The green chemistry revolution is providing an enormous number of challenges to those who practice chemistry in industry.docx

8/20/2019 The green chemistry revolution is providing an enormous number of challenges to those who practice chemistry i…

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The green chemistry revolution is providing an enormous number of challenges to those who practicechemistry in industry, education and research. With these challenges however, there are an equal number of opportunities to discover and apply new chemistry, to improve the economics of chemicalmanufacturing and to enhance the much-tarnished image of chemistry.

WHAT I !"##$ %H#&I T"'(

In the )**+s, the concept of green chemistry was initiated in both the and#urope, and has since been adopted widely by the chemical industry. !reen chemistry deals withdesigning chemical products and processes that generate and use fewer /or preferably no0 ha1ardoussubstances. 2y applying the principles of green chemistry, companies embrace cleaner and more efficienttechnologies, with an a priori commitment to a cleaner and healthier environment. The green chemistrymessage is simple3 ee4 prevention, not cure .

!reen chemistry offers an alternative to the traditional environmental protectionagenda, mainly because it deals with avoiding ha1ards, rather than treating andsolving e5posure problems.

Green Chemistry is the utilisation of a set of principles that reduces or eliminates the use orgeneration of hazardous substances in the design, manufacture and application of chemicalproducts


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)+. )esign for )egradation

%hemical products should be designed so that at the end of their function they brea4 down intoinnocuous degradation products and do not persist in the environment.

)). +eal time analysis for #ollution #revention

Analytical methodologies need to be further developed to allow for real-time, in-processmonitoring and control prior to the formation of ha1ardous substances.

);. Inherently (afer Chemistry for $ccident #revention

ubstances and the form of a substance used in a chemical process should be chosen tominimi1e the potential for chemical accidents, including releases, e5plosions, and fires.

(ustainable development

In the modern conte5t, the terms 9sustainable development6 and 9greenchemistry6 have been around for less than )> years. :iscussion of sustainability began,essentially, when the )* @ $ %ommission on #nvironment and :evelopment /usually referredto as the 2runtland %ommission0 noted that economic development might lead to a deterioration,not an improvement, in the quality of people6s lives. This led to the now commonly accepteddefinition of 9sustainable development6

A sustainable society is one that meets the needs of the current generation withoutsacrificing the ability to meet the needs of future generations. ustainable development is astrategic goal. It can be reached using various approaches, and this is where green chemistry

comes in. green chemistry is Cust one step /albeit an important one0 along the road tosustainability.

What is sustainable energy?

There are three basic demands an energy source must meet to be characteri1ed assustainable3

• The long-term availability of the energy source that guarantees to meet any present orfuture consumption needs.

• The energy source must be replenishable without human intervention.

• The amount of energy consumed to e5ploit the available resources should not e5ceed theamount of energy these resources produce /ie, the energy efficiency of the source0.

What is +enewable %nergy?

"enewable #nergy is simply energy from a source that isn6t used up when the energy isconsumed. The supplies of solar and wind are not limited. The supply of oil and other

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hydrocarbons is limited for all practical purposes. When we have e5tracted the last barrel fromthe ground or tar sands D it6s finished for these millennia.

Adopting a geological sense of time /billions of years0, we can say that all energysources are renewable. If we could wait long enough, more oil would be created. The geological

sense of time doesn6t help us to ma4e good, sustainable energy decisions since our rate of consumption far e5ceeds the rate of replenishment.

Eeeping a human sense of time /generations0, our most renewable sources of energyare3 solar, wind, hydroelectric, hydro4inetic, geothermal, and hydrogen fuel cells. %arbon-basedrenewables include3 biomass, biodiesel, waste to energy, anaerobic digestion and landfillgas. These eleven energy sources are listed by most . . states in their "enewables 7ortfolio

tandards /"7 0.

(ustainable vs +enewable %nergy / $re hey the (ame?

&any use the terms sustainable and renewable energy without any distinction.

%ontrary to the common belief, there is a difference between them. Although renewable energysources share all the sustainable energy characteristics described above, they are clean sourcesthat do not pollute the environment during consumption and have minimum impact on humanhealth and the ecosystems. Therefore, the term FsustainableF is wider and includes all types of renewable energy.

ypes of (ustainable and +enewable %nergy (ources

ources such as the sun, the wind, and the earthGs heat can be characteri1ed as bothsustainable and renewable energy sources, since they have minimum impact on the environment3

(olar %nergy0

The sun provides us with a completely clean and environmentally friendly source of energy. Technology is currently focused on manufacturing high efficiency solar modules toe5ploit this energy.

