SCIENCE & TECHNOLOGY LOOK AT THIS! Tsapatsis (from left), postdoc Shubhajit Ghosh, and Ph.D. student Jungkyu Choi examine the porous structure of MFI zeolite.
SOWING THE SEEDS OF ORIENTED FILMS Technique allows systematic manipulation of pore orientation in molecular sieve thin films MICHAEL FREEMANTLE, C&EN LONDON
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ICROMETER-THIN MOLECULAR
sievefilmsare exciting considerable research interest as potential materials for chemosensors, catalysts, microelectronic devices, and separation membranes. Before such applications can be developed, however, a number of scientific and technical hurdles need to be overcome. "Zeolite films in particular hold promise for practical applications, but their fabrication imposes challenges, including control of thickness, grain size, and pore orientation," says Michael Tsapatsis, professor of chemical engineering and materials science at the University of Minnesota, Twin Cities. Zeolites are naturally occurring or synthetic aluminosilicate materials with porous framework structures that can host water and a variety of molecules and cations. They are used as hosts for catalysts in industrial processes and also for water purification
and softening, laundry detergents, soil treatment, and the preparation of medical-grade oxygen. Over the past decade, Tsapatsis and coworkers have been investigating the development and use of molecular sieve materials, including synthetic zeolites with 10-membered aluminosilicate rings, variously known as MFI, ZSM-5, or silicalite-1. Pellets of microcrystalline MFI powder are widely used for catalytic processes in the chemical and petrochemical industries. MFI films are also being developed as membranes for gas separation (C&EN, Oct. 3,2005, page 49). "We are particularly interested in preparing M F I films that can be used as membranes because the size of MFI pores is close to that of many industrially important molecules," Tsapatsis says. Zeolite membranes are invariably prepared as composite materials, mainly with porous supports of stainless steel or alumina.
A key problem is that the composite is not reproducible, observes Joaquin Coronas, an expert on MFI gas-separation membranes who is associate professor of chemical engineering at the University of Zaragoza, Spain. "For instance, one can find MFI-type zeolite membranes with not only different values of permeability and selectivity but also with different qualitative behaviors. Some silicalite membranes can separate butane isomers at high temperatures, while others only do so at low temperatures. A few very high quality membranes can separate xylene isomers, but some of these cannot separate butane isomers." The discrepancies in performance arise principally from variations in the porosity and chemical composition of the support, Coronas says. "Quite often, the zeolite is synthesized inside the support pores, and so it is not possible to know the thickness of the membrane," he explains. "In some cases, the zeolite precursor gel attacks the support, and undesirable reactants are incorporated into the zeolite phase." The Tsapatsis group has been working on methods for systematically controlling the pore orientation of continuous MFI zeolite films. "The performance of a zeolite membrane depends on its microstructure," Tsapatsis says. "The pore structure of MFI is anisotropic. We have shown that the membrane performance depends strongly on the preferred crystallographic orientation, so it is important to control the way the crystals, and therefore the pores, are oriented in the membrane." THE DIRECTIONS of channels or pores in MFI zeolite crystals relate to three mutually perpendicular crystallographic axes: a, b, and c. Pores in the a-direction are arranged in a zigzag pattern and interconnected with straight pores in the b-direction. Pores in the c-direction follow a "tortuous path," Tsapatsis observes. Films that consist of crystals with their b-crystal axis perpendicular to the support surface are said to be b-oriented. "Since MFI-type zeolite has an anisotropic structure with straight pores along the b-axis, b-oriented membranes should provide the fastest permeation pathways," Coronas
"Zeolite films in particular hold promise for practical applications, but their fabrication imposes challenges/' WWW.CEN-0NLINE.ORG
