Going Green with Zeolites - These inorganic materials are playing an increasing role in tackling environmental challenges.
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Going Green
with Zeolites
arely in our technological soci- There are 40 known naturally occurring
ety does the discovery of a new zeolites and more than 150 synthetic
These inorganic class of inorganic materials re- ones.
.. sult in such a wide scientific in- Zeolite structures can be visualized by
materials are terest and kaleidoscopic development of taking a neutral SiO, framework and iso-
applications as has happened with the ze- morphously substituting A10,- for SO,.
playing an olite molecular sieves,” declared Donald The resulting structure (Figure 1) then ex-
W. Breck in 1974 (1). That was twenty hibits a net negative charge on the frame-
increasing role years after the commercial introduction work aluminum. This negative charge is
of synthetic zeolites. During that time, balanced by cations (for instance, Na+,
in tackling the number of areas utilizing zeolites had K+, or NH4+) that reside in the interstices
grown phenomenally. Since then, the va- of the framework. Many of these cations
environmental riety of applications and zeolite types are mobile and free for exchange. This
available has continued to increase. In- ion-exchange property accounts for the
challenges. deed, growth seems to be limited only by greatest volume use of zeolites today. For
the imagination of the people looking for instance, Zeolite A, synthesized with
solutions. sodium as the cation, has widely replaced
Zeolites, already well established in environmentally harsh phosphates as de-
such environmentally driven applications tergent water softeners; it functions by
as the production of lead-free octane en- exchanging its sodium for the calcium
hancers for gasoline and as phosphate- and magnesium ions that cause hard
free ion exchangers for detergent water water and poor laundry performance.
softening, now are being used or consid- The type of cation present not only in-
ered for a variety of other important envi- fluences the ion-exchange properties of a
ronmental services. To put the potential zeolite, but also is a factor in its adsorp-
of zeolites into DersDective. this article
L A
tive and catalytic properties. High-purity
looks at the properties of zeolites, and synthetic zeolites exhibit uniform pore
Bonnie K. Marcus sizes that can be further tailored to specif-
how the materials can be used.
and William E. Cormier, ic molecular dimensions by changing the
Zeolyst International Zeolite properties nature of the cation after synthesis. Some
Zeolites are members of a family of zeolite properties that are determined dur-
minerals called tectosilicates, which in- ing synthesis include:
cludes dense-phase materials such as the structure;
feldspars and the various forms of silica. silica-to-alumina ratio;
Zeolites are microporous, high-internal- pore size; and
surface-area crystalline materials with an framework density (that is, atoms
open, three-dimensional framework con- per unit cell).
sisting of tetrahedral A104-5 and The pore size is the two-dimensional
units linked through shared oxygens. opening of the zeolite and is determined
CHEMICAL ENGINEERING PROGRESS JUNE 1999 47ONTHF H O W O N
-
A I - - 0 Si - 0 - A I - - 0 - Si - 0 - A I - Range
2.2-8 8,
6.6-1 1.8
500-1,000”C
Ion-exchange capability Up to 700 milliequivalents/lOO g
M Figure 1. Zeolite structure.
Up to 900 m2/g
I1 to =25 wt.%
by the number of tetrahedral atoms
joined together. The structure is built
up further by connecting the tetrahe-
dral atoms in a three-dimensional quaternary ammonium compounds as tions except the time of formation,
array. This array can lead to larger structure-directing agents. This re- which is thousands of years in nature.
inner cavities connected by pore sults in materials with dramatically Commercially, zeolites must be pro-
openings. In some zeolites, there are different properties and many new duced in hours or days; this, thus, re-
no cavities, but a series of one-, structure types. quires optimizing the other variables
two, or three-dimensional channels Environmentally benign zeolite to change the window of formation.
through the structure. catalysts have wide potential as solid Progress in zeolite synthesis has
In addition, various post-synthesis acids in many commercial processes, been ongoing since commercial zeo-
modifications, such as hydrothermal due in part to the ability to “tune” lites were first introduced 50 years
treatment, coating, and impregnation, their acidity. In addition, zeolites can ago. Most zeolites are made from the
may further alter catalytic and ad- be formulated to carry active materi- same reagents: alumina, alkali
sorptive properties. als such as catalytic metals. These cations, and silica. Small variations in
Because of this ability to cus- properties, coupled with the size and conditions can cause major differ-
tomize properties, zeolites are com- shape features of the zeolites, allow ences in the structures that form. In-
mercially valuable as adsorbents and the materials to be used as catalysts deed, in a number of cases, new zeo-
molecular sieves - selectively ad- in extremely selective reactions, such lites have been discovered while at-
mitting some molecules while ex- as in the manufacture of para-xylene. tempting to make known zeolites. A
cluding others whose size, shape, or High silica zeolites’ ability to remove classic zeolite example is Zeolite X,
polarity preclude adsorption. hydrophobic organic compounds which was first made inadvertently in
The first generation of zeolites from many environments is expand- an attempt to repeat the synthesis of
(such as A, X, Y, and mordenite) gen- ing their use as specialty adsorbents Zeolite A.