Wind0

This is another clean energy source with high efficiency. However, the wind turbineswill have to be placed in windy locations in order to be effective. :enmar4 and !ermany are theworld leaders in this field.

1iomass0

7lants, trees, switchgrass, corn, and other biofuels are converted into ethanol which inturn, is burned to generate power. witchgrass is thought to have higher energy efficiency thancorn.Hydrogen can also be made from biomass, although this is not a very efficient way toe5ploit biomass Cust yet. &any suggest the use of buffalo urine as a biofuel . In this case, huge

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quantities of urine would have to be produced, which is almost impossible. Anyway, any 4ind of urine could be used to produce hydrogen, which is a better fuel.

1enefits of +enewable %nergy *se

• ittle to $o !lobal Warming #missions

• Improved 7ublic Health and #nvironmental uality

• A Jast and Ine5haustible #nergy upply

• Kobs and Lther #conomic 2enefits

• table #nergy 7rices

• A &ore "eliable and "esilient #nergy ystem

%nergy is neither renewable nor sustainable /a different loo-.

1y (teven (mithWinthrop Professor, Plant Energy Biology, ARC Centre of Excellence7osted on ;) April ;+))Miled under #nergy , &inerals

The pressure is on to reduce greenhouse gas emissions to slow climate change. The way proposed by most people is to switch away from fossil fuels to alternatives such as wind, solar,

tidal and geothermal. uch alternative energy sources are often described as 9renewable6 or 9sustainable6. This terminology implies to most people that such alternatives can meet our energydemands in perpetuity, without polluting the environment. This is wrong, and will lead to seriouserrors in policy ma4ing.

#nergy generated for human use cannot be 9green6, 9clean6, 9renewable6 or 9sustainable6. Thesewords are all part of the 9greenwashing6 or 9sugar-coating6 vocabulary used for the benefit of corporate or political interests, or simply words of misunderstanding. They have no foundation inrigorous scientific language or thought.

7ut simply the #arth can be considered as an open thermodynamic system in terms of energy buta closed system as far as matter is concerned. The sun continues to radiate energy to the #arth,and energy is re-radiated to space, more-or-less at the same rate. Lver a very long period of time/many millions of years0 there is a progressive increase in entropy and a net loss of energy fromthe #arth to the rest of the universe but this natural process is not significant on time scalesrelevant to humans.

However, humans increasingly wish to convert solar radiation into different forms of energy suchas electricity or fuel, that can do wor4. This can only be achieved by creating devices or

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machines to convert one form of energy into another and the resources for those devices comefrom the #arth6s crust. Those devices have a finite life span and depend on yet further infrastructure /transport, cities, factories, universities, police, etc. 0 to maintain and operate them,which in turn has a finite life span. %ontinued mining, refining and manufacturing is required.

The amount of energy captured from the sun by such devices can never be enough to restore the#arth to its original condition. This is determined by the second law of thermodynamics. o the process of mining, building and manufacturing, to convert and use energy, ine5orably depletesand degrades the #arth6s mineral resources. It is irreversible and unsustainable. It ma4es nodifference whether we consider solar, wind, hydro, coal, bio, nuclear or geothermal energy. Theyare all unsustainable according to the laws of physics.

The second law of thermodynamics also tells us that we cannot completely recycle resources thathave been e5tracted from the #arth and refined for use /such as metals, helium or phosphatefertiliser0. The greater the percentage we try to recycle, so the energy cost increasesdisproportionately. o whether the resources that we want to use are still in the ground or are in

circulation above ground, human industry will inevitably dissipate and lose those resources.The more people we have on the planet, and the more energy we use, the faster and moree5tensive is the degradation of #arth6s resources. Humanity is li4e a huge organic machine, usingenergy to mine and deplete minerals. The more energy that is put into the system, the faster thedegradation occurs. $uclear fusion energy, if it comes to be, might be particularly efficient atdegrading our resources and environment /one effect of such technology may be to convert our lithium reserves into helium which will escape the #arths atmosphere and be lost forever0.

#nergy for human use is as unsustainable and non-renewable as mining. o to tal4 about9renewable energy6 or 9sustainable energy6 is an o5ymoron, as is 9sustainable mining6 or

9sustainable development6. The more energy we use, the less sustainable is humanity. Thesooner that people realise this, the sooner we can embar4 on the process of reducing energyconsumption, rather than clutching at the straws of alternative energy sources to perpetuate theunsustainable.

