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OCR for page 3471
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 3471-3478, March 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Geology, Mineralogy, and Human Welfare,"
held November 8-9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA.
Synthetic zeolites and other microporous oxide molecular sieves
JOHN D. SHERMAN
Research and Development Department, UOP LLC, 25 East Algonquin Road, Des Plaines, IL 60017-5017
ABSTRACT Use of synthetic zeolites and other micro-
porous oxides since 1950 has improved insulated windows,
automobile air-conditioning, refrigerators, air brakes on trucks,
laundry detergents, etc. Their large internal pore volumes,
molecular-size pores, regularity of crystal structures, and the
diverse framework chemical compositions allow "tailoring" of
structure and properties. Thus, highly active and selective cat-
alysts as well as adsorbents and ion exchangers with high
capacities and selectivities were developed. In the petroleum
refining and petrochemical industries, zeolites have made pos-
sible cheaper and lead-free gasoline, higher performance and
lower-cost synthetic fibers and plastics, and many improvements
in process efficiency and quality and in performance. Zeolites
also help protect the environment by improving energy effi-
ciency, reducing automobile exhaust and other emissions, clean-
ing up hazardous wastes (including the Three Mile Island
nuclear power plant and other radioactive wastes), and, as
specially tailored desiccants, facilitating the substitution of new
refrigerants for the ozone-depleting chlorofluorocarbons banned
by the Montreal Protocol.
Relationships of Synthetic Zeolites to Natural Zeolites and
Other Minerals. Only 6 of the >63 natural zeolites commonly
occur in large beds: analcime (ANA),* chabazite (CHA), clinop-
tilolite (HEU), erionite (ERI), mordenite (MOR), and phillipsite
(PHI) (14; ferrierite (PER) occurs in a few large beds. Each of the
seven also has been synthesized, but only mordenite and ferrierite
are manufactured in large quantity. Significantly, synthetic mor-
denite has large pores whereas natural mordenite has small pores
(2~.
Besides mordenite and ferrierite, the principal synthetic (alu-
minosilicate) zeolites in commercial use are Linde Type A (LTA),
Linde Types X and Y (Al-rich and Si-rich FAU), Silicalite-1 and
ZSM-5 (MFI), and Linde Type B (zeolite P) (GIS). Other
commercially available synthetic zeolites include Beta (BEA),
Linde Type F (EDI), Linde Type L (LTL), Linde Type W
(MER), and SSZ-32 (MTT). All are aluminosilicates or pure
silica analogs.
Recently, new nonaluminosilicate, synthetic molecular sieves
became available commercially. They include aluminophosphates
(family of AlPO4 structures); silicoaluminophosphates (SAPO
family); various metal-substituted aluminophosphates tMeAPO
family, such as CoAPO-50 (AFY)~; and other microporous
framework structures, such as crystalline silicotitanates.l
Most current commercial applications use aluminosilicate zeo-
lites or their modified forms. Undoubtedly, commercial uses both
for zeolites and other molecular sieves will continue to grow.
Development of Synthetic Zeolites and Other Microporous
Oxides. The first zeolite mineral (stilbite) was described in
Sweden by Baron Cronstedt in 1756 (3-5~. Highlights of the
history of adsorption studies of zeolites were reviewed by Breck
(6~.
PNAS is available online at www.pnas.org.
By 1926, the adsorption characteristics of chabazite were
attributed to tiny pores (~5 A in diameter) that allowed small
molecules to enter but excluded larger ones: hence, the term
"molecular sieve" (7~.
By 1945, Barrer classified zeolite minerals into three classes
depending on the size of the molecules adsorbable rapidly, slowly,
or not appreciably at room temperature or above (8, 9~. However,
zeolites did not find any significant commercial use until synthetic
zeolites were discovered and developed (large, mineable deposits
of natural zeolites were not discovered until the late 1950s).
Barrer's 1948 synthesis of small-port mordenite at high temper-
atures and pressures heralded the era of synthetic zeolites (10~.
From 1949 through the early 1950s, the commercially signifi-
cant zeolites A, X, and Y were discovered by Milton and Breck
at the Tonawanda, New York, laboratories of the Linde Air
Products Division of Union Carbide Corporation. I~hese zeolites
were synthes~zed from readily available raw materials at much
lower temperature and pressure than used earlier. Many of the
new synthetic zeolites had larger pore size than most of the known
natural zeolites, allowing applications involving larger molecules.
In addition, many had larger pore volume, giving higher capacity.
In 1953, Linde Type A zeolite became the first synthetic zeolite
to be commercial~zed as an adsorbent to remove oxygen ~mpurity
from argon at a Union Carbide plant (11~. Synthetic zeolites were
introduced by Union Carbide as a new class of industrial adsor-
bents in 1954 and as hydrocarbon-conversion catalysts in 1959.
New zeolites and new uses appeared steadily through the 1960s.
An explosion of new molecular sieve structures and compositions
occurred in the 1980s and 1990s from the aluminosilicate zeolites
to the microporous silica polymorphs to the microporous alu-
minophosphate-based polymorphs and metallo-silicate composi-
tions (12~. Molecular sieves now serve the petroleum refining,
petrochemical, and chemical process industries as selective cat-
alysts, adsorbents, and ion exchangers.
Many zeolites can be synthes~zed with SiO2 higher or lower
than in nature for the same framework type. Higher SiO2
generally gives greater hydrothermal stabil~ty, stronger-acid cat-
alytic activity, and greater hydrophobicity as adsorbents. Con-
versely, lower sio2 gives greater cation exchange capacity and
higher adsorbance for polar molecules. Controlling the synthesis
process optimizes a zeolite for different applications.
Abbreviations: SAPO, silicoaluminophosphate; 8R, eight-ring; EB,
ethylbenzene; PSA, pressure swing adsorption; VSA, vacuum swing
adsorption; tpd, U.S. tons of 02 per day; 3D, three-dimensional.
*The three-letter International Zeolite Association Structure Com-
mission code for the framework topology of each zeolite is given in
parentheses at the first mention of that zeolite in this paper (the full
list is at http:/fwww-iza-sc.csb.yale.edu/IZA-SC/~.
lThe molecular sieves of AlPO4s, SAPOs, MeAPOs, etc., were
discovered in the 1980s by scientists in the Tarrytown, NY labora-
tories of Union Carbide Corporation's Catalysts, Adsorbents and
Process Systems (CAPS) group. In 1988, Union Carbide Corpora-
tion's Catalysts, Adsorbents and Process Systems and the Process
Division of UOP of AlliedSignal merged to form a partnership
company, called UOP, which is jointly owned by AlliedSignal and
Union Carbide. UOP LLC has continued to develop both the
materials and their applications.
3471
OCR for page 3472
3472 Colloquium Paper: Sherman
Many synthetic zeolites have framework topologies not found
to date among the natural zeolites. The natural zeolite faujasite
has the same framework (FAU) and similar framework compo-
sition to the Type Y synthetic zeolite but is rare in nature.
Where both natural and synthetic forms of the same zeolite are
available in commercial quantity, the variable phase purity of the
natural zeolite and the chemical impurities, which are costly to
remove, can make the synthetic zeolite more attractive for specific
applications. Conversely, where uniformity and purity are not
important, the cheapness of a natural zeolite may favor its use.
