Znd. Eng. Chem. Res. 1991,30, 1675-1683
1675
REVIEWS
Zeolites and Molecular Sieves: Not Just Ordinary Catalysts Mark E. Davis’ Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
The materials properties and applications of zeolites and molecular sieves are reviewed. Emphasis is placed upon the relation between the physicochemical properties and the macroscopic function of zeolites and molecular sieves. Uses in fields other than adsorption and catalysis are outlined, and future areas of interest are discussed. Zeolites and other molecular sieves are finding widespread application in a diversity of areas. Thus, they are becoming of interest to more than just those working in separations and catalytic chemistry. This review is intended for those with little to no background in molecular sieve science. It is hoped that the information provided is sufficient to define zeolites and molecular sieves in such a manner as to allow one to appreciate the unique features of these materials.
Definition of Zeolites In 1756 the Swedish mineralogist A. F. Cronstedt observed that the mineral stilbite gave off steam when heated. This result led him to coin the term zeolite, which is derived from the two Greek words “zeo”, to boil, and “lithos”, stone. Since then, approximately 40 mineral, or natural, zeolites have been discovered. Today, it is known that zeolites are hydrated, crystalline tectoaluminosilicates. Zeolites are constructed from TO4 tetrahedra (T = tetrahedral atom, e.g., Si, Al); each apical oxygen atom is shared with an adjacent tetrahedron. Thus, the framework ratio of O/T is always equal to 2. To understand the physicochemical properties of zeolites, this review begins by describing the crystal chemistry of the framework. From the valency of silicon it follows that silicon atoms generally prefer to form bonds with four neighboring atoms in a tetrahedral geometry. If a Si04 entity could be isolated, its charge would be -4 since silicon is +4 and each oxygen anion carries a -2 charge. However, a SiOl unit in a framework is neutral since an oxygen atom bridges two T atoms and shares electron density with each (see Figure 1). Thus, a defect-free pure SiOz framework will not contain any charge. If aluminum is tetrahedrally coordinated to four oxygen atoms in a framework, the net charge is -1 since aluminum carries a +3 valency. When tetrahedra containing silicon and aluminum are connected to form an aluminosilicate framework, there is a negative charge associated with each aluminum atom and it is balanced by a positive ion (M+ in Figure 1) to give electrical neutrality. Typical cations are alkali metals, e.g., Na+, K+, alkaline earth metals, e.g., Ca2+,Ba2+,and the proton, H+. Tectoaluminosilicates do not in general have ratios Si/A1 l are allowable and are most commonly observed. Figure 2 illustrates an example of a common cage structure, i.e., the sodalite cage, and how it is constructed of silicon, aluminum, and oxygen atoms. Figure 2 shows also how several zeolites are comprised of these sodalite cages. A typical convention is to draw the framework topology by using short straight-line segments. Each line represents a bridging oxygen atom while the intersections locate the T atoms. Notice that the rings that are formed consist of equal numbers of T and oxygen atoms. It is common to name the rings according to the number of T atoms (or oxygen atoms) in the ring. For example, the sodalite cage is comprised of 4- and 6-membered rings. The aperture of the 6-membered ring is near 3 A, so only small molecules such as HzO and NHBcan access the interior of sodalite cages. However, notice that larger rings are formed in structures like zeolite A and faujasite. (The size of these apertures or pores is discussed below.) Figure 3 shows the typical cubic morphology of zeolite A as well as its cubic array of 8-membered rings. Since the aluminosilicate framework is hydrophilic, it adsorbs moisture from ambient air into its voids (e.g., isosteric heat of ad, sorption of water on zeolite A (NaA) is larger than 15 kcal/mol (Breck, 1974)). Thus,it should now be clear that zeolites are hydrated, crystalline aluminosilicates. Many zeolite structures exist. There are natural zeolites, synthetic analogues of natural zeolites, and synthetic zeolites with no natural counterparts. In all, there are approximately 70 structures (Meier and Olson, 1987). Figure 4 illustrates some typical framework projections containing various rings (pores) of different sizes (the aluminofiosphates VPI-5, A1PO4-5 and AlP04-11 are discussed brlow). Notice that the sizes range from approximately 4 to 12 A and that the topologies may contain channels and/or cages. For more complete listings of the various structures see any of the following references: Breck (1974),Barrer (1978),Barrer (1982),Szostak (1989), Meier and Olson (1987). What makes zeolites unique is that their pores are uniform in size (see Figure 5) and that they are in the same size range as small molecules (see Figure 4). Thus, zeolites are molecular sieves (McBain, 1932) since they can discriminate molecules on the basis of size. Molecules smaller than the aperture size are ad-
0888-5885/91/2630-1675$02.50/00 1991 American Chemical Society
1676 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991
r
L
i o
o
0 '
I
\
-
'0
\
: "
I I
I.'