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SCIENCE & TECHNOLOGY explains. "They should be faster than a-oriented ones which, in turn, should be faster than c-oriented membranes." MFI films that are b-oriented perform best in the separation of xylene isomers and in other aromatic separations, according to Tsapatsis. "In the b-oriented MFI structure, straight pores with a diameter of about 0.55 nm run along the b-crystallographic axis," he says. "That means that the molecules to be separated by the boriented membrane travel through the straight pores. Films with the c-orientation perform better for separating butane isomers and other linear/branched hydrocarbon separations." The orientation of MFI pores depends on how the films are prepared. In the so-called in situ method of synthesis, zeolite crystals nucleate on the support surface that is directly in contact with an alkaline solution containing the zeolite precursors. However, the success of in situ methods in yielding uniformly oriented MFI films is limited, according to the Minnesota team. An alternative synthesis method, known as the secondary (or seeded) growth technique, decouples zeolite nucleation from zeolite growth by depositing a seed layer of zeolite crystals on the support surface. The technique enables one of the crystallographic directions to be oriented perpendicular to the support. "We were the first academic group that, simultaneously with the ExxonMobil zeolite membrane group, reported in 1996 on the synthesis of zeolite membranes using secondary growth," Tsapatsis says. One type of film they synthesized was c-oriented MFI films. "We used randomly oriented seed layers that were deposited by dip-coating the porous alumina substrate in a colloidal suspension of 100-nm MFI crystals." The technique relies on the use of structure-directing agents in the growth solution. The agents become incorporated at the pore intersections of the crystals in the films during growth. The structure-directing properties of the agents are thought to arise from their geometric shape, size, and charge distribution. The Tsapatsis group showed that secondary growth from a seed layer in a growth solution made from tetraethyl orthosilicate (TEOS), water, and Ul
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tetrapropylammonium (TPA) hydroxide as a structure-directing agent leads to the development of films as thin as 1 Jim that can be used as membranes. TPA
TPA trimer
i
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TPA
ported that seeds that are b-oriented rather than randomly oriented can be used to prepare b-oriented zeolite membranes on porous alumina supports {Science 2003, w 300, 456). The seeds are formed by 5 using TPA as a structure-directing g agent and a trimer of TPA as the ^ agent for secondary growth from | the seed layer. o The group also demonstrated £ that the alumina-supported b-orio ented membranes can be used to separate xylene isomers with high flux—that is, low resistance to gas or vapor flow—and high selectivity. Last year, the Minnesota group reported that the same technique can be extended to grow b-oriented MFI films on rough, porous stainless steel supports precoated with a layer of mesoporous silica (Jnd. Eng. Chem. Res. 2 0 0 5 , 44, 9086). That work was carried out in collaboration with chemistry professor Wilhelm Schwieger's group at the University of Erlangen-Nuremberg, in Germany.
"THE ABILITY to synthesize thin, oriented zeolite films is a key step in moving this technology forward," comments chemical engineer Richard D. Noble of the University of Colorado. "There are several steps described in this paper that need to be considered, such as the initial state of the support layer and the SEEDED GROWTH The orientation of the seed layer as well tetrapropylammonium as its attachment to the surface. This (TPA) cation is used to = TPA paper also raises an important point prepare zeolite seeds of the mass-transfer resistance of with straight pores the support." perpendicular to the surface of an alumina According to Tsapatsis, to make porous support. TPA a practical membrane, it is necessary trimer is used to grow to grow the selective molecular sieve the b-oriented film layer on a high-flux porous support. (bottom left) from the "Commercially available porous seeds. The a-oriented stainless steel membranes are such film (bottom right), supports," he says. "Stainless steel with zigzag pores as a support material for zeolite = TPA trimer perpendicular to the membranes possesses the imporsurface, employs TPA tant advantage of ductility, and it is trimer for seeding compatible with the most commonly and TPA monomer for used plant equipment parts. growth. "Sealing at high temperatures is less problematic than with ceramics, and its better heat conductivity can be advantageous for membrane reactor ap"We call these the first generation of plications involving heat transfer to or from oriented films," Tsapatsis says. "Their orithe reaction region," he adds. "However, entation is not uniform throughout the film to date, highry/>-xylene-selective, stainless thickness, but it gradually develops as the steel supported MFI membranes have not film becomes thicker." been reported. The b-oriented MFI films In 2003, Tsapatsis and coworkers reWWW.CEN-0NLINE.ORG