Hence, natural and synthetic zeolites seldom compete for the
same applications.
Structure and Properties of Synthetic Molecular Sieves. Zeo-
lites have the chemical formula M2/nOAl2O3xSiO2yH2O, where
the charge-balancing nonframework cation M has valence n, x is
2.0 or more, end y is the moles of water in the voids. The Al and
Si tetrahedral atoms, or T-atoms, form a three-dimensional (3D)
framework of A1O4 and SiO4 tetrahedra linked together by
shared oxygen ions. Although an SiO4 tetrahedra is charge-
balanced, an A1O4 tetrahedra has a negative charge balanced by
a positive charge on M. Related pure SiO2 frameworks, such as
silicalite-1 (MFI), are charge-balanced and do not need non-
framework cations.
Variants involve Ge substitution for Si in the framework or
involve substitution of Fe, Co, Mn, Zn, Ti, or Mg for Al. In the
related aluminophosphates (AlPO4), each negatively charged
A1O4 tetrahedron is balanced by a positively charged PO4 tetra-
hedron, and nonframework cations are not needed. Still other
variants include the silicoaluminophosphate (SAPO) structures
in which Si substitutes some P in the AlPO4 framework; each
added Si needs a nonframework cation to balance the charge on
the framework.
The pore geometry and volume in a specific microporous oxide
are determined by the specific topology of the particular 3D
framework. The lower the T-atom density per volume of the
zeolite crystal, the higher the void fraction inside the crystal. The
void fraction is 50% for NaX and 47% for NaA. The size of the
largest pore in a zeolite is determined by the number of oxygen
ions rimming the pore and its shape; e.g. a planar, circular
eight-ring (8R) pore rimmed by eight oxygen ions has a diameter
of 4.1 4, as in Linde Type A zeolite, whereas the elliptical 8R pore
of NaP zeolite (GIS) is 4.5 x 3.1 4.
Applications in separation and purification processes often
used the ability of zeolites and other molecular sieves to exclude
molecules too large to enter the pores and admit smaller ones.
Similarly, shape-selective catalysis takes advantage of the ability
of the pores to favor the admission of smaller reactant molecules,
the release of smaller reaction product molecules, or the restric-
tion of the size of transition-state complexes inside the micro-
pores of the zeolite (13~.
Petroleum Refining Processes for the Production of Fuels
Catalytic Cracking. The prime goal in petroleum refining is
efficient conversion of crude oil into high-quality fuel compo-
nents. Desired fuel fractions in order of increasing molecular
weight are gasoline, aviation jet fuel, and diesel fuel. Gasoil and
asphalt, with even higher molecular weighs, are most often further
processed by thermal cracking, catalytic cracking (to make gas-
oline) (14), and catalytic hydrocracking (to make jet fuel). A
lower-boiling fraction, light straight-run naphtha, rich in pentanes
and hexanes and some butane, is further processed by catalytic
hydroisomerization.
Strong acid catalytic activity of X and Y zeolites was discovered
in 1957 by Rabo and was related to their crystallinity (154. This
discovery laid the basis for zeolites in cracking, hydrocracking,
and isomerization of hydrocarbons. From the early 1960s on, use
of synthetic zeolites in catalysis and in related adsorption sepa-
ration processes has dramatically transformed petroleum refining
by vastly increasing the yield of high-quality fuels and reducing
capital and operating costs, energy requirements, and adverse
Proc. Natl. Acad. Sci. USA 96 (1999J
environmental impact. Zeolites also played a major role in
allowing the efficient reformulation of gasoline to the present
lead-free gasoline.
In modern petroleum refineries in the world, gasoil and other
heavier fractions from the crude oil fractionation unit are fed to
fluid catalytic cracking units, which use small, fluidizable catalyst
particles containing Type Y zeolite or other zeolites, or to
hydrocracking units, which use fixed beds of larger catalyst
particles also containing zeolites. The fluid catalytic cracking and
hydrocracking units convert higher-molecular-weight hydrocar-
bons to lighter ones suitable for gasoline, light fuel oils, olefins,
and other uses.
In both fluid catalytic cracking and hydrocracking, zeolite
catalysts provide vastly superior combinations of strong acid
catalytic sites, uniformity of pore structure, and stability, all of
which provide improved selectivity, yield, durability, and cost over
nonzeolite alternatives. In addition, these zeolites have provided
much higher yield of gasoline and other high-quality fuels per
barrel of crude oil, significantly reducing crude oil imports to the
U.S. (>400 million barrels a year) and to other countries.
Hydrocracking. I~he early 1960s saw increasing demand for
high-octane gasoline for the high-compression-ratio engines in
new high-performance cars. Demand also grew for diesel fuel for
diesel-electric locomotives and low-freeze-point jet fuel. These
needs were met by rapid growth in hydrocracking of the more-
refractory crude fractions that were not converted to gasoline and
lighter products in the catalytic cracking units. This growth was
accompanied by the pioneering development by Roland Hanford
at Union Oil, now Unocal, of new, zeolite-based hydrocracking
catalysts with dramatically improved activity and selectivity.
Hydrocracking grew rapidly in the 1960s and 1970s inside and
then later outside the U.S. Worldwide hydrocracking capacity
should grow from ~2.5 million barrels per day in 1990 to ~3.5
million in 2000 (16~.
In hydrocracking, hydrocarbon molecules and hydrogen gas
pass over the zeolite catalyst, which converts higher-molecular-
weight petroleum fractions to lower-molecular-weight fuels (17~.
For example, UOP's Unicracking process (developed jointly by
the Molecular Sieve Department of Union Carbide, now part of
UOP, and Unocal) uses base- or noble-metal hydrogenation-
activity promoters impregnated on combinations of zeolite- and
amorphous-aluminosilicates for cracking activity (18~. The spe-
cific metals chosen and the proportions of the metals, zeolite, and
nonzeolite aluminosilicates are optimized for the feedstock and
desired product balance. The Isocracking process of Chevron also
uses hydrocracking catalysts, some containing zeolites to increase
the cracking function of these dual-function catalysts (19~.
The zeolites most frequently used in commercial hydro-
cracking catalysts are partially dealuminated and low-sodium, or
high-silica, Type Y zeolites in hydrogen or rare-earth forms.
Other zeolites and mixtures of zeolites also are used. The zeolites
often are imbedded in a high-surface-area amorphous matrix,
which serves as a binder. The metals can reside inside the zeolite
and on the amorphous matrix.
Catalytic Dewaxing. Catalytic dewaxing yields various grades
of lube oils and fuel components suitable for extreme winter
conditions. Paraffinic (waxy) components, which precipitate out
at low temperatures, are removed. In the UOP Catalytic Dew-
axing process, the first stage saturates olefins and desulfurizes and
denitrifies the feed via hydrotreating (20~. In the second stage, a
dual-function, non-noble-metal zeolite catalyst selectively ad-
sorbs and then selectively hydrocracks the normal and near-
normal long-chain paraffins to form shorter-chain (nonwaxy)
molecules. Alternatively, as in the recently commercialized Chev-
ron Isodewaxing process, the dewaxing results from isomerizing
the linear paraffins to branched paraffins by using a SAPO-11
molecular sieve catalyst containing platinum (21, 22~.