0
AI
0'
j
\
O
O
A\,
0
Figure 1. TOz units in zeolites and aluminophosphates.
mitted to the crystal interior (adsorbed) while those larger are not. As is illustrated below, these properties are exploited in the applications of zeolites.
Applications of Zeolites Although zeolites were discovered in 1756,their largescale, commercial application did not begin until the 19509. In 1953,Kumins and Gessler (1953)reported the synthesis of ultramarine blue, which has the sodalite structure. However, it was the syntheses of zeolite A (Milton, 1959a) and zeolite X (Milton, 1959b) that really initiated the largescale use of zeolites as adsorbents and catalysts. (The first international conference on molecular sieves was held in London in April, 1967 (Breck, 1974).) Since those times, the field of zeolite science has continued to grow. During the year 1970,there were approximately 200 patents and 750 publications on zeolites, while in 1985 the numbers have risen to approximately 700 patents and 2000 publications (per year!) (Newsam, 1988). Zeolites are useful as adsorbents primarily because they (i) are molecular sieves, (ii) contain large void fractions (zeolites A and X have almost a 50% void fraction), and (iii) are hydrophilic. Natural gas is dried by contact with zeolites since they adsorb water. This application and several others are generally classified as purification op-
erations and rely on surface selectivity for polar or polarizable molecules such as water and C 0 2 and sulfurcontaining molecules. Bulk separations can also be accomplished with zeolites. For example, zeolite A is used to separate linear from branched hydrocarbons since only the linear ones can be adsorbed. The largest use of zeolites in terms of tons per year is as detergent builders. Referring to Figure 1, a zeolite that has a Si/Al ratio equal to 1 will have a large ion-exchange capacity. Zeolite A has Si/Al= 1, and M+ is most typically Na+. When zeolite A is used as a detergent builder, the sodium ion exchanges for other hard-water ions, e.g., Ca2+. This use is so large that in 1980 of the 325 million pounds of zeolites sold in the U.S.,247 million pounds were used aa detergent builders (45.5million pounds as catalysts and 32.5 million pounds as adsorbents) (Layman, 1982). Zeolites are also useful as catalysts and catalyst supports. If the balancing cation in the zeolite is H+, then the framework is a solid acid that can reveal shape-selective properties due to confinement of the acidic proton within the zeolite pore architecture. The first report of a shape-selective reaction was by Weisz and Frilette (1960). However, a clearer example of shape selectivity was presented shortly thereafter by Weisz and co-workers (Weisz et al., 1962). Zeolites A and X were ion exchanged with calcium salts (creates acid sites by hydrolysis of the water of hydration around the calcium ions located within the zeolite, e.g., Ca(H20),2+ Ca(H20),1(OH)+ + H+) and were contacted with primary and secondary alcohols in the vapor phase. (For further details on the more standard methods of creating acidity in zeolites see, for example, Breck (1974)or Dwyer (1984).) Figure 6 illustrates the results obtained by Weisz and co-workers. Both the primary and secondary alcohols were dehydrated on CaX, whereas only the primary one reacted on CaA. Since the secondary alcohol is too large to diffuse through the pore of CaA, it cannot reach the active sites within the CaA crystals. This type of shape selectivity is called reactant shape selectivity. Other types of shape selectivity are possible and are illustrated in Figure 7 (Csicsery, 1984). Product shape selectivity can occur when reaction products of different sizes are formed within the interior of the zeolite crystals,
-
SODALITE
OXYGEN.
