Chapter 2
The Emergence of Shape-Selective Catalysis: Adventure, Basic Science, and Technology
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Paul B. Weisz
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Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104-6392, and Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802
Many publications, patents and books now exist that deal with shape-selective zeolite catalysis. This is an account of the adventure of its evolution. It recalls the basic interests that drove the exploration, the steps taken and the bridges crossed on the pathways from science to technology. It will also remind us of human and social elements that interplay, as always, in the evolution of any major technological innovation. This historical review serves furthermore to remind us of basic scientific elements that became recognized or evolved. It may stimulate thoughts toward more science and other technologies that may be sprungfromthese roots.
Scientific research generally explores vertically deeper down into one's specialty. The rarer form connects elements horizontally from different columns of experience and specialized knowledge. When such combination is made of several such basic elements, we have the makings for innovation and - if effectively developed - for the growth of new technologies for society. A past example is the transistor. It sprung from a seed in basic solid state physics at the Bell Telephone Laboratories. It led to the exponential growth of a multitude of new technologies for society. It began with smaller radios, but now embraces nearly all-human experience from information technology to space travel. There is an analogy here with the emergence of zeolite catalysis and zeolite technology. It grew exponentially from a laboratory experiment to embrace the major chemical industries world-wide, the energy supply system, the raw materials for polymers andfibers,the automobiles, the clothes we wear, the construction materials we use, and more. Molecular structure selectivity was the basic focus of the search and discovery and success surrounding shape-selective catalysis.
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Current address: Foxdale Village A - l , 500 East Marylyn Avenue, State College, PA 16801; E-mail:
[email protected] 18
© 2000 American Chemical Society
In Shape-Selective Catalysis; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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The Personal Adventure At age fifteen I passed a bookstore on the way to school. Two titles in the window mystified me. One was "Indigo". I learned much later about that early brilliant dye, a natural product importedfromAsia, and about the time, art and skill it took for man to duplicate its molecular structure which nature's enzymes could tirelessly and continuously duplicate in Indigofera tinctora. The other title was "Catalysis". That, my teacher told me, is a substance that would cause transformations of chemical structures to proceed by "just being there". I was even more mystified. After an earlier career as a physicist, I joined what is now Mobil Corporation. I wasfreeto browse the tasks and the problems of the industry. In retrospect this was a most important circumstance, rarely duplicated in industry. I soon recognized that catalysis was clearly a major and key involvement. Moreover, I was impressed by the tremendous number of chemical species in petroleum oil, and the very limited "selectivity" that the man-made catalysts were able to exercise by "just being there". Why, even a single hydrocarbon component exposed to the catalyst would produce a plethora of products. What a contrast to nature's enzymes, the kind that could make indigo with precision! Our working environment was that of acid catalysis on porous solids. You needed hundreds of square meters per gram of surface area on the particles. Crystals, even as small as 1 micron particle size could offer only some 3 m of surface area per gram. Decades of research had evolved some porous clays and finally synthetic silica/alumina amorphous solids of sufficient porosity to provide sufficient surface area and acidity to operate catalytic cracking operations. In the 1950's, synthetic zeolites 4A and 5A became availablefromthe Linde Division of Union Carbide, for absorbing moisture. They also sorbed certain light gases, and did so reversibly. Could we build catalytic activity inside of these particles and then have catalysis, and have it specific to only those certain gases? Zeolites are crystals! And 4A and 5A are salts of Na (4A) and Ca (5A), not acids. The usual methods of generating acidity, using acids or ammonium salts with subsequent thermal decomposition to exchange the cations on the solids for protons proved to destroy the zeolites. Rather than quit outright we tried catalysis on the "salts" as is, anyway. We obtained observable catalytic activity on 5A, the Ca-zeolite (1,2,3): At 260°C it dehydrated 60 % of 1-butanol. Moreover, it, dramatically, did not