Light Paraffin Hydroisomerization. Lead was added to gaso-
line to increase its octane number, especially for vehicles, intro-
duced in the early 1960s, that had modern high-compression
OCR for page 3473
Colloquium Paper: Sherman
ratio, high-performance engines. The subsequent U.S.-legislated
reduction of lead in gasoline required increased use of the
catalytic hydroisomerization of the light straight-run naphtha
fraction mentioned earlier.
Some versions of UOP's hydroisomerization processes use
highly active zeolite-based, Pt-containing hydroisomerization cat-
alysts, such as UOP I-7, which contains modified synthetic
(large-port) mordenite. In the presence of hydrogen at moderate
conditions, such catalysts optimize isomerization and minimize
hydrocracking (23~. Linear paraffins in the feed convert to
branched paraffins with higher octane number. The Sud-Chemie
HYSOPAR catalyst used in CEPSA's CKS ISOM process also
uses a zeolite for the hydroisomerization of light naphtha."
To further increase octane level, products from a hydroisomer-
ization unit can be sent to the Molex process, where the remaining
lower-octane, linear paraffins are separated from the other
compounds by using a zeolite adsorbent and a liquid desorbent;
the Molex process is an example of UOP's Sorbex simulated-
moving-bed technology (24~. The extracted linear paraffins are
recycled to the hydroisomerization unit, and the remaining
higher-octane fraction is recovered for gasoline blending. The
combination of the hydroisomerization and Molex processes
boosts the research octane number of a typical feed from 68-70
to 89-92.
Alternatively, if a refinery can use the linear paraffins, it need
not recycle them to a hydroisomerization unit. For example, the
paraffins may be added to the feed of an ethylene steam cracker,
thus increasing the efficiency of the cracker and leading to lower
energy consumption and a purer product. Linear paraffins also
are used as intermediaries in some food processing.
UOP's once-through zeolitic isomerization process (formerly
known as the Shell Hysomer process) also uses a strongly acidic
zeolite with a noble metal to hydroisomerize the light naphtha
(25~. Refiners with idle catalytic reformers or hydrotreaters can
convert this equipment to use this process. To achieve higher
octane levels, UOP's TIP total isomerization process uses the
once-through isomerization process combined with UOP's IsoSiv
process, which uses size-selective zeolite adsorption of the unre-
acted linear paraffins so that they can be recycled and converted
to extinction (32~. Both the TIP and IsoSiv processes originally
were developed at Union Carbide's Molecular Sieve Department,
now part of UOP.
Petrochemicals Processing for Aromatics Production
and Derivatives
Ethylbenzene Synthesis. Styrene monomer, made by dehydro-
genating ethylbenzene (EB), is the basic chemical for all poly-
styrene products. Ethylbenzene is made by using various catalysts
to alkylate benzene with ethylene. Until 1980, nearly all EB was
produced by liquid-phase alkylation reactions using aluminum
chloride catalyst. In 1980, vapor-phase alkylation using a heter-
ogeneous catalyst was introduced to eliminate many problems of
waste disposal and special metallurgy involving aluminum chlo-
ride. In 1990, liquid-phase zeolitic technology began to replace
the Mobil/Badger vapor-phase process, based on ZSM-5 zeolite.
The 1990 Lummus/UOP ethylbenzene liquid-phase process,
using highly stable, poison-resistant zeolite catalysts manufac-
tured by UOP, operates at low benzene-to-olefin ratio and high
selectivity to EB.§ The UOC-4120 catalyst from UOP, used
initially for both alkylation of benzene with ethylene and transal-
kylation of polyethylbenzenes and benzene to produce more EB,
operated successfully for 7 years, with >5 million metric tons of
capacity installed or ordered. The extremely low xylene content
tFloyd, F. M., Gilbert, M. F., Pascual, M. P. & Kohler, E., Middle East
Petrotech 98: Second Middle East Refining and Petrochemicals
Conference and Exhibition, September 14-16, 1998, Bahrain.
§Woode, G. B., Zarchy, A. S., Morita, M. & Shinohara, K., Sud-
Chemie Group 1998 International Styrene Symposium, June 14-18,
1998, Sapporo, Hokkaido, Japan.
Proc. Natl. Acad. Sci. USA 96 (1999) 3473
of the EB product permits the production of the highest-purity
styrene monomer and lowers the costs in the styrene production
unit. Current designs use the EBZ-100 catalyst for transalkylation
and the EBZ-500 catalyst for alkylation. The new Mobil/Badger-
Raytheon EBMax process commercialized in 1995 uses an
MCM-22 (MWW) zeolite catalyst for liquid-phase alkylation;
Mobil's TRANS-1 modified MFI catalyst for vapor-phase
transalkylation of polyethylbenzene and cracking of C6 and C7
naphthenes; and TRANS-4 catalyst for liquid-phase transalkyla-
tion of polyethylbenzenes (26~.
Cumene Synthesis. More than 95~o of the 7 million metric tons
per year of cumene is used worldwide as the principal chemical
for production of phenol and its acetone byproduct. The phenol
yields phenolic resins, bisphenol-A, caprolactam, and other prod-
ucts. Phenolic resins are used extensively to bond plywood and
composition board. Both phenol and acetone are used increas-
ingly in the production of polymers such as epoxy, polycarbonate
resins, and nylon-6.
Most cumene is made by alkylating benzene with propylene
over an acid catalyst, mostly solid phosphoric acid and minor
AlCl3. Recent awareness of the negative environmental impact of
spent-catalyst disposal has spurred a search for more benign
alternatives. The world's leading technology (90% open market)
for producing cumene is the UOP Catalytic Condensation pro-
cess, which uses inexpensive solid phosphoric acid catalyst. Its
high-purity cumene product has set the standard. However, side
reactions over the solid phosphoric acid catalyst result in a 4-5%
loss in cumene yield.
The UOP Q-Max process, commercial~zed in 1996, uses the
new, environmentally benign QZ-2000 zeolite catalyst for direct
alkylation of benzene with propylene and incorporates a second
step (transalkylation) to react the diisopropylbenzene, a byprod-
uct of the first step, with benzene to form additional cumene.t It
produces a higher-quality cumene product (>99.97% purity) at
overall cumene yield of 99.7% and lower investment cost.
The Mobil-Badger process using aluminum chloride catalyst
also yields cumene. Recently, Mobil/Raytheon also developed a
zeolite catalyst based on the relatively new zeolite MCM-22.
Similarly, Dow and Kellogg developed "3D-DM" catalysts based
on dealuminated forms of mordenite (27~: Enichem, a catalyst
based on zeolite Beta, and CD-Tech/Lummus, a catalytic distil-
lation zeolite catalyst.
para-Xylene Production from MLxed Cs Aromatics. Polyester
fibers have revolutionized clothing. Many people in the U.S. take
for granted wash-and-wear and permanent press clothing, and
ironing of shirts is out of fashion. Demand has grown in devel-
oping countries because of the great comfort of fabrics made from
cotton-polyester fibers blended in any proportion for any climate,
as well as low cost, excellent durability, and ease of washing with
little water and detergent (M. M. Sharma, personal communi-
cation).