S I L I C O N OR A L U M 1 NUM
SODALITE CAGE
FAUJASITE
Figure 2. Schematic of zeolite frameworks. The synthetic faujasites are zeolites NaX and NaY (difference between NaX and NaY is the Si/Al: NaX N 1.1, NaY N 2.4).
Ind. Eng. Chem. Res., Vol. 30,No.8,1991 1677
#PORE V O L W qDIAMETER)
I
5
AMORPmXlS SILICA
lo
DIAMETER(A)
100
Figure S. Pore size distribution.
i I iC@H
200
300 Temperature ("c)
Figure 6. Dehydration of alcohols by zeolites (after Weka et al.,
(8)
1962).
Figure 3. Zeolite A (A) scanning electron micrograph showing cubic morphology; (B) echematic of cubic array of &membered rings. 14-
'2-
a&
PORE S I Z E ,
Figure 4. Correlation between pore size of molecular sieves and the diameter (a) of various molecules.
and only certain ones can escape. For example, the diffusion coefficient for p-xylene in ZSM-5 is approximately
103times faster than that of either 0- or m-xylene. Thus, p-xylene selectively diffuses out of ZSM-5(see Figure 7). Recently, the first direct evidence for this type of shape selectivity has been reported. Anderson and Klinowski (1989) have shown by in situ magic angle spinning NMR that tetramethylbenzenes are formed from methanol in the pores of ZSM-5 but do not diffuse out of the crystals to be observed by gas chromatography. Transition-state shape selectivity occurs when the space within the zeolite is not sufficient to allow the larger of several possible transition-state intermediates to form and has the potential to yield the most stereospecific synthesis of products. However, it is also the most difficult to exclusively prove. Examples of transition-state selectivity do exist (Kucherov et al., 1988), but proven cases are rare. Recently, zeolites have been shown to act as solid bases (Martens et al., 1988; Hathaway and Davis, 1989a-c). Martens et al. formed neutral metallic sodium particles within zeolites by decomposition of occluded sodium azide. The sodium zeolites were shown to perform base-catalyzed reactions. However, these materials are converted to s6dium oxide upon exposure to oxygen. Hathaway and Davis synthesized zeolite-supported cesium oxide by decomposing occluded cesium acetate in air. These air-stable materials were shown to be excellent base catalysts with activities equal to or in excess of other well-known base catalysts, e.g., MgO. Shape-selective base catalysis is of great current interest, and many groups are now working in this area.
--
1678 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 -
n m+--
100
/v
-z
80
8
REACTANT SELECTIVITY
9
60
v)
K W
>
8
40
20
-
I
-
*
-1
2
i
6
4
8
I 10
TIME (h)
PRODUCT SELECTIVITY 100
80
8
v
z 0 v)
6C
K W
>
z
TRANSITION STATE SELECTIVITY
Figure 7. Schematic representation of the types of shape selectivity
8
40
20
exhibited by zeolites (after Csicaery, 1984).
In addition to their acidlbase chemistries with zeolites, these structures can serve as hosts for small metal particles. Transition-metal ions, e.g., platinum and rhodium, can be ion exchanged into zeolites and then reduced to their zerovalent state to yield zeolite-encapsulated metal particles. These particles can perform shape-selective catalysis. For example, Figure 8 illustrates the shape-selective hydrogenation of olefins by rhodium encapsulated in zeolite Y (Joh et al., 1989). Specifically, cyclohexene and cyclododecenecan be hydrogenated by rhodium supported on nonmicroporous carbon (Figure 8b) but only cyclohexene can be hydrogenated by rhodium encapsulated in zeolite Y (Figure 8a) since cyclododecene is too large to adsorb into the pores of zeolite Y. Also, metal centers may act in coordination with acid sites on the zeolite to perform bifunctional catalysis. For further information on zeolite catalysis see, for example, Jacobs (1977), Holderich et al. (1988), Chen et al. (1989), Perot and Guisnet (1990), and Biswas and Maxwell (1990). Finally, the possibility of performing oxidation-reduction reactions in zeolites may be possible. Rossin et al. (1987) showed that Co2+ could be incorporated into a tetrahedral atom position in the framework of ZSM-5. Thus, the possibility of having an isolated redox center within a microporous environment appeared feasible. Recently, the framework Co2+ Cos+ redox reaction has been demonstrated (Schoonheydt et al., 1989; Iton et al., 1989; Montes et al., 1990) for cobalt-containing molecular sieves.