2uantifying %nvironmental Impact0 %fficiency, % factors, and $tom %conomy

#veryone agrees that green chemistry and green manufacture are good things. Thewebsites and pamphlets of all the maCor chemical companies emphasi1e their concern for theenvironment. They all say that their processes and products are efficient, green, andenvironmentally friendly . 2ut how should we compare these processes( How should we Cudgesuch claims(

Mirst, let us define some 4ey terms. Lne method for quantifying a reaction6sefficiency is by e5amining the reactant conversion , the product selectivity , and theproduct yield over time. The reactant conversion is the fraction of reactant moleculesthat have transformed to product molecules /regardless of which product it is0. Theselectivity to product 7 is the fraction /or percentage0 of the converted reactantthat has turned into this specific product 7. The yield of 7 is simply conversion N


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selectivity. High conversions in short time spans mean smaller and safer reactors.imilarly, high selectivity means less waste, and simpler and cheaper separation

units. Thus, conversion, selectivity, and yield are all measures of the reactionefficiency.

In addition, there are specific rulers for measuring the greenness or eco friendlinessof processes and products. Lne such measure is the #-factor, introduced by "oger heldon in)**=. A reaction6s #-factor is the quotient 4gwaste B4g product /here waste is everything formed inthe reaction e5cept the desired product0. The waste can be gases such as %L; or $L5, water,commoninorganic salts /e.g., $a%l, $a; L=, or /$H=0; L=0, heavy metal salts, andBor organic compounds. This is surprising, as we are used to thin4ing of such chemicals as

pollutants. In fact, #-factors increase substantially when going from bul4 chemicals to finechemicals and specialties. This is partly because finechemicals production often involvesmultistep syntheses, and partly because stoichiometric reagents are more often used for

producing fine chemicals and pharmaceuticals.

The concept of ATL& #%L$L&' , introduced by 2arry Trost in )**), is similar to that of the #-factor. Here one considers how many and which atoms of the reactants areincorporated into the products. With these two concepts, we can evaluate chemical reactions toget a quantitative result.

$tom economy

!enerations of chemists, especially organic chemists, have been educated to devisesynthetic reactions to ma5imi1e yield and purity. Although these are worthy goals, reactions may

proceed in )++O yield to give a product of )++O purity and still produce more waste than product. In simplistic terms, #quation ;.)3

A P 2 Q % P : P #

In which A and 2 react to give product % in high yield and high purity, also leads tothe formation of by-products /or waste0 : and # in stoichiometric quantities. Mor many years


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phenol was manufactured via the reaction of sodium ben1ene sulfonate /from ben1enesulfonation0 with sodium hydro5ideR the products of this reaction are sodium phenolate/which is hydrolysed subsequently to phenol0, sodium sulfite and water. #ven if the reaction

proceeds in quantitative yield, it is evident from loo4ing at the molecular weights of the product/sodium phenolate0 and unwanted by-products /sodium sulfite and water0 that, in terms of

weight, the reaction produces more waste than product. Historically,however, the chemist wouldnot consider the production of this aqueous salt waste to be of any importance when designingthe process.

The atom economy concept proposed by Trost is one of the most useful toolsavailable for design of reactions with minimum waste. The concept is that for economic andenvironmental reasons reactions should be designed to be atom efficient, i.e. as many of thereacting atoms as possible should end up in useful products. In the e5ample shown above all

the carbon atoms present in the starting material are incorporated into the product, giving acarbon atom efficiency of )++O, but none of the sulfur ends up as useful product and hence theatom efficiency for sulfur is +O. Lverall, the atom efficiency of the reaction is defined as theratio of the molecular weights of desired product to the sum of the molecular weights of allmaterials produced in the process. In the above e5ample the atom efficiency would be ))?B;?+ or ==.?O.

#5amining the #-factor for this reaction, we see that for every three molesof ben1ophenone we produce one mole of chromium sulfate and si5 moles of water. Themolecular weight of ben1ophenone is ) ;.; g mol), so every 4ilogram of ben1ophenone contains>.= moles of ben1ophenone. This means that for every 4ilogram of ben1ophenone we generate>.= B< S ). < moles /or +.@)@ 4g0 of

chromium sulfate and )+.* moles /or +.)*@ 4g0 of water. The overall #-factor is therefore given by #q.


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$early a whole 4ilogram of waste for every 4ilogram of product "emember, this is forthe ideal case of )++O yield and )++O selectivity. In real life, the #-factor is usuallymuch higher, because product yields are less than )++O and the reagents are often used ine5cess. Murthermore, in many cases one needs to neutrali1e acid or base side products, so theoverall waste amounts are even higher.