The worldwide annual production of 12-15 million tons p-
xylene is expected to rise to ~17-18 million tons in 10 years. Most
p-xylene is used to make purified terephthalic acid, which is
reacted with ethylene glycol to make the poly-(ethylene tereph-
thalate), the basis of polyester fibers. I~he p-xylene is separated
from mixed Cs aromatics (containing o-, m-, end p-xylenes and
EB) by using either crystallization or adsorption processes. Since
1971, the new UOP's Parex adsorption separation process cap-
tured 60% of the worldwide p-xylene production. Another large
and growing use for p-xylene is in the manufacture of poly-
(ethylene terephthalate) for bottles recyclable and environmen-
tally benign.
The Parex process uses the Sorbex technology mentioned
earlier (28~. Its critical ingredient is a special, p-xylene-selective
adsorbent. Ion-exchanged forms of synthetic FAU zeolite are
~Jeanneret, J., Greer7 D., Ho, P., McGehee, J. & Shakir, H., 22nd Annual
DeWitt Petrochemical Review, March 18-20, 1997, Houston, TX.
OCR for page 3474
3474 Colloquium Paper: Sherman
used with desorbent liquids to recover >97~op-xylene at >99.9%
purity from a raffinate containing EB and o- and m-xylenes.
Xylene Isomerization. The raffinate from the Parex unit can go
to an Isomar unit (29), licensed by UOP, for isomerization to a
near-equilibrium mixture of xylenes, which are recycled to the
Parex unit. The Isomar unit itself also uses UOP zeolite acid
catalysts, such as the Pt-bearing I-9 catalyst, which converts EB to
xylenes, and the I-100 catalyst, which deaRylates EB to benzene.
Both provide efficient EB conversion with excellent xylene
retention.
Disproportionation of Toluene and Transallylation of Tolu-
ene and Trimethylbenzenes. Recent strong demand for p-xylene
has begun to exceed the supply of mixed xylenes. Incorporating
the Tatoray process (originated with Toray Industries in Japan
and further developed and licensed by UOP) into the aromatics
complex in a refinery can more than double the yield of p-xylene
from a naphtha feedstock (30~. The zeolite-based TA-4 catalyst
has two principal functions, disproportionation of toluene into the
more-valuable benzene and mixed xylenes, and transalkylation of
toluene and trimethylbenzenes to mixed xylenes. The mixed
xylenes then are added to the Parex unit to produce more
p-xylene.
p-Xylene Synthesis from Toluene. Xylenes can be produced by
the zeolite-catalyzed disproportionation of toluene alone. Mobil
developed the MTPX (Mobil toluene topara-xylene) process for
internal use, and the licensed MSTDP (Mobil selective toluene
disproportionation) process, based on ZSM-5 (MFI) "product
shape-selective" zeolite catalysts (31~. The toluene disproportion-
ation generates mixed xylenes inside the catalyst, but the overall
relative yield of p-xylene is greater than the thermodynamic
equilibrium allows because thep-xylene diffuses more rapidly out
of the zeolite than do the o- and m-xylenes. UOP's recent PX-Plus
process also uses a zeolite catalyst for p-xylene synthesis by
shape-selective disproportionation of toluene.
Aromatics from Light Hydrocarbons. UOP's Cydar process
converts low-value LPG (propane, butanes) or light feedstocks
containing olefins and paraffins to high-value, easily transport-
able, petrochemical-grade liquid aromatic products, particularly
BTX (benzene, toluene, and xylenes). It uses a single gallium-
modified zeolite catalyst developed by BP and UOP in conjunc-
tion with UOP's CCR continuous catalytic regeneration system
(32, 33~. Acidic sites on the zeolite catalyze dehydrogenation,
oligomerization, and cyclization. The shape-selectivity of the
zeolite cavities helps promote the cyclization reactions and limits
the size of the rings (344.
M-Forming. Catalytic reforming produces a high-octane liquid
reformate product rich in aromatics and hydrogen gas; light
hydrocarbon gases, such as LPG; and C6 to Cg paraffins. Mobil's
M-Forming process selectively hydrocracks linear and singly
branched paraffins in gasoline reformate fractions to LPG by
size-selective catalysis by using medium-pore ZSM-5 zeolite (35~.
Olefins produced from paraffin cracking alkylate the aromatics
and also form some aromatics by oligomerization.
Other Aromatics Produced by Sorbex Separations. Some other
applications of the Sorbex zeolite-based simulated-moving-bed
technology (36) are MX the Sorbex process, m-xylene from EB
and o- and p-xylenes; the Cymex process, m- and/or p-xylene
from a mixture of cymene isomers; and the Cresex process, m-
and/or p-cresol from mixtures of cresol and xylenol.
Petrochemicals Processing for Olefins Production
Light Olefin Production by Methanol-to-Olefins (MTO) Pro-
cess. Only ~110 of the ~2,500 billion cubic meters of natural gas
produced annually is wasted (burned in flares). About 103 billion
cubic meters per year of natural gas are processed to make
liquefied natural gas, but at high cost. Altematively, natural gas
can be converted first to syngas (CO and H2) and then to the
more-valuable, easily shipped methanol. However, the methanol
market is too small for the available natural gas.
Proc. Natl. Acad. Sci. USA 96 (1999J
The new UOP/HYDRO MTO process provides the means to
efficiently convert methanol to even more valuable light olefins
(ethylene, propylene, and butenes), which have large, commod-
ity-type petrochemical markets: Ethylene and propylene repre-
sent the largest, together accounting for 120 million MTA, and
growing (37~. This process uses the product-shape-selective UOP
MTO-100 catalyst based on a unique molecular sieve. During the
late 1980s, Norsk Hydro, assisted by Sintef, started independent
work, and UOP and Hydro agreed on joint development of the
process, now available from UOP for commercial licensing.
Norsk Hydro is running a large (0.75 metric tons/day) UOP/
HYDRO MTO demonstration plant in Porsgrunn, Norway.
Olefin Isomerization. The 1990 Clean Air Act increased the
demand for blendable ethers in motor fuels and created a demand
for isobutene to make methyl tertiary butyl ether and for isopen-
tene to make tertiary amyl methyl ether. In anticipation, the UOP
I-500 catalyst, based on a SAPO structure, and two new processes
were developed: Butesom for isobutene isomerization and Pen-
tesom for pentene isomerization (38-41~. In both processes, coke
progressively accumulates on the catalyst and is periodically
removed by a simple carbon burn-off in the reactor. The Lyondell
IsoPlus process (42, 43) uses a ferrierite (FER) zeolite for the
isomerization of olefins to isoolefins, and a Mobil patent (44)
describes using a medium-pore zeolite catalyst (for example,
ZSM-5) for similar applications.