-
When Is a Molecular Sieve Not a Zeolite? Strictly speaking, a zeolite is an aluminosilicate. Thus, molecular sieves with framework atoms other than silicon and aluminum should not be called zeolites. This raises the question of whether pure silica molecular sieves should be called zeolites. The point is a minor one and opinions
0-
02
04
06
08
1
12
TIME (h)
Figure 8. Shape-selective catalytic hydrogenation by rhodium encapsulated in zeolite Y (after Joh et al., 1989): (a) rhodium encapsulated in zeolite Y; (b) rhodium on the surface of carbon (not microporous).
vary. However, substitutions of ions other than A13+ and Si4+into a pure Si02or aluminosilicate frameworks give molecular sieves that are normally called metallosilicates. For example, claims of iron, boron, chromium, beryllium, gallium, germanium, cobalt, vanadium, and titanium substitution are available (see Szostak (1989) for a fairly comprehensive list). These materials can have unique properties. Titanium-substituted high silica ZSM-5 (TS-1) is an extremely interesting catalyst for the oxidations of organic substrates using aqueous H202 (Notari, 1988). TS-1is being used commercially in Italy for the production of hydroquinone and catechol from phenol. Recently, it has even been shown to oxidize alkanes at very low temperatures (Huybrechts et al., 1990; Tatsumi et al., 1990). In 1982 a major breakthrough in molecular sieve science was announced. Wilson et al. (1982) disclosed a novel class of crystalline, microporous aluminophosphate materials. Referring to Figure 1, if tetrahedra containing aluminum and phosphorus are connected in a strict Al/P = 1, a neutral framework is obtained. The aluminophosphate or AlP04lattice is the "3-5" analogue of the "4-4" pure SOz. Thus, the AlPO, materials do not contain framework charge. The designation AlPO,-n has been used, and the number n denotes a particular structure type. Novel structures such as A1PO4-5and AlP04-11 (referto Figure 4) have been synthesized as well as AlP04 analogues of zeolites, e.g., AlP04-20has the same framework topology as sodalite (refer to Figure 2). A good overview of AlPO, molecular sieves is given by Wilson et al. (1983). Of
Ind. Eng. Chem. Res., Vol. 30, No. 8,1991 1679 particular importance is the aluminophosphate material VPI-5 (Davis et al., 1988). The largest ring size in natural and synthetic molecular sieves contains 12 T-atoms. Thus, the free diameter available for adsorption is bounded by -10 A. VPI-5 (refer to Figure 4) is the first molecular sieve to contain rin consisting of greater than 12 T atoms (pore size -12.3 The 18 T-atom rings allow for the adsorption and reaction of molecules larger than other molecular sieves (Davis et al., 1989). To date, VPI-5 has not been synthesized as an aluminosilicate. To introduce framework charge into the AlP04 molecular sieves, additional elements are incorporated into the framework. For example, if a portion of PMis substituted by Si4+,then an anionic framework can be obtained. Thus, the microporous solid is a silicoaluminophosphate (SAPO-n). Elements such as silicon, magnesium, iron, titanium, cobalt, vanadium, zinc, manganese, gallium, germanium, beryllium, and boron have been combined with the A1P04’sto give a vast number of element-substituted aluminophosphate-based molecular sieves. This area has been reviewed by Flanigen et al. (1988). Zeolites and the A1P04-based molecular sieves provide numerous microporous solids with a broad spectrum of hysicochemical properties. Variations in pore size (- 3-12 ), pore shape (circular, elliptical, etc.), dimensionality of pore system (1-D, 2-D, 3-D), presence or absence of cages, properties of acid sites (strength, location, distribution, etc.), surface properties (hydrophilic-hydrophobic), void volume (up to -50%), and framework composition are possible. However, most of the aforementioned materials are not commercially available and are at best still laboratory curiosities.
