has the potential to result in new synthetic strategies for tailoring new framework compositions for specific applications in the fields of catalysis, adsorption, and nanotechnology," the researchers reported at the recent American Chemical Society annual meeting in New York and in the Journal of Materials Chemistry.
Silica materials similar to PSU-1 exist and are small particles with nanoscopic pores. Some have hexagonal, close-packed pores. Others are cubic with three-dimensional linkages. These tailored materials, which appear powder like, are usually created by producing a template in the shape of the required pore. The silica forms around the template, which is then removed either with organic solvents or by heating until the template material calcines.
PSU-1 has a more complex pore structure than cubic or hexagonal. The pore, referred to as a cage, has a central large hollow area with smaller tubes connecting the central pore spaces. Manufacturing a template to create this structure is possible, but expensive and time-consuming.
"We prepared two gels and two templates and mixed them together to see what kind of material might come up with this hybrid template," says Komarneni. "We were surprised to get a really new structure, not like the two starting structures."
The two sets of templates and gels mixed together – one forms large pores and one forms small pores – created the cage-like structure. Altering the size of the templates alters the sizes of the pores, which have sizes of 4.6 and 5.4 nanometers, while the powders are 30 to 40 micrometers in diameter.
The researchers add another twist by using microwaves to synthesize the material in liquid. Microwaving takes a much shorter time than conventional heating techniques, creates a more stable material and the 30 to 40 micrometer particles are much bigger than the previously produced 1 to 2 micrometer particles.
"We can tell it is a cage with passageway structure because very small molecules will block the flow through the particles and that will not happen in the hexagonally arranged pores of a silica particle," says Komarneni. "What we do not know is how many tubes branch off from each central cage."
The National Science Foundation-supported Penn State Materials Research Science and Engineering Center supported this work.
Journal of Materials Chemistry