Oxygenates Removal Unit. Zeolite adsorbents are used in a
UOP oxygenate removal unit down to >1 ppm total of trace
oxygenates (e.g., DME, methanol, and methyl tertiary butyl
ether) from C4 streams. Depending on the flow scheme, the C4
stream generally goes to a motor fuel alkylation (sulfuric acid or
hydrofluoric acid) process or is recycled to a dehydrogenation-
etherification complex, which has a UOP Oleflex unit and a
methyl tertiary butyl ether unit. The advantages of the oxygenate
removal unit is that it minimizes the acid consumption otherwise
associated with these oxygenates, thus minimizing the acid neu-
tralization wastes, a significant environmental benefit (B. V.
Vora, personal communication). In dehydrogenation, the oxy-
genate removal unit improves catalyst stability and lowers costs of
methyl tertiary butyl ether production.
Petrochemicals Processing for Detergents Production
Linear ParafD'ns for Biodegradable Detergents. Petroleum
derivatives account for most of the total surfactant production
and household detergents. During the 1940s and 1950s, sodium
dodecylbenzene sulfonate was the most widely used synthetic
detergent. However, the dodecyl paraffin side group on the
benzene ring is highly branched and not easily biodegraded. In the
early 1960s, environmental concerns led to development of linear
allylbenzene sulfonate (LAS) detergents, which are both biode-
gradable and cost-effective.
The key to the manufacture of the linear paraffins required to
make linear alkylbenzene (LAB) and, hence, LAS is the use of
size-selective synthetic zeolites that adsorb linear paraffins but
exclude branched paraffins, naphthenes, and aromatics from
mixtures spanning a range of boiling points, as in kerosene (C~2
to C~. Although anticipated by the work of McBain (7) and
Barrer (8-10), such a class separation of molecules spanning a
range of boiling points was virtually impossible before develop-
ment of the synthetic molecular sieves by Union Carbide in the
1950s. Two different processes, one vapor phase and the other
liquid phase, are used.
The vapor-phase IsoSiv process was developed at Union Car-
bide originally for octane improvement. To produce linear par-
affins for detergents, kerosene feed, pretreated to acceptable
quality and the desired carbon number range, passes at elevated
temperature and just over atmospheric pressure through a bed of
zeolite adsorbent that adsorbs just the linear paraffins. Just
enough hexane vapor follows the kerosene feed to displace the
nonadsorbed feed and isomeric hydrocarbons from the void
spaces in the adsorber vessel. The effluent from this step is
OCR for page 3475
Colloquium Paper: Sherman
combined with the adsorption effluent stream. The linear par-
affins adsorbed in the zeolite are desorbed by purging the bed in
the opposite direction with hexane. The hexane in the effluent is
separated by distillation and is recycled. The remaining linear
paraffins comprise the desired product.
The liquid-phase Molex process, mentioned earlier, is most
often used to produce plasticizers (C6-C~O), LABs (C, 0-C~s), and
detergent alcohols (C~3-C22+, but usually heavier than Cud. To
make linear paraffins for LAB, increased linearity and low
aromatics content are desired. The new high-purity Molex pro-
cess has improved product purity to 99.7% and reduced aromatics
content to 0.05 wt %. In addition, the new OF ADS-34 zeolite
adsorbent provides improved long-term separation performance
in the Molex process.
The linear paraffins made in the Molex process can be sent to
UOP's Pacol and DeFine processes (45) for catalytic conversion
to monoolefins. These pass to a UOP Detergent Alkylate process
(46>, which uses a hydrofluoric acid catalyst, or to a Detal process
(offered for license in 199511), which uses a more environmentally
friendly solid, heterogeneous catalyst to produce LAB from the
monoolefins plus benzene. Unreacted linear paraffins are recy-
ded to the Pacol and DeFine units and are converted to extinc-
tion. These catalytic processes use nonzeolite catalysts.
Linear Olefins for Detergent Alcohols. As discussed previ-
ously, LAB accounts for half of the detergent intermediate
market. Detergent alcohols made from linear olefins are another
quarter. Detergent alcohols are made from C~o-Ci5 alpha olefins
derived from ethylene or from C~o-C~s internal olefins derived
from lower-cost kerosene feed.
Linear olefins of improved purity are increasingly sought.
Linear paraffins from the new high-purity Molex zeolite adsorp-
tion process are sent to the Pacol and DeFine processes to convert
them to a mixture of monoolefins plus unreacted linear paraffins.
The mixture is fed to UOP's Olex process, which uses a zeolite
adsorbent in a Sorbex simulated-moving-bed process to separate
the linear olefins and the unreacted paraffins, which are recycled
back to the Pacol and DeFine units to extinction. The Olex
process now uses the new UOP ADS-32 zeolite adsorbent, which
provides improved capacity and rates. The linear olefins product
has improved product purity (reduced aromatics and diolefins) as
a result of improvements in the zeolite adsorbents and in the
nonzeolite catalysts and operating conditions in the Molex-Pacol-
DeFine-Olex sequence of processes.
Separation and Purification Process Applications
Molecular sieve adsorbents are used in many other separation
and purification applications: (i) petroleum refining processes,
used to remove CO2, chlorides and mercury from a variety of
streams; to dry and purify liquids and gases in diverse applica-
tions; to treat alkylation unit feed to reduce acid consumption,
regenerator use, and corrosion, and to treat refinery hydrogen to
prevent corrosion in downstream equipment; to dry and desul-
furize refined products; and to dry and purify feed and recycle
hydrogen in isomerization units; (ii) petrochemicals, used to dry
hydrocarbon liquids, cracked gas, and hydrogen; to dry and purify
natural gas liquids, ethane and propane feedstocks in ethylene
and polymer plants; and in ethylene, propylene, butadiene,
bu~lenes, amylenes, and various other comonomers and solvents;
(iii) natural gas treating, used to dry and desulfurize natural gas
to protect transmission pipelines and to remove undesirable
impurities from home cooking and heating gas and to desulfurize
ethane, propane, and butane and for H2O and CO2 removal
before cryogenic processing; (iv) industrial gas production and
purification, used to remove H2O and CO2 from air before
Imai, T., Kocal, J. A. & Vora, B. V., Second Tokyo Conference on
Advances in Catalytic Science Technology, August 21-26, 1994, Tokyo.
Proc. Natl. Acad. Sci. USA 96 (1999) 3475
liquefaction and separation by cryogenic distillation, for pressure
swing adsorption (PSA) separation of air, and in PSA purification
of hydrogen by using zeolites and other adsorbents, such as
activated carbon; (v) specialty and fine chemicals and pharma-
ceuticals, used for drying; for removal of impurities, including
odors; and for other applications in the manufacture of specialty
and fine chemicals and pharmaceuticals. Common features of
these uses are now summarized.
Preprocessing of Gases before Cryogenic Separations. Deep
drying and CO2 removal are required before cryogenic liquefac-
tion and subsequent separation processing to prevent formation
of ice and dry ice, which would plug up the cryogenic processing
equipment. Several synthetic zeolites exhibit great affinity for
polar compounds such as H20 and CO2 and have high adsorption
capacity at ambient temperature. They are used extensively in
processing natural gas to make liquefied natural gas or to recover
hydrocarbon liquids or helium; in processing air to make 02, N2,
and Ar in cryogenic air-separation plants; and in treating ethylene
and other olefins formed in ethylene steam-cracking plants before
separation in cryogenic distillation separation units. These pre-
treatments work to perfection: Passing ambient air over such
zeolite adsorbents at room temperature makes the air drier
(-60°C dew point or lower) than in the coldest part of Alaska in
the depth of winter.