erally have a low silica-to-alumina in numerous fields. Some of the factors that determine
ratio and a high ion-exchange capaci- The ability to tune a number of the type of synthetic zeolite produced
ty. Most boast a high affinity for other properties of zeolites (see Table are (2):
water and, so, are widely used as des- 1) is adding to the interest in the ma- composition of the gel;
iccants. They also are employed to terials. Another attraction is that zeo- silica and alumina source;
adsorb other polar molecules in sepa- lite powder can be formed into extru- other materials present (such as
ration and purificdon applications. dates, beads, monoliths, and other OH- and other anions, cations (inor-
Second generation zeolites (e.g., shapes. ganic or organic), and organic
ZSM-5 and silicalite) take advantage reagents);
of quaternary ammonium ions and al- The subtleties of synthesis time of reaction;
kali cations during synthesis to create The greatest challenge in the de- temperature and heat-up rate;
new structures and different chemical velopment and commercialization of pressure; and
compositions that are higher in silica- new and modified zeolites is their synthesis conditions (like the
to-alumina ratio. The highly siliceous synthesis. Natural zeolites are found order of mixing, gel aging, and
materials are effective for adsorbing worldwide and were formed by ig- stirring).
organic molecules, even in low-con- neous or sedimentary solution pro- The optimization of the synthesis
centration, high-humidity, and high- cesses. Some of the parameters that process for a known zeolite is not a
temperature applications. A third gen- controlled the type of zeolite formed trivial task and can involve many iter-
eration of alumino-silico-metal phos- are the composition and pH of the so- ations. To commercially synthesize a
phates (AlPO,, SAPO, MeAlPO, etc.) lution, as well as the temperature, new zeolite type, substantial expertise
is synthesized without alkali cations pressure, and time of formation. It is in both chemistry and chemical engi-
present, using organic amines and possible to replicate all of the condi- neering is required. Scale-up from the
48 JUNE 1999 CHEMICAL ENGINEERING PROGRESSZedies Used Advantages
Beta Excellent yields in continuous reactor
alpha-terpinyl alkyl ethers Eliminates use of HCI, H,SO,, AICI,, toluene,
sulfonic acid, boron trifluoride etherate, and
acidic cation resins as catalysts
Cumene synthesis Dealuminated mordenite; MCM-22; Lower impurities
beta; V; omega Trensalkylation function
Lower benzene-to-propylene ratio allows higher
capacity, greater unit efficiency
High selectivity
Regenerable, nonhazardous, noncorrosive
Direct oxidation of benzene to ZSM-5 Eliminates cumene as an intermediate
Enables possible use of M,O as oxidant
Caprolactam (via oxidation) Titanium-framework-substituted Dramatic reduction in number of processing
ZSM-5 (TS-1) steps and waste streams
Possible further reduction by using another
zeolite in the last step of the process
Gasoline from methanol ZSM-5 Produces methanol from coal, natural gas, or
biomass and then converts it into liquid fuel
Conservation of crude oil, elimination of many
waste streams
ities of zeolites can provide effective such as ozone and peroxyacetyl ni- ogy is “lean NO,,” a catalytic process
environmental solutions by minimiz- trate; and nitrous oxide, N,O, is of that uses methane and higher hydro-
ing the output of pollutants and by concern as a greenhouse gas, because carbons as reductants, making it ideal
secondary treatment of effluents pro- it absorbs in the infrared region. for automotive and other mobile
duced. Areas in which zeolites show NO,, naturally produced by light- sources (56).
strong environmental potential are: ning, bacterial action in the soil, and Just as vehicle engines and fuels
reduction of atmospheric oxides volcanic eruptions, also is generated are being redesigned to reduce the
of nitrogen (NO,,; by human activity - primarily the creation of pollutants, so, too, zeolites
cutting emissions of volatile or- combustion of fossil fuels at temper- are being developed and deployed to
ganic compounds (VOCs), including atures higher than about 1,OOO”C handle the new concentrations, com-
from cold starting of automobiles; (3). binations, and conditions of combus-
and Selective Catalytic Reduction tion products. For example, efficient,
process improvements in the (SCR), in which ammonia is added to oxygen-rich, lean-burn diesel engines
chemical process industries (CPI). stack gases as a reductant, is the tra- give off less CO, (a greenhouse gas).
Tables 2 and 3 list some of these ditional method of NO, removal at Metal-exchanged zeolite catalysts
opportunities. Now, let’s look at them stationary sources such as power have proven to be a less costly and
in more detail. plants. SCR is effective at lower tem- more effective option for NO, re-
peratures, but NO, conversions are moval from these engines’ exhausts
Reduction of better at higher temperatures (4). Cat- than three-way catalytic converters
atmospheric NO, alysts can extend the temperature (7,8).