OACOB
pACOB
8.
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k - CO
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ZEOLITE p-ACOB
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- CO
o-ACOB hv’ZSM-% OAOA + BB
- CO
f
EXTERNAL SURFACE (ZSU.5)
CH,
CH,
INTERNAL SURFACE (ZSU.5)
Molecular Sieves for Advanced Materials Applications The molecular sieving abilities combined with the ‘tunability” of the properties mentioned previously give molecular sieves unique characteristics that &ow for their use in a variety of interesting new studies. Below are described several laboratory reports that illustrate the diverse uses of molecular sieves. Separations. Zeolites are currently being employed to separate hydrocarbons on a huge scale. More recently, they have been explored for use as separation media for the separations of fructose-glucose mixtures (Ho et al. 1987), amino acids (Ching and Ruthven, 1989), and antibiotics (Shirvastava and Prakash, 1989). Also, zeolites have been formulated into composites that serve as selective membranes. When high silica ZSM-5 was added to a silicone rubber membrane, there was an increased flux and selectivity for the permeation of ethanol from aqueous ethanol solutions (te Hennepe et al., 1985). Zeolites have also been incorporated in sol-gel-derived,silica glassy, thin films that serve as selective membranes for surface acoustic wave based chemical sensors (Bein et al., 1989). Recently, Davis et al. (1991) illustrated that zeolite films can be grown directly on metal surfaces. This result may open new areas in chemical sensors and surface-modified electrodes. Photochemical and Electrochemical Reactions. Turro (1986) and co-workers have used zeolites to modify the photochemical reaction pathways of organic molecules. For example, Figure 9 illustrates how ZSM-5 can alter the photolysis products of o- and p-methylbenzyl benzyl ketone. Again, the molecular sieving properties of the zeolite are the underlying reason for the altered selectivity. Zeolites have been used as modifiers for electrode surfaces. The advantages of these composites and many examples of use are provided in the review by Rolison (1990).
~~
\ ~_____
t
EnERNAL SURFACE (ZSU.5)
Figure 9. Photochemical reaction pathways in the gas phase and on ZSM-5(after Turro, 1986).
Electrochemical as well as photochemical applications of zeolite catalysts have been reviewed also by Krueger and Mallouk (1991). Very recently, Creasy and Shaw (1990) have performed gas-phase electrocatalysis with zeolitemodified electrodes. Their results open the possibility of performing shape-selective gas-phase electrocatalysis. A particularly interesting example of photochemical and electrochemical reactions involving zeolites is the attempts at artificial photosynthesis by Mallouk and co-workers (Krueger et al., 1991). In natural photosynthesis, a portion of the light energy absorbed is stored as electrochemical potential energy and involves light-induced electron transfer. Each forward electron-transfer step is faster than the reverse step, and the ultimate electron acceptor is well separated from the electron donor. Figure 10 illustrates the approach of Mallouk and co-workers. Notice that zeolite L causes the physical separation of the ruthenium complex and the benzylviologen and that a two-step electron process occurs. Importantly, the Rus+-benzylviologen+ state exists for 35 ps. Thus a long-lived charge-separated state is created. Confinement. Recently, the well-ordered arrays of nanosized void spaces in molecular sieves have been exploited to serve as hosts for interesting guests. Quantumconfined materials show properties that are distinct from
1680 Ind. Eng. Chem. Res., Vol. 30, No. 8,1991
,-
Zeolite L channel
35 ps
Figure 10. Schematic of artificial photosynthesis system (after Krueger et al., 1991).