Because the adsorbed impurities are strongly held on the
zeolite adsorbents, they are regenerated for subsequent reuse in
thermal-swing processes that pass a hot regeneration gas over the
spent zeolite to heat it and to carry away the adsorbed com-
pounds. The zeolite then is cooled to ambient temperature and
is used to treat more gas.
Removal of Impurities from Gases and Liquids Down to Low
Levels. Because zeolites bind strongly to polar compounds,
including hydrogen sulfide, mercaptans, organic chlorides, CO,
and to mercury, they can purify many streams in petroleum
refineries, petrochemical plants, natural gas production plants,
and chemical plants. In refineries, zeolite adsorbents remove
impurities detrimental to downstream processing, including cat-
alyst poisons (e.g., oxygenates and sulfur), corrosive agents, and
chloride compound byproducts from processes using chloride
catalyst promoters (e.g., catalytic reformers).
In natural gas production, zeolite adsorbents are used to dry
the gas to prevent freezing and corrosion in pipelines, to remove
sulfur compounds from the gas or LPG fractions to prevent
corrosion in burners, and to remove compounds that are obnox-
ious or toxic (such as the odoriferous hydrogen sulfide and
mercaptans in natural gas that form sulfur dioxide pollutants
when bumed for home cooking and heating). Worldwide, >1,000
units process tens of billions of cubic feet of natural gas daily.
Zeolites are used in the preparation of very high-purity fluids
for special uses: e.g., gases used in the manufacture of electronics
or gases and liquids used in modern analytical laboratory instru-
ments.
Air Separation by Pressure Swing Adsorption (PSA) Pro-
cesses. The following draws primarily from reviews (47-52) plus
the author's personal reflections from direct involvement in
zeolite adsorbent development over the last 30 years. Many
zeolites adsorb N2 more strongly than O2 tthe possible use of
zeolites in air separation was indeed the principal impetus for the
pioneering work of Milton (11~. Also, because zeolites adsorb
more of both N2 and O2 from air with increasing pressure, air can
be separated by using a PSA process. The air is passed at an
elevated pressure through a bed of zeolite particles that adsorb
the N2 more strongly and hold it on the bed but allow O2 to pass
through the bed. Then, the adsorbed N2 is discharged from the
feed end of the bed as the pressure in the bed is lowered. Many
variations on the process cycle were developed to improve
efficiency and capital and operating costs.
The PSA and vacuum-swing adsorption (VSA) processes use
zeolite adsorbents to produce O2 of 90-94% purity (the balance
is primarily argon). The O2 is used, for example, in the manu
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3476 Colloquium Paper: Sherman
facture of steel, glass, pulp, paper (in delignification and bleach-
ing), and chemicals and in nonferrous metal recovery, waste
incineration, and bioremediation. Zeolites provide benefits in
energy efficiency, process efficiency, improved processing rates
and product quality, and environmental impact.
Over the last 25 years, improvements in the PSA and VSA O2
processes were driven by the development of zeolite absorbents
with improved N2 capacity and selectivity. Zeolites such as NaX
and CaA made possible the development of the first economical
PSA O2 process at a relatively small scale tup to ~15 U.S. tons of
O: per day (tpd)] in the early- to mid-1970s. Second-generation
absorbents, such as CaX (53), and third-generation absorbents,
such as LiX (54), LiCaX or LiSrX (55, 56), and MgA (57),
together with improved (vacuum PSA or VSA) processes have
dramatically reduced both capital and operating costs. Of the
third-generation Type X zeolites, only LiX has been used com-
mercially as of 1997.* *
From the mid-1980s to the mid-199Os, these improvements
provided a 5-fold reduction in adsorbent inventory and a nearly
2-fold reduction in power requirements. The commercial viability
of a simple two-bed VSA system expanded to well over 100 tpd,
and that of an even simpler one-bed system expanded to well over
40 tpd, allowing use of these noncryogenic systems in many
applications formerly served by the cryogenic distillation of air.
For the delivery of 02 of 90-94% purity, single-bed units are
more economical because of lower capital costs (although with
higher energy costs) than liquid O2 delivery in the 4-57 tpd range.
Two-bed units have lower energy costs (but higher capital costs)
and are more economical than either liquid O2 delivery or on-site
cryogenic plants in the 57-235 tpd ranged In 1994, VSA plus
PSA O2 production was estimated to be 4-4.5% of the world
demand for O2, the fourth largest chemical at 39 billion pounds
in 1995 (51~. In 1996, PSA/VSA O2 production was estimated to
be >3,500 tpd in the USA and >10,000 tpd (>265,000 Nm3O2 per
hour) worldwide (47~. Assuming a value of $20 per ton of O2, this
production corresponds to a total market value of more than $75
million per year, and growing.
Manufacturing Industries and Consumer
Products Applications
Small Oxygen Concentrators for Medical Use (Medox). In the
U.S., a dozen companies manufacture small-scale PSA oxygen
concentrators for patients with emphysema and chronic obstruc-
tive pulmonary disease. As with the large-scale PSA O2 units,
these small PSA concentrators use zeolites to produce 90 and
95% pure oxygen, the balance mainly argon and nitrogen. They
can dramatically improve the quality of life.
The PSA units are engineered to be small (about the size of a
small end table), readily transportable, (weighing ~40 pounds),
quiet, and reliable. They use 3-7 pounds of zeolite adsorbent to
produce between 3 and 6 liters per minute of oxygen. Users are
freed from needing high-pressure cylinders of oxygen delivered or
stored in their homes. Use of these concentrators has grown
substantially over the last 20 years (S. R. Dunne, personal
communication). In 1996, home medical equipment reimburse-
ments (from U.S. Medicare) associated with oxygen concentra-
tors totaled $1.1 billion (58~. Most are PSA units, the rest being
primarily membrane units, which produce much lower oxygen
concentration.
Automotive Air-Conditioning and Stationary Refrigerant Dry-
ing. Zeolite desiccants remove water and acids formed by break-
down of the refrigerant mixed fluid thus protecting the system
from freeze-up and corrosion. UOP supplied zeolite 4AXH-5
desiccant for automotive use and 4A-XH-6 for stationary refrig-
erant drying. These desiccants dominated the refrigerant industry
* *Notaro, F., Schaub, H. R. & Kingsley, J. P., Second Joint China/U.S.
Chemical Engineering Conference, May 22, 1997.
TiNotaro, F., Schaub, H. R. & Kingsley, J. P., Second Joint China/U.S.
Chemical Engineering Conference, May 22, 1997.
Proc. Natl. Acad. Sci. USA 96 (1999'
for use with refrigerants R-12 and R-22 and the associated
mineral oil lubricants. However, the 1987 Montreal Protocol
heralded their demise because the very long life of fugitive R-12
emissions in the atmosphere became linked to ozone depletion
and global warming.