Oxides of nitrogen are a major tar- range for NO, reduction (see Figure N,O is produced in large quanti-
get of clean air policies: NO, is a 2). Platinum and vanadium catalysts ties by nitric acid and adipic acid
major contributor to acid rain (rank- are effective up to 450°C; ion-ex- plants. Although many zeolites have
ing second only to sulfur com- changed zeolites are applicable to been used to decompose N,O at high-
pounds); NO, compounds foster the 600°C. er temperatures, cobalt or copper
creation of photochemical oxidants Another emerging zeolite technol- ZSM-5, mordenite, and beta zeolites
50 JUNE 1999 CHEMICAL ENGINEERING PROGRESSA variety of materials such as Process improvements ter streams, salts, and heavy metals.
carbons and silicas effectively ad- in the CPI Frequently, the catalyst is not
sorb VOCs in high concentration The applications of zeolites for se- reusable. Zeolites are being exten-
streams. As low concentration lective chemical synthesis are grow- sively studied as heterogeneous cata-
streams become more regulated, ing rapidly. Using the materials as lysts that can be recovered and recy-
however, more-selective materials catalysts contributes to the “green- cled with greater ease and less ex-
are needed. Combinations of materi- ing” of process technology in two pense, leading to less waste and fewer
als like carbon and zeolites, or mix- ways: first, by replacing many haz- byproducts (14). Their use also pro-
tures of zeolites, increasingly are ardous acidic catalysts such as HF, vides process advantages such as im-
being investigated for concentration HCl, and H,SO,; second, by eliminat- proved selectivity, higher activity, and
of VOCs. Once concentrated, the ing some intermediate steps in certain reduced corrosion. In addition, many
VOCs can be recovered for reuse or processes to reduce overall waste out- processes can gain efficiency and
thermally destroyed. New, more hy- put and energy use. economy by using zeolites to com-
drophobic zeolites such as ZSM-5 Fine chemicals. The production of bine several catalytic steps.
types, dealuminated faujasites, and flavors, fragrances, and other fine Even greater returns can be
betas are especially effective in re- chemicals often involves homoge- achieved by the use of zeolites in
moving VOCs from dilute or humid neous catalysts that generate large continuous reactors. For example, the
streams. quantities of wastes such as washwa- traditional synthesis routes for many
fragrances and flavors include strong ,
acids, many steps, low yields, and
much waste. Acidic zeolites have the
potential to improve such processes,
but they now are limited to batch re-
actors. The ability to form zeolites
into different shapes has improved
their compatibility with many reactor
designs, such as the one for alpha-ter-
piny1 alkyl ether (15). This continu-
ous reactor uses beta zeolites, and has
yielded excellent results with much
less waste.
Petrochemicals. The synthesis of
cumene is one example in which sev-
eral different zeolite catalysts have
found application, replacing environ-
mentally unfriendly catalysts such as
solid phosphoric acid. Zeolite cata-
lysts have been found to yield fewer
impurities, to have higher capacity, to
give greater unit efficiency, and to
provide higher selectivity in cumene
production. And, zeolites are nonhaz-
ardous, regenerable, and noncorro-
sive. Table 4 compares current com-
mercially available cumene synthesis
catalysts.
Cumene is an intermediate for
making phenol and acetone. Avoiding
its use promises further environmen-
tal benefits. The direct oxidation of - -
benzene to phenol reduces waste out-
put and the use of hazardous materi-
als. A number of companies have de- ~ ~- ~~~~
veloped processes with oxygen or ni-
tric acid as the oxidant. A recent de-
52 * JUNE 1999 * CHEMICAL ENGINEERING PROGRESSNaOH
NH3 H202 TS-1
1 1 1
Ammonium Sulfate
Cyclohexanone Cyclohexanone
Sodium Sulfate
I
.L J
NH3 NH3
-+ Ammonium Sulfate -+ Ammonium Sulfate
Oleum Oleum
$. t
Raw Caprolactum Raw Caprolactum
(-83%) (-90%)
a. Raschig Process b. Enichem Process
Figure 3. How Raschig and ammoximation routes to caprolactam compare.
velopment employs N,O as the oxi- er way in which waste may be re-
dant in a direct oxidation process duced and natural resources con-
with a zeolite as catalyst (16). Com- served. The Mobil methanol-to-gaso-
mercialization of this process could line process, now commercialized in
cut waste and energy consumption, New Zealand, is one such process.
and would improve the economics of Methanol may be produced from
phenol production, as well. coal, natural gas, or biomass, and
Zeolites also can reduce wastes in then converted to liquid fuels or
the production of caprolactam, an in- chemical feedstocks using a zeolite
termediate for nylon-6 and other syn- catalyst (18). Future possible uses of
thetic fibers. The traditional Raschig zeolites might include conversion of
process generates multiple waste flare gases at remote wells into a
streams. In contrast, the Enichem more easily transportable, waxy syn-
ammoximation process, which uses a crude that could be reprocessed into
titanium-framework-substituted fuels where needed.
ZSM-5 zeolite (TS-l), hydrogen per-
oxide, and ammonia, dramatically re- A green light
\ duces the number of process steps Additional methods for manipu-
lating zeolite crystalline structure
and the volume of waste (17) (see
Figure 3). Employing another zeolite continue to be developed, and zeolite
in the last step of the Enichem pro- applications are expanding exponen-
cess may lead to further cuts in waste tially. Zeolites’ chemical inertness
production. and unique, modifiable properties
The use of cheaper and more read- make them ideal for green chemistry
ily available feedstocks is still anoth- applications. na
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