their bulk characteristics. For example, the thermodynamic properties of 4He confined within zeolites have been investigated (Kato et al., 1987), and implications for superfluid behavior have been discussed (Doi, 1989). Nanoscale particles of semiconducting materials have also been synthesized within the cages of zeolite Y and sodalite. Cadmium selenide (Moller et al., 1989a),cadmium sulfide (Herron et al., 1989), lead sulfide (Moller et al., 1989b), and gallium phosphide (MacDougallet al., 1989) have been encapsulated in the cages of zeolite Y. Also, silver salts have been imbibed into the cages of sodalite (Stein et al., 1990a). In fact, Stein et al. (1990b) have assembled silver and sodium bromosodalites into semiconductor quantum supralattices one atom at a time. These quantum-confined clusters typically reveal optical properties that are sufficiently different from the bulk to attract interest as nonlinear optical materials. Besides use of the cages within zeolites as confinement areas, molecular sieves with only channel voids have found application as well. Selenium loaded into the channel system of m o 4 - 5appears to form a helical chain (Parise et al., 1988), and it has been speculated that VPI-5 could contain two or three helices (Stucky and MacDougall, 1990). Also, the synthesis of polymers such as polypyrrole (Bein and Enzel, 1989), polythiophene (Enzel and Bein, 1989a),and polyaniline (Enzel and Bein, 1989b) has been accomplished within the pores of zeolites. Ozin et al. (1990) have formed films of silver-containing sodalites for use as high-resolution optical storage media. The silver sodalites were mixed into a solution containing poly(methy1methacrylate), and a film was spin-coated onto a fused silica disk. Since the silver clusters are subnanometer sized, the resolution is limited by the size of the laser beam that would be used in a device. The recent work by Stucky and colleagues nicely illustrates the use of molecular sieves as confining hosts. Noncentrosymmetric structures can exhibit frequency doubling or second harmonic generation (SHG), i.e., conversion of coherent light of frequency w into light frequency 2w. Several organics that have molecular nonlinear optical properties and have noncentrosymmetric structures do exist. 2-Methyl-4-nitroaniline(MNA) has these properties. 4-Nitroaniline (NA) does not because in spite the same molecular hyperpolarizability as MNA, NA crystallizes in a centrosymmetric manner (cannot have SHG with centrosymmetric structure). A1PO4-5 has an acentric structure. When NA is adsorbed in A1PO4-5,the noncentrosymmetric structural features of A1PO4-5 (acentric host) are imposed on the NA (or created by the guest-host interaction) to give a SHG (Cox et al., 1988). However, NA residing on the outside surface of Awo4-5yields no SHG. Interestingly, MNA shows the opposite effects. MNA adsorbed within the pores of A1PO4-5 reveals no SHG, whereas adsorption on the outside surface does produce
SHG. Thus, as stated by Cox et al., MPo4-5 switches the SHG of NA and MNA on and off, and these results are a dramatic demonstration of how inclusion can influence nonlinear optical properties. Two reviews on molecular sieves as hosta for optoelectric guest materials have appeared (Ozin et al., 1989; Stucky and MacDougall, 1990). These papers give further examples of interesting new guest-host composite materials using molecular sieves and the production of quantum confinement affects. Future As illustrated previously, molecular sieve science is growing at a rapid rate. Many new and interesting uses for molecular sieves are being explored, and the list of chemical reactions that employ molecular sieve catalysts continues to increase. Rather than speculate on future applications of molecular sieves, four important topics involving the plausible syntheses of new molecular sieving materials are discussed. Anion Exchangers. Throughout the discussions in this paper, the cation-exchange properties of molecular sieves have been illustrated. To date, molecular sieves that contain all framework atoms (other than oxygen) in tetrahedral coordination are either neutral (pure defect-free SiOz or AIPOJ or are cation exchangers. Thus, the synthesis of an anion exchanger (positive lattice charge) would be of great interest. Referring to Figure 1, if silicon and phosphorus could form a tectosilicophosphate, it would have a positive framework charge. However, this is unlikely. According to Pauling’s bonding rules (Pauling, 1929),the bond strength on the lattice oxygen anion should not exceed its valency, i.e., 2. In Figure 11 this principle is illustrated for silica, zeolites, and AlPOis. If tetrahedral silicon is adjacent to tetrahedral phosphorus, then the total bond strength on the bridging oxygen atom exceeds 2. However, if silicon is octahedrally coordinated, then the configuration does not violate this principle. Octahedral silicon adjacent to tetrahedral phosphorus is a known configuration (Mudrakovskii et al., 1985). Thus, silicophosphates will most likely be layered materials rather than three-dimensional structures. Layered anion exchangers already exist (Reichle (1985) and references therein). The concepts illustrated with silicon and phosphorus will likely apply to other element combinations as well. Thus, it is doubtful that a true, completely tetrahedral, positive framework can be synthesized. There is a report that is contrary to this belief. Dyer et al. (19871 claim to have substituted P6+into zeolite frameworks to produce anion exchangers. While the materials synthesized do in fact have anion exchange capacity, there is no proof provided for the claims of (i) framework substitution and (ii) the existence of a complete tetrahedral framework. More likely, these novel materials are not completely
Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 1681 TOTAL BOND STRENGTH
CONFIGURATION
2.00
0
0.75
1
\
,P.*,/O
0
0
I\
1.75
A\,
0
2.00
2.25
0
I
0.67
1.25
1.92
Figure 11. Bond strengths on bridging oxygen atoms.