The #-factor and atom economy can be used for comparing reaction alternatives, but we should remember that there are different types of waste. The reaction e5ample above hastwo by-products3 chromium sulfate and water. Lbviously, water is !LL: WA T#, whilechromium sulfate is 2A: WA T#, so evaluating a synthetic protocol on the basis of only the

amount of waste produced is insufficient. To solve this problem, heldon put forward the conceptof the environmental quotient. 2y multiplying the #-factor by , an arbitrarily assigned ha1ardquotient, this measure ta4es both the amount and the nature of the waste into account.

Assigning absolute -values to waste streams is difficult, because cases differ according to location and type of waste. $evertheless, the # gives a better measure of theenvironmental impact of a process than the #-factor or the atom economy alone.


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reen Metholodies

7ropene o5ide is an important bul4 chemical, used for ma4ing propyleneglycol/propane-),;-diol0, polyethers, glycerol /propane-),;,


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It is less polluting, but couples the epo5ide production to that of styrene, a huge-volume product. Thus, this route depends heavily on the styrene mar4et price. Another alternative, the A"%LBL5irane process, uses a molybdenum.

catalyst, and couples the epo5idation of propene to isobutene o5idation.The t-butanol that this generates as a by-product is then used as a gasoline additiveand a starting material for ma4ing methyl t-butyl ether /&T2#0. The disadvantageshere are that &o is a to5ic heavy metal, and &T2# use is being phased out in the and #urope.


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(olvents

solvents are very useful. hey can be used to). 7lace reagents into a common phase where they canreact;. :issolve solids so that they can be pumped from place to

place million tons of solvents /with a fuel valueof ; billion dollars0 each year<;. "eaction of lost solvents in air with nitrogen o5ides insunlight to produce ground level o1one. &iscarriages caused by ethers of ethylene glycol?. 2irth defects from e5posure to solvents=@. Mires and e5plosions that may result from use

. &onetary cost

Aromatic solvents /ben1ene, toluene, etc0, chlorinated and polychlorinatedsolvents /carbon tetrachloride, chloroform, dichloromethane, etc0 and other organic solvents/:& L, :&M, petroleum ether, diethyl ether, acetone, etc0 are used in great quantities in manylaboratory and analytical techniques. The persistent solvents /non-biodegradable0 are difficult torecycle and their disposition is very e5pensive.

There are many smaller commercial sources of solvent emissions, includingdry-cleaning shops, printing establishments, metal-cutting fluids in machine shops, auto bodyrepair shops, and many others. The consumers also emit solvents from their homes when theyuse paint remover, oil-based paints, adhesives, spot remover, charcoal lighter fluid, aerosolcans of personal care products, hair spray, nail polish, and gasoline-powered tools, among many.The use of chlorinated solvents minimi1es the ris4 of fire, but causes to5icity problems, such as


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liver damage and cancer. If other volatile solvents are used, all equipment in the plant must bee5plosion-proof, which includes placing every electrical switch in a heavy metal container. Thelimits being set on these volatile organic compound /JL%0 emissions from various productsare the subCect of some controversy in the nited tates.

A tiered approach to this problem from the least changeto the most change might be the following3

). 7lace something on the plant outlet to destroy or recover the solvent.;. #nclose the operation so that the solvent is not lost.. se water.?. se no solvent.@. witch to another process that eliminates the need for the solvent.

. %an we do Cust as well without the product made with thesolvent(

Mor laboratory use and, perhaps, for small-volume production, another first stage can be added. &icroreactors, which require much less solvent, are being studied for fluorination,:$A chips, and other uses.

Lne of principles of !reen %hemistry is to promote the idea of greenersolvents /non-to5ic, benign to environment0, replacement in cases that can be substituted withsafer alternatives, or changes in the methodologies of organic synthesis, when solvents are notneeded.

Green (olvents and $lternative 3ethods

!reen solvents have been characteri1ed for their low to5icity, higher low solubilityin water /low miscibility0, easily biodegradable under environmental conditions, high boiling

point /not very volatile, low odour, health problems to wor4ers0 and easy to recycle after use.

Ionic &i4uids in 5rganic (ynthesis. $re they Green Chemistry?

Ionic liquids are mi5tures of anions and cations, molten salts, with melting point

around )++ o%, which can be used as alternative solvents in organic synthesis. Although the ionicliquids do not comply full with green chemistry principles, they are very promising asalternatives to organic solvents. uitable for the construction of ionic liquids are

bul4y asymmetric ions with a delocali1ed charge, although cation and anion need not both adhereto these requirements.