The leading contender to replace R-12 was R-134a refrigerant,
but R-134a was found to be unstable in the presence of the
4AXH-5 desiccant, leading to acids, sludge, deterioration of the
desiccant, and possible failure of the refrigerant system. A UOP
team developed the new XH7 zeolite desiccant, which is com-
patible with the new R134a lubricant systems, to meet the critical
legislated deadlines. The deadlines for original automotive equip-
ment manufacture and fleet testing were set at 3 years before
system production, an extremely short development time for such
a complex application.
Another new zeolite desiccant, XH9, was developed a couple
of years later; in addition to automobiles, it is widely used in
refrigerators (home refrigerators, supermarket freezers, and dis-
play cases) and stationary air conditioners. Because XH7 and
XH9 desiccants are also compatible with systems using R12
refrigerant and mineral oil lubricants, dryers using the new
desiccants can be profit into R12 systems before total conversion
to R134a systems. The XH7 desiccant today holds almost all of
the automotive air-conditioning market formerly held by the
4AXH-5 desiccant before the advent of the new refrigerants.
Consumers benefited because the availability of current systems
was not disrupted and any danger to the environment was
alleviated: hence, the American Chemical Society Heroes of
Chemistry Award in March 1998 to the UOP team (A. P. Cohen,
S. L. Correll, P. K. Coughlin, and J. E. Hurst).
Worldwide, millions of pounds of zeolite desiccants are in-
stalled in air conditioning units in passenger cars and light trucks.
For stationary systems, most refrigerators in the U.S. and many
elsewhere use zeolite desiccants to dry and remove acids from the
refrigerants. The new desiccants, together with hermetically
sealed systems with internal pumps, have extended the service life
of refrigeration units by at least two to three times.
Air Brake Dryers for Heavy Trucks and Locomotives. Most
goods produced worldwide are moved to market by heavy trucks
and locomotives whose brake systems are actuated mostly by
clean dry air at high pressure. Air brake systems are engineered
to be fail-safe: When the air supply system fails, the brakes engage
and prevent the trucks or locomotives from moving.
A key element of the air supply systems of a truck is a PSA
dryer, typically using a single packed bed of ~3 pounds of
molecular sieve to dry the compressed gas; locomotives require
more absorbent. The air compressor on a truck runs for 1-3 min
at a time. The compressed gas is dried and passed to a reservoir
that in turn supplies air pressure to the brakes to prevent the
unintended actuation of the brakes while the truck is moving.
When the reservoir is full, a signal shuts off the compressor; the
dryer is Repressurized, and a little dry air is bled back through the
dryer to partially regenerate the bed of molecular sieve (S. R.
Dunne, personal communication). These dryers have signifi-
cantly improved the reliability and safety of braking systems for
large trucks and locomotives.
Insulated Glass Windows. Most insulated glass produced
worldwide is manufactured with desiccant contained in channels
(or in matrices) that separate the panes of double-, triple-, or
quadruple-paned windows. The desiccant scavenges moisture
and other trace compounds, such as solvents or plasticizers, that
may evolve during manufacturing. Although the sealants used for
the manufacture of insulated glass windows are excellent, a finite
amount of moisture still leaks into the windows over time.
Desiccants, primarily zeolites, prevent fogging, mists, or forma-
tion of dew between the windowpanes because they lower the dew
point of the gases inside the windows to levels far below the lowest
expected surface temperature of the glass. Insulated glass pro-
vides aesthetic features, improved human comfort, and energy
savings that make them a truly economical and beneficial addi
OCR for page 3477
Colloquium Paper: Sherman
lion to both commercial buildings and homes (S. R. Dunne,
personal communication). Residential and nonresidential dual-
pane (insulating) windows and patio doors containing zeolite
absorbents have a total window area of ~46 billion square feet
worldwide. The estimated present energy savings in heating
during winter and cooling during summer from the use of these
insulating windows is equivalent to 450 million barrels of oil per
year.
Environmental Protection Applications
In addition to the benefits already listed, many other applica-
tions provide environmental benefits.
Builders for Phosphate-Free Laundry Detergents. In the
1960s, growing public awareness of eutrophication of natural
waters led to efforts to reduce the inflow of plant nutrients,
especially phosphate and ammonia or nitrate. Dead algae sinks to
the bottom of a pond or lake, where it depletes the oxygen in the
water. Too much growth of algae depletes oxygen so much that
fish die. As a result, many states, particularly those bordering the
Great Lakes, banned phosphate in laundry detergents.
The prime function of phosphate "builders" in laundry deter-
gent powders is removal of the hardness ions Ca2+ and Mg2+ in
the wash water by complexing. Zeolite ion exchangers in powder
form also can provide this service by removing Ca2+ and Mg2+
ions from the solution and replacing them with soft ions such as
Na+. Zeolite NaA was known to have high selectivity and
capacity for calcium, and its application as a builder in heavy-duty
powder detergents was developed in the 1970s, primarily by
scientists at Henkel (59, 60, ft) in Germany and Procter and
Gamble (61, 62) in the U.S. Round NaA zeolite particles, a few
micrometers across, are small enough to pass through the open-
ings in the weave of the fibers in clothing and are not filtered out
to form encrustations on the cloth. Recently, zeolite P (GIS), as
maximum aluminum P or MAP, was developed by scientists at
Unilever (and Crossfield) (63) as an alternative builder for the
same applications, and debate on the relative merits of NaA and
NaP zeolites continues (64~. Today, the conversion of the USA
detergent market to zero-phosphate formulas is virtually com-
plete. In Europe, one-third of the powder detergents are zeolite
based, and Canada is ~50% converted. Latin America and many
of the Pacific region countries continue to use phosphates (654.
In 1987, the Kao Corporation in Japan introduced Attack, a
compact powder detergent that has higher bulk density and
higher surfactant level and needs lower dosage. Use of compact
powders in the Japanese laundry market grew to >90%. In the
U.S., from 1990 to 1994, the use of compact powders grew from
2% to >90%. All compact powders in the U.S., Europe, and
Japan have no phosphates. Zeolites used as builders in compact
powders serve as particle-formation aids. This use of zeolites has
facilitated changes in the process of detergent manufacture from
spray drying and to alternative processes such as agglomeration.
This shift, in turn, has led to increased use of zeolites in laundry
detergent powders.
Automotive Emissions Control. New stable zeolites have been
used successfully in diverse automotive emissions control prob-
lems. A very serious challenge in automotive emissions control
today is control of nitrogen oxides (NOX) emitted from lean-bum
diesel engines. Many catalyst developers and academic research-
ers are using zeolites with a wide array of added base and precious
metals as catalysts to enable hydrocarbon storage, NOX reduction,
and oxidation of both hydrocarbons and CO. Today, zeolites are
used commercially for enhanced hydrocarbon oxidation in con-
ventional diesel engines and NOX reduction. Four-way catalysts,
which provide NOX reduction, HC oxidation, CO oxidation, and
particulate control, are being developed.
tTSchwuger, M. J. & Smolka, H. G., 49th National Colloid Symposium,
June 1975, Clarkson College, Potsdam, NY.