we prepared are the best available for xylene isomer separations, and we currendy deposit them on stainless steel tubular supports that are appropriate for industrial use." In a recent paper, the Minnesota group and Zhiping Lai, an assistant professor of chemical engineering at Nanyang Technological University, in Singapore, showed that uniformly a-oriented MFI zeolite films can be prepared by seeded growth (Angew. Chem. Int. Ed. 2006,45,1154). To grow the films, the group uses an alkaline TEOS solution and a trimer of TPA as the structure-directing agent for seed formation. The support is a layer of mesoporous silica on the polished side of a porous alumina disk. Films are grown from the seed layer with the TPA monomer as the structure-directing agent. "Until now, no reports existed on the synthesis of uniformly a-oriented MFI films," Tsapatsis says. "We cannot predict how these films are going to perform as membranes. However, now they are available for the first time, and based on our previous findings, we expect that their behavior will be different from existing membranes." T h e Minnesota researchers plan to systematically test and optimize the performance of the a-oriented films and then determine potential applications for the IVAN AMATO, C&EN WASHINGTON membranes. They intend, for example, to investigate their use for the separation of ATE LAST MONTH, THE UNIVERSITY xylene isomers, saturated and unsaturated of Zurich hosted a symposium in hydrocarbons, and linear and branched celebration of the 100th birthday hydrocarbons. ofone ofits most illustrious alumni, Albert Hofmann. More than 60 "The Tsapatsis group may claim to be the years ago, on a spring day while World War first to establish absolute control of the crysII raged in Europe and the Pacific, Hofmann tallographic orientation of MFI-type zeolite synthesized perhaps the most mind-bending membranes," Coronas says. "Their seeded chemical of all time: the psychomimetic growth strategy allows one to prepare thin zeolite membranes in a reproducible way agent LSD, formally known as D-lysergic acid diethylamide. Hofmann got some of that can potentially afford the expansion his first inklings of the drug's powers while of zeolite membranes to many interesting riding his bike home from his laboratory at separation and reaction applications." the pharmaceutical giant Sandoz in Basel, Tsapatsis points out that several recent Switzerland. developments by other groups have contributed to this level of control. "Our work Hofmann, born on Jan. 11,1906, first benefited greatly from recent progress in synthesized LSD in 1938 as part of a systemthin mesoporous films and zeolite nanoparatic investigation of dozens of derivatives of ticle synthesis and extensively uses recently alkaloids made by ergot, a poisonous funintroduced particle monolayer deposition gus that infects grains and grasses. Other techniques," he says. ergot alkaloid derivatives from Hofmann's research program had shown strong effects The a- and b-oriented films, synthesized on blood pressure and vasodilation, and from oriented seeds, belong to what the these led to clinically effective medicines for Minnesota group calls the second generatreating migraines and postchildbirth bleedtion of oriented MFI films. The researching in mothers. Initial tests of lysergic acid ers are currently comparing the permeation diethylamide, then designated as LSD-25, properties of the a- and b-oriented films. showed nothing of pharmacological interThe aim, they note, is to provide "the first est, so the compound was shelved. set of data for gas and vapor permeation through zeolite membranes of a given strucNot for good, as it turns out. Five years ture type with drastically different preferred later, Hofmann, still fascinated by LSDorientations." • 25's chemical structure, decided to revisit
TRIP OF A CENTURY
Albert Hofmann, inventor of the mind-altering drug LSD, celebrates his 100th birthday
L
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the compound. On April 16,1943, while synthesizing a batch no larger than a few hundredths of a gram, he unintentionally experienced humanity's first LSD trip. "I was seized by a peculiar sensation of vertigo and restlessness," he later wrote in a memoir titled "LSD-My Problem Child." "Objects as well as the shape of my associates in the laboratory appeared to undergo optical changes," he recounted. "With my eyes closed, fantastic pictures of extraordinary plasticity and intensive colour seemed to surge towards me. After two hours, this state gradually subsided, and I was able to eat dinner with a good appetite." To follow up his suspicion that this fantastic cognitive experience was caused by accidental ingestion of the LSD-25 he had made, a few days later Hofmann deliberately took 150 jig of the substance. It was a minuscule amount that he assumed would have only subtle but sufficient effects to confirm his hypothesis. W h a t followed was the world's first bad LSD trip, an all-day psychopharmacological ordeal that included frightening hallucinations, physical symptoms such as numb limbs, a metallic taste on the tongue, feelings of suffocation, and outof-body sensations. "I lost all control of time," he recalls in his memoir. "Space and time became more and more disorganized, C & E N / FEBRUARY 2 7 . 2006
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