tetrahedral frameworks. Thus, the synthesis of a true cationic, tetrahedral framework would be a significant breakthrough. Frameworks with Three-Membered Rings. Recently, Brunner and Meier (1989) have shown that there appears to be a correlation between the minimum framework density (FD) defined as the number of T atoms per cubic nanometer and the smallest ring size in the framework. The FD is related to the void volume of the crystal: as the FD decreases, the void volume or capacity for adsorption increases. The minimum known FD is 12.5 and corresponds to the void occupying slightly over half the crystal volume. All of the materials with FD > 12 contain rings consisting of four or more T atoms. There are however two known exceptions: lovdarite (Meier and Olson, 1987) and ZSM-18 (Lawton and Rohrbaugh, 1990). Lovadarite and ZSM-18 both possess 3-membered rings, some of which have T atoms shared by rings with higher numbers of T atoms. If the Brunner and Meier correlation between FD and ring size is correct, then zeolites with all the T atoms in 3-membered rings must be synthesized to achieve materials with higher capacities. Meier has offered several hypothetical structures based upon 3-membered ring building units with FDs in the range 9-10 (Meier, 1986; Davis, 1989). The plausibility of synthesizing zeolites with very low FDs leads one to speculate on several types of materials that could be quite significant. A small-pore, very large capacity zeolite could be useful in the storage of methane and other light hydrocarbons. Also, zeolites with pores comprised of rings that contain greater than 12 T atoms and are multidimensional may be possible. Meier (1986) speculated that all structures with greater than 12 T-atom rings based upon 4-membered rings or larger will have one-dimensional pore systems. This is because hypothetical frameworks with multidimensional extra-large pores yield FDs below 12, in violation of the correlation. However, the correlation is satisfied if the frameworks are based upon 3-membered rings. An example of a 16-membered ring framework with 3-dimensional pore systems derived by Meier using 3-membered rings and a FD of 9.3 can be found elsewhere (Davis, 1989).
