Going Green With Zeolites

7/27/2019 Going Green With Zeolites

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by the number of tetrahedral atomsjoined together. The structure is builtup further by connecting the tetrahe-dral atoms in a three-dimensionalarray. This array can lead to largerinner cavities connected by pore

openings. In some zeolites, there areno cavities, but a series of one-,two, or three-dimensional channelsthrough the structure.

In addition, various post-synthesismodifications, such as hydrothermaltreatment, coating, and impregnation,may further alter catalytic and ad-sorptive properties.

Because of this ability to cus-tomize properties, zeolites are com-mercially valuable as adsorbents andmolecular sieves selectively ad-

mitting some molecules while ex-cluding others whose size, shape, orpolarity preclude adsorption.

The first generation of zeolites(such as A, X, Y, and mordenite) gen-erally have a low silica-to-aluminaratio and a high ion-exchange capaci-ty. Most boast a high affinity forwater and, so, are widely used as des-iccants. They also are employed toadsorb other polar molecules in sepa-

ration and purification applications.Second generation zeolites (e.g.,

ZSM-5 and silicalite) take advantageof quaternary ammonium ions and al-kali cations during synthesis to createnew structures and different chemicalcompositions that are higher in silica-to-alumina ratio. The highly siliceousmaterials are effective for adsorbingorganic molecules, even in low-con-centration, high-humidity, and high-temperature applications. A third gen-

eration of alumino-silico-metal phos-phates (AlPO4, SAPO, MeAlPO, etc.)is synthesized without alkali cationspresent, using organic amines and

quaternary ammonium compounds asstructure-directing agents. This re-sults in materials with dramatically

different properties and many newstructure types.Environmentally benign zeolite

catalysts have wide potential as solid

acids in many commercial processes,due in part to the ability to tunetheir acidity. In addition, zeolites canbe formulated to carry active materi-als such as catalytic metals. Theseproperties, coupled with the size andshape features of the zeolites, allowthe materials to be used as catalystsin extremely selective reactions, such

as in the manufacture ofpara-xylene.High silica zeolites ability to removehydrophobic organic compounds

from many environments is expand-ing their use as specialty adsorbentsin numerous fields.

The ability to tune a number ofother properties of zeolites (see Table1) is adding to the interest in the ma-terials. Another attraction is that zeo-lite powder can be formed into extru-dates, beads, monoliths, and othershapes.

The subtleties of synthesisThe greatest challenge in the de-

velopment and commercialization ofnew and modified zeolites is theirsynthesis. Natural zeolites are foundworldwide and were formed by ig-neous or sedimentary solution pro-cesses. Some of the parameters thatcontrolled the type of zeolite formedare the composition and pH of the so-lution, as well as the temperature,

pressure, and time of formation. It is

possible to replicate all of the condi-

tions except the time of formation,which is thousands of years in nature.Commercially, zeolites must be pro-

duced in hours or days; this, thus, re-quires optimizing the other variablesto change the window of formation.

Progress in zeolite synthesis has

been ongoing since commercial zeo-lites were first introduced 50 yearsago. Most zeolites are made from thesame reagents: alumina, alkalications, and silica. Small variations inconditions can cause major differ-ences in the structures that form. In-deed, in a number of cases, new zeo-lites have been discovered while at-

tempting to make known zeolites. Aclassic zeolite example is Zeolite X,

which was first made inadvertently inan attempt to repeat the synthesis ofZeolite A.

Some of the factors that determinethe type of synthetic zeolite producedare (2):

composition of the gel; silica and alumina source; other materials present (such as

OH- and other anions, cations (inor-ganic or organic), and organic

reagents); time of reaction; temperature and heat-up rate; pressure; and synthesis conditions (like the

order of mixing, gel aging, andstirring).

The optimization of the synthesisprocess for a known zeolite is not atrivial task and can involve many iter-ations. To commercially synthesize anew zeolite type, substantial expertisein both chemistry and chemical engi-

neering is required. Scale-up from the

CHEMICAL ENGINEERING PROGRESS JUNE 1999

ON THE HORIZON

Property RangeChannels 2.28 Cavities 6.611.8 Thermal stability 5001,000CIon-exchange capability Up to 700 milliequivalents/100 gSurface area Up to 900 m2/gWater capacity 1 to 25 wt.%Water affinity Hydrophilic to hydrophobic

Table 1. Zeolite properties.O

OAl

_

- O - Si - O - Al

_

- O - Si - O - Al

_

-

-

O

O

-

-

Si-

Si-

Figure 1. Zeolite structure.


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laboratory can be tricky, and com-mercial production usually demandsprecise control of temperature, pres-

sure, and other variables.

Improvingzeolite performance

New types of zeolite structures arecontinuing to be discovered; they un-doubtedly will lead to additional ap-plications and may offer advantagesin existing services. But, significantscope exists for enhancing current ze-olites performance for specific du-ties. Most often, this involves post-

synthesis modifications including

pore-size tailoring, surface treat-ments, acidity tuning, and changes insilica-to-alumina ratios. These typesof modifications have led to manymaterials with improved performanceas adsorbents, molecular sieves, andcatalysts.

Today, software modeling pack-ages that contain the known zeolitestructure types are available. Theyallow a user to create defects in or

incorporate a variety of het-eroatoms, such as iron, boron, andgallium, into a framework and simu-

late the altered structure. Separately,reaction intermediate and productmolecules can be introduced into the

structure model to assess the possi-bility of a reaction occurring. Thesethree-dimensional pictures simulat-ing molecules diffusing into thepore can be extremely powerful; themodels are as topically accurate aspossible, all the way down to the in-trinsic physics involved. Modelingcapabilities are growing better everyyear and, while models are not yetpredictive, they provide useful in-sights for tailoring materials for spe-

cific applications.