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%ommon or other otherwise valuable ionic liquid cations /a0 and anions /b0. ", al4yl group.

In the scientific literature there are a large number of research papers for the useof ionic liquids in synthetic routes and various applications.

(ynthesis of 6bmim761 87. (tep !0 nucleophilic substitution in either acetone ordichloromethane 9)C3:. (tep "0 anion metathesis in either ethyl acetate or acetonitrile.

! butyl ; methylimidazolium tetrafluoroborate 9abbreviated to 6bmim761 87:

5rganic (ynthesis in Water

Although water is considered a problem for organic synthesis and the purification processes and drying in final products is very cumbersome, in recent years water is considered agood solvent for organic reactions. A good e5ample id the synthetic routes of the :iels-Alder reactions in which the hydrophobic properties of some reagents ma4es water an ideal solvent.

Water as a solvent accelerates some reactions because some reagent are not soluble and providesselectivity. The low solubility of L5ygen is also an advantage for some reactions where metalcatalysts are used.

nfortunately due to reactivity and solubility issues water is not a suitable solventfor many chemical transformations, and as such the scope for applications is somewhat limited.


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Having said this, the low solubility of substrates in water is not always an unsurpassablerestriction, with several on-water reactions reported in the literature.

echni4ues for 5rganic (ynthesis in #erfluorinated #hases

In some new methodologies chemists use perfluorinated diphasic solvents todissolve a catalyst with very long perfuorinated chain. These catalysts can be very effective and

provide high yields in some types of reactions where the catalysts play an important part.Another advantage is that after the reaction the catalyst can be separated and recycled.

(upercritical carbon dioxide and supercritical water

A supercritical liquid is at a temperature and pressure above its critical point,where distinct liquid and gas phases do not e5ist. The supercritical liquid can effuse throughsolids li4e a gas, and dissolve materials li4e a liquid. In addition, close to the critical point, smallchanges in pressure or temperature result in large changes in density, allowing many propertiesof a supercritical fluid to be Ffine-tunedF. upercritical liquids are suitable as a substitute for organic solvents in a range of industrial and laboratory processes. %arbon dio5ide and water arethe most commonly used supercritical fluids. upercritical %L; and water are considered

green solvents in many industrial processes, providing high yields in many reactions, and thereare many e5amples of their use in the scientific literature.

5rganic (ynthesis with Carbonic esters

%arbonic esters, such as :&%, dimethyl carbonate /%H


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1iosolvents

An alternative approach to solvent substitution is to produce solvents from biomass. $otonly does this alleviate demand for depleting resources, but depending on the biomass used thesource can be replenished fairly rapidly and with little net pollution caused. Jolatile bio-derived

solvents still retain the JL% ha1ards presented by many petroleum chemicals, and so thissolution is not comprehensive. The diversity of the potential feedstoc4s available but their dissimilarity to petroleum means that a traditional organic solvent may not be feasibly supplanted

by an e5act bio-derived version. However, the desirable physical properties characteristic of thatsolvent will undoubtedly present themselves in the form of an easily accessible related

biosolvent.

$pplication of Green (olvents

Water


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Lf all the alternative reaction media, water has perhaps been used most e5tensively asmodifications to reagents are rarely required, and often the organic substrates do not even need to

be soluble in the aqueous phase for the reaction to proceed smoothly. Water has proven itself as avery useful solvent for many types of reactions, including :ielsDAlder, aldol, other carbonDcarbon bond-forming reactions including %DH activation processes,

epo5idation reactions and alcohol o5idations. &ore recently, the hydrophobic effect has beenused in its most e5treme form, where the organic substrates are hydrophobic and insolubleR suchreactions are said to proceed 9on water6.


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3icrowave $ssisted +eactions

'igh emperature, (uperheated or


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bond networ4 brea4s down. In this temperature range, water can be calledhigh temperature, superheated or near critical /$%W0. It has a lower polarity density, viscosity,and surface tension than water at room temperature. However, U /hydrogen bond acceptor ability0remains constant with changing temperature, and diffusivity and specific heat

capacity increase.

What is Catalysis and Why is it Important?

we saw that one of the 4ey obCectives of green chemistry is waste minimi1ation.&oreover, we learned that a sustainable process is one thatoptimi1es the use of resources, while still leaving sufficient resources for futuregenerations. %atalysis is an important tool in both cases. In fact, as far as chemistry isconcerned, catalysis is the 4ey to sustainability

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The green chemistry revolution is providing an enormous number of challenges to those who practice chemistry in industry.docx