Proc. Natl. Acad. Sc'. USA 96 (1999) 3477
For gasoline-fueled vehicles, the most-serious problem is the
cold-start period. Over 90~o of the hydrocarbon emission by a car
during a cold start occurs within the first 3 min of engine
operation. A hydrocarbon trap must contain an adsorbent that
captures most of the hydrocarbons during this period. Once
captured, most of the hydrocarbons must be held by the adsorber
until the catalytic converter has heated up enough to be capable
of oxidizing them. Then the adsorber must release the hydrocar-
bons to be oxidized, rendering them harmless to the atmosphere.
The adsorber also must be mechanically, thermally, and hy-
drothermally durable enough to withstand the harsh environment
of the exhaust gas stream. Especially since 1995, major advances
have been achieved in the development of hydrocarbon traps.
Improved hydrothermal stability of the molecular sieve adsor-
bent, improved chemistry of the wash coating, and the addition
of a noble metal catalyst directly onto the adsorber brick are
technical milestones that enable successful implementation of
hydrocarbon traps in emissions control (S. R. Dunne, personal
communication).§§
Several major automakers have demonstrated excellent
emissions reduction and system durability. General Motors
achieved 50,000-mile-aged converter performance that sur-
passes Environmental Protection Agency requirements for use
on a car designated as a low-emissions vehicle: i.e., a vehicle
must emit nonmethane hydrocarbons at a weighted rate
<0.075 grams per mile in the U.S. federally mandated test
protocol. Mercedes Benz has achieved emissions that beat the
ultralow-emissions vehicle standards of <0.04 grams per mile
in the same testing (S. R. Dunne, personal communication).
The total U.S. car and light truck production rate is ~12 million
vehicles per year. Over the next 10 years, emissions control
technologies of all kinds will be implemented to keep the auto
manufacturers in compliance with the law. Europe has a larger
vehicle production rate and more eligible vehicles.
Radioactive Waste Management. A special use category that is
small in both the quantity of both molecular sieves and the gases
and liquids processed is radioactive waste management, in which
zeolites and other new molecular sieve ion exchangers (66, 67),
adsorbents, and catalysts have been used for >25 years. Although
only small quantities are used, the past and future environmental
benefits are large indeed.
UOP's IONSIV zeolite ion exchangers were used for the
radioactive waste cleanup at l~hree Mile Island; the West Valley
commercial nuclear fuel reprocessing site; and the Hanford,
Savannah River, Oak Ridge, and other U.S. Department of
Energy nuclear waste storage sites (67~. Most recently, and of
special interest, is the current use of the new IONSIV IE-911
crystalline silicotitanate (CST) ion exchangers for the cleanup of
the radioactive wastes in the Melton Valley tanks at Oak Ridge
(68) and the planned use elsewhere in similar and other appli-
cations. The effectiveness of CST was discovered (69) by re-
searchers at Sandia National Laboratories and Texas A & M
University, and its further product and manufacturing process
development and commercial manufacture was carried out in
1994-1995 by UOP under a Cooperative Research and Devel-
opment Agreement with Sandia (7O, 71~. In 1996, this work
earned an R&D 100 Award for the Sandia, Texas A & M, and
UOP researchers (72~.
Other Smaller Environmental Applications. Many smaller
applications of zeolites in catalysis and adsorption, although
important and beneficial to mankind, are not discussed here
because of space limitations. Noteworthy are the growing uses of
zeolites and other molecular sieves as adsorbents in separations
of volatile organic compounds and other pollutants, in desiccant
cooling and dehumidification, and as ion exchangers for pollution
abatement and toxic waste management.
§§Dunne7 S. R. & Taqvi7 S. M.7 AIChE Annual Meeting7 Session on
Environmental Catalysis7 November 19977 Los Angeles7 CA.
OCR for page 3478
3478 Colloquium Paper: Sherman
Conclusions
Likely Future Applications. Use of zeolites as catalysts in the
manufacture of some fine chemicals should expand. New Zeolite
and other microporous oxide catalysts should be developed with
improved selectivity and new functionalities, perhaps for strong
base and oxidation catalysis, chiral synthesis, and possibly, mem-
brane reactors. Desiccant cooling and dehumidification, and
sorption heat pumps, may achieve serious success. New ion
exchange applications of new microporous oxides also may be
expected. Experience has taught that the availability of new
materials normally precedes by many years the discovery of all of
their useful properties and the conception and development of
new uses.
Impacts of Molecular Sieves on Human Welfare. From these
numbers, Roth estimated the value of fuels and chemicals pro-
duced using catalysts in 1989 to be $891 billion per year, or 17%
of the U.S. gross national product, and judged the corresponding
worldwide product values for fuels and chemicals to be $2.4
trillion per year (73~. Of course, not all catalysts are based on
zeolites, but for petroleum-based fuels and petrochemicals, most
catalysts are now zeolitic. Thus, the impact of zeolites in these
areas is clearly great. Likewise, the use of zeolites in catalytic
converters to reduce undesirable emissions from vehicles also
represents a significant present market (with large growth po-
tential) and substantial benefits to mankind in pollution abate-
ment.
As described earlier, synthetic zeolites play critical roles in the
production of fuels, petrochemicals, and other products essential
to modern societies; in pollution avoidance or abatement; in
energy efficiency; and in the efficient use of natural raw materials.
They also contribute to the quality and performance of the
ultimate products because of the greater purity and uniformity of
the intermediates made by using zeolites.
The many benefits achieved from the applications of zeolites
and other molecular sieves are the fruits of the basic investments
made decades ago, and into the present, In the research areas of
mineralogy, geology, geochemistry, structure and properties of
natural zeolites, exploratory materials synthesis, materials char-
acter~zation methods and their application, and exploratory re-
search on structure, property, and functionalities. The basic
concepts and understanding from these efforts, coupled with
creative consideration of how the properties and functionalities so
discovered might be of service to solve the needs of mankind,
continue to create new benefits.
We all contribute by standing on the shoulders of giants. In the field of
synthetic zeolites and related matenals, and their applications, my per-
sonal giants include R. M. Barrer, R. M. Milton, D. W. Breck, E. M.
Flanigen, J. A. Rabo, L. B. Sand, P. E. Pickert, C. G. Gerhold, D. B.
Broughton, G. T. Kerr, G. H. Kuehl, J. J. Collins, G. E. Keller, W. M.
Meier, and J. V. Smith. In various ways, the prior work of each has had
specific impact on my own work over the years. I also gratefully acknowl-
edge the contributions of my many friends and colleagues at UOP,
especially P. T. Barger, J. C. Bricker, A. P. Cohen, N. A. Cusher, S. R.
Dunne, G. J. Gajda, S. H. Hobbs, J. A. Johnson, D. C. Kaminsky, P. J.
Kuchar, R. L. Patton, M. W. Schoonover, B. V. Vora, S. T. Wilson, and
C. M. Yon, the thoughtful secretarial support of Sharon Lambert, and the
skilful editorial support of Sandy Weiss. I thank J. V. Smith and the
National Academy of Science for organizing this colloquium and inviting
this contribution and thank UOP for supporting this endeavor and many
others. Finally, I thank my wife, Carol, for patience and understanding.
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Representative terms from entire chapter:
synthetic zeolites