convert iso-butanol or a secondary alcohol. It even cracked n-hexane but did not crack 3methylhexane, and the activity was actually not much lower than that of a then prevailing industrial catalyst ( Table la). Not only did we have charge selectivity but we clearly had molecular shape selectivity among the emanating products (Table I b). Conventional catalytic cracking of paraffins always yields a great deal more isobutane then η-butane. On cracking hexane, we saw no isobutane (2). It left us with excitement over witnessing the extraordinarily selective principle at work in our hands. But, even before resolving the question as to the possible usefulness of converting only linear molecules with extraordinary selectivity, we had to admit to having rather limited activity, thermal and acid stability of this zeolite. Two directions for progress were indicated: 1) exploring the potential for non-acid catalytic modifications of the zeolite, and 2) searching for other zeolites that may be more acid stable. We undertook both. 2
In Shape-Selective Catalysis; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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20 As to exploration of non-acidic catalysis on zeolites, could we include catalytic metal "sites" within the zeolites? Platinum ions were too large to penetrate Azeolite channels. But we succeeded to incorporate some tenths of percents of platinum by including catiomc platinum complex to the normal crystallization ingredients to create the Α-zeolite. The catalyst did hydrogenate 1-butene while leaving isobutene entirely unconverted (1X3) from a mixture of both (Table lia)!. Similarly we could hydrogenate linear pentadienes in admixture with isoprene (3,4) (Table lib). Perhaps even more dramatically, this catalyst would catalytically burn η-butane or n-butene in admixture with air to carbon dioxide cleanlyfroma mixture with iso-butane, leaving the latter entirely untouched (3) (Table He)! Looking beyond the one synthetic Α-zeolite, there was always nature, but natural zeolites were rare minerals then. My partner in exploration Vincent Frilette and I began a search for zeolite mineral samples and testing their capability to perform acid catalysis. The most exciting discoveries occurred with two particular samples: mordenite pebblesfromthe beaches of Newfoundland that Vince located, and a sample of gmelinite which I purchased (along with desmine, epistilbite and others) in a mineral store near the Sorbonne, on the west-bank of Paris. With these we obtained the highest acid catalytic activity we had ever seen, corresponding to rate constants near 10,000 times those of conventional silica/alumina cracking catalysts. We pronounced them "superactive" catalysts (5,6). In this process of exploration of natural zeolites, we learned that extraordinary acid and thermal stability could be had if the zeolite had a silicon-to-aluminum atom ratio of at least 2.0, preferably 3.0 or larger. Another remarkable case of shape-selectivity occurred with mordenite. My early and long time co-explorer N.Y. Chen was able to introduce platinum by removing its sodium by conversion to H-mordenite, then introducing the Pt-ion and thereafter reloading sodium. To assure non-availability of Pt-sites to large molecules, "external" Pt was poisoned using a catalytic poison of large molecular size (7). Remarkably, this catalyst would hydrogenate ethylene but not propylene, in spite of its ability to sorb both (Table III). We learned that this "engineered" zeolite structure was unable to emit propane when or if it was produced; propane has a slightly wider effective diameter than propylene, inasmuch as propylene has one stiff double bond! In 1966, the Encyclopedia of Chemistry (Reinhold Publ. Co.) decided to add a new entry "Catalysis, Shape-Selective" to their new edition (8). It was noteworthy from a historical perspective that in the Encyclopedia this now followed the entry "Catalysis" which ended with "...enzymes are part of such large molecules that they can probably best be classified as heterogeneous catalyst. They are characterized by an astonishing specificity...,". It added drama to the stepfromenzyme to man-made catalyst for catalytic selectivity. The Path toward Technology. We had shape-selective catalysis, a new capability at a very basic level. But where could this catalysis, selective to linear molecules, fit into our society, anyway? Furthermore, if such applications were identified, where would we find or produce such zeolite material, with thermal and acid stability and in sufficient quantity for industrial operation? And must we always be limited to selectivity toward strictly linear molecular structures? Any further progress clearly required a multidimensional effort.
In Shape-Selective Catalysis; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
21 Table I. Shape selective paraffins cracking over Ca-A zeolite fas is) n-Hexane and 3-me-pentane, 500°C, 1 atm, t = 7 sec (a) Activity:
% conversion on Ca-A on silica/alumina n-hexane 3-me-pentane
9.2