Some research groups have suc-ceeded in creating new zeolite struc-tures using designer organicmolecules as structure-directingagents to yield a desired pore size orshape. Many of these agents, howev-er, pose economic hurdles beingeither very expensive, not availableon a commercial scale, or very diffi-cult to synthesize. In some cases,though, materials that were first dis-

covered using designer organicmolecules have been commercializedusing other organics that are more

readily available, easier to handle,and less expensive.

EnvironmentalapplicationsFour interrelated forces are driving

clean technologies in industrializedcountries:

1. regulatory pressures to decreasethe output of waste streams andbyproducts or face fines and possibleshutdowns;

2. the need to remain cost compet-itive with third-world manufacturersunencumbered by stringent environ-

mental regulations;

3. social demands for a clean envi-ronment and sustainable productionmethods; and4. increased knowledge of the

causes, effects, and dispersion ofpollution.

It is widely recognized that thegains made by the current generationof environmental technologies arebeing offset by population growth.Consequently, many new and creative

pollution-prevention and -abatementmethods are required.

The unique properties and selectiv-


Application Zeolites Used AdvantagesSelective catalytic reduction of NOx Copper ZSM-5; mordenite Good for high dust applicationsExtended temperature rangeCheaperHigher selectivityMordenite is particularly stable in acid streams

Lean NOx Copper, cobalt ZSM-5; beta Uses fuel hydrocarbons as reductantsNo ammoniaNo special handlingCheaper

Lean-burn (oxygen rich) Copper, cobalt ZSM-5; beta More effective than three-way catalyticdiesel-engine NOx removal converter for NOx

Removal of N2O Cobalt, copper ZSM-5; mordenite; N2O decomposes over zeolites at higherferrierite; beta; ZSM-11 temperatures (400C)

VOC removal in dilute, high-volume, High silica, hydrophobic zeolites Effective where carbon is nothumid streams Systems available from several vendors

VOC removal during automotive High silica, hydrophobic Achieved 3570% reductioncold starts medium- and large-pore zeolites

Table 2. Environmental applications of zeolites.


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velopment employs N2O as the oxi-

dant in a direct oxidation processwith a zeolite as catalyst (16). Com-mercialization of this process couldcut waste and energy consumption,and would improve the economics ofphenol production, as well.

Zeolites also can reduce wastes inthe production of caprolactam, an in-termediate for nylon-6 and other syn-thetic fibers. The traditional Raschig

process generates multiple wastestreams. In contrast, the Enichemammoximation process, which uses at i tanium-framework-substi tutedZSM-5 zeolite (TS-1), hydrogen per-oxide, and ammonia, dramatically re-duces the number of process stepsand the volume of waste (17) (seeFigure 3). Employing another zeolitein the last step of the Enichem pro-cess may lead to further cuts in wasteproduction.

The use of cheaper and more read-

ily available feedstocks is still anoth-

er way in which waste may be re-

duced and natural resources con-served. The Mobil methanol-to-gaso-line process, now commercialized inNew Zealand, is one such process.Methanol may be produced fromcoal, natural gas, or biomass, andthen converted to liquid fuels orchemical feedstocks using a zeolitecatalyst (18). Future possible uses ofzeolites might include conversion of

flare gases at remote wells into amore easily transportable, waxy syn-crude that could be reprocessed intofuels where needed.

A green lightAdditional methods for manipu-

lating zeolite crystalline structurecontinue to be developed, and zeoliteapplications are expanding exponen-tially. Zeolites chemical inertnessand unique, modifiable properties

make them ideal for green chemistry

applications. CEP


B. K. MARCUS is Technical Market DevelopmentManager for Zeolyst International,Conshohocken, PA ((610) 6514679; Fax: (610)9414588; E-mail: [email protected]). Sheis responsible for custom zeolite development,testing, and marketing in the U.S. Shepreviously worked for UOP and Sun Oil. Shehas specialized in inorganic synthesis ofzeolites and development of new catalyticmaterials, holds over 30 patents in the U.S. andEurope, and has authored ten publications onzeolites. She has a MS in chemistry from

Fordham Univ.W. F. CORMIER is Technical Director of Zeolyst

Internationals R&D Center, Conshohocken,PA ((610) 6514623; Fax: (610) 9414588; E-mail:[email protected]). His responsibilitiesinclude new product development, zeolitecharacterization and testing, and scale-upfrom bench to pilot scale to commercialproduction. Previously, he worked for PQ Corp.,Katalistiks Intl., and Mobil R&D. He holds 14patents and is the author of 11 articles. Hereceived BS and MS degrees in chemicalengineering from Worcester Polytechnic Inst.,and is a member of AIChE.

NitritePreparation

a. Raschig Process b. Enichem Process

HydroxylamineSulfate

Preparation

AmmoniaOxidation

NH3

NH3

NH3

Oleum

NaOH

O2

NH3 H2O2 TS-1

SO2

NOx

SO2

Cyclohexanone

Oxime Synthesis

OximeRearrangement

Ammoximation

ReactionCyclohexanone

Ammonium Sulfate

Ammonium Sulfate

Raw Caprolactum(~83%)

Raw Caprolactum(~90%)

Sodium Sulfate

NH3

Oleum

OximeRearrangement

Cyclohexanone

Ammonium Sulfate

Figure 3. How Raschig and ammoximation routes to caprolactam compare.

Going Green With Zeolites

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