The existence of lovdarite and ZSM-18 lends hope that frameworks based upon 3-membered rings can be synthesized. Very recently, Annen et al. (1991) have synthesized a new zincosilicate molecular sieve that contains 3-membered rings. This material has many advantages over lovdarite and ZSM-18 in that it does not contain beryllium (toxic) and highly specific structure-directing agents (as do lovdarite and ZSM-18, respectively), and it provides a basis for new avenues toward synthesizing structures with 3-membered rings. Chiral Zeolite. An object is chiral if it cannot be superimposed on its mirror image by translation and rotation. Quartz can be chiral. There are right- and left-handed quartz crystals (d- and I-quartz, respectively). These materials have been implicated as possibly playing a role as adsorptive substrates that served as templates in the abiotic evolution of biopolymers (Bernal, 1951). Stereospecific adsorption of various organic molecules such as vitamin B12and d,l-camphor have been reported to occur on quartz. Resolution of organic racematics from solution is feasible by adsorption onto powdered d- or 1-quartz. Thus, the separation of racemic mixtures and the possibility of asymmetric catalysis (if an active center is incorporated into the crystal) exist when using chiral inorganic crystals. To date, none of the zeolite structures are chiral. However, Newsam et al. (1988) have shown that zeolite /3 is a highly intergrown hybrid of two distinct structures: the so-called polymorph A and B. Polymorph A is chiral. I t contains a 3-dimensional pore system consisting of 12 T-atom rings. The 12 T-atom ring intersections between the straight channels in the a and b directicins follow a 4-fold screw operation along c and define a helix (right or left handed: P4,22 or P4322,respectively). Newsam et al. immediately recognized the consequences of this structure and stated that it might have potential for effecting chiral catalysis (asymmetric synthesis) or chiral separations (resolution of racemates). Thus, the synthesis of polymorph A of zeolite /3 should open new areas in asymmetric chemistry. One can imagine that shape-selective asymmetric synthesis would be possible. Also, it would be yet another step closer to truly mimicking enzyme catalysis. Zeolite Membranes. An inorganic, molecular sieving membrane would be useful in many areas of separations and catalysis. It could operate at high temperatures and in corrosive environments. To date, no proven example of a true molecular sieving membrane is available. However, Ishikawa et al. (1989) appear to be on the right track. Porous Vycor was reacted with sodium hydroxide, sodium aluminate, and tetraethyl orthosilicate to yield a modified Vycor tube that shows preferential separations of alcohols from water. The Vycor reaction conditions are such that one might expect the formation of zeolite A. The authors state that they lack direct evidence showing that in fact a zeolite was formed in the pores of the Vycor. However, the modified Vycor showed a separation factor for ethanol from water of 1633, whereas the pure Vycor revealed none (separation factor = 1). The results of Ishikawa et al. are impressive. Further work in this area is necessary and could yield significant progress toward the goal of a molecular sieve membrane for large-scale chemical production applications.
Acknowledgment I thank Profs. T. Bein, T. Mallouk, G. Ozin, S. Suib, and G. Stuckey for sending preprints of their work. Also, I thank the Chemical Engineering Department at Stanford University and especially Prof. Michel Boudart for hwting
1682 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991
a half-year sabbatical during 1990 in which the writing of this article was initiated.
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Received for review October 15, 1990 Revised manuscript received February 15, 1991 Accepted March 5,1991
KINETICS AND CATALYSIS Restrictive Liquid-Phase Diffusion and Reaction in Bidispersed Catalysts S. Y. Lee,t J. D. Seader,**tC h u g H. Tsai,t and F. E. Massotht Departments of Chemical Engineering and Fuels Engineering, University of Utah, Salt Lake City, U t a h 841 12
The effect of bidispersed pore-size distribution on liquid-phase diffusion and reaction in NiMo/A120g catalysts was investigated by applying two bidispersed-pore-structuremodels, the random-pore model and a globular-structure model, to extensive experimental data, which were obtained from (1)sorptive diffusion measurements a t ambient conditions and (2) catalytic reaction rate measurements on nitrogen-containing compounds. Transport of the molecules in the catalysts was found to be controlled by micropore diffusion, in accordance with the random-pore model, rather than macropore diffusion as predicted by the globular-structure model. A qualitative criterion for micropore-diffusion control is proposed: relatively small macroporosity and high catalyst pellet density. Since most hydrotreating catalysts have high density, diffusion in these types of catalysts may be controlled by micropore diffusion. Accordingly, it is believed in this case that increasing the size of micropores may be more effective to reduce intraparticle diffusion resistance than incorporating macropores alone.
Introduction The efficiency of a porous catalyst is often limited by the diffusion rate of reactants in the catalyst, especially under circumstances in which diffusion rate is relatively
* Author to whom correspondence should be addressed. Department of Chemical Engineering. Department of Fuels Engineering.
slow compared to surface reaction rate. In order to evaluate the interactions between diffusion and reaction within a catalyst, a good understanding of the diffusion process within the catalyst is crucial. Since intraparticle diffusion rates are strongly affected by the pore structure of the catalyst, the use of a pore-structure model combined with a mass-flux equation is widely adopted to analyze diffusion and reaction in catalysts. An ideal pore-structure
0888-5885/91/2630-l683$02.50/00 1991 American Chemical Society
