Chapter 5 Techniques in Green Chemistry

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Chapter 5 Techniques in Green Chemistry
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Chapter 5 Techniques in Green Chemistry

• 5.1 The performance of Catalysts in Chemical
  reaction
• 5.2 Green Chemistry and Catalysis
• 5.3 The Design of High Efficient and Safe
  Catalyst
• 5.4 Changing Starting Material for Chemical
  reaction
• 5.5 Changing Reagents
• 5.6 Changing the solvent of Chemical Reaction
• 5.7 Process Control and Process Intensification
• References

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5.1 The performance of Catalysts in Chemical
reaction
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Samples for the Application of Catalysts
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The application of catalysts to chemical reaction

• Accelerate the chemical reaction rate
• Accelerate selectively the one of the several
  thermodynamically possible reactions and yield
  selectively the special products.
• Namely, the use of catalysts can control the
  selectivity for special products.
• Control the enantioselectivity of reaction

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Catalyst has been called as molecular machine

• Synthesize the special conformation of chiral
  isomers
• Incorporate with reaction conditions, and control
  the selectivity of chemical reaction.

• It is the most important of high selectivity and
  atom economy in bios-process, and all the
  reactions in biomass are catalyzed by enzyme.
• high specificities, selectivity and atom economy.

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Catalyst has been called as molecular machine

• Hence, those enzyme have been called as Molecular
  Machine

A hand-over-hand mechanism for kinesin
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Catalyst has been called as molecular machine

• Recent investigations have reported that not only
  enzyme acts as molecular machine but also the
  common catalysts own the similar functions.
• The classical example is the metal
  cyclopentadiene(????) complex which was used as
  the catalyst in olefin polymerization

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Ziegler-Natta catalyst mechnsim? ?
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Catalyst has been called as molecular machine

• Enzyme and other traditional chemical catalyst.
• If they exhibit high specificity, selectivity,
  yield and atom economy, they should be considered
  as the molecular machine with special functions
  in chemical reactions.

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Chapter 5 Techniques in Green Chemistry

• 5.1 The performance of Catalysts in Chemical
  reaction
• 5.2 Green Chemistry and Catalysis
• 5.3 The Design of High Efficient and Safe
  Catalyst
• 5.4 Changing Starting Material for Chemical
  reaction
• 5.5 Changing Reagents
• 5.6 Changing the solvent of Chemical Reaction
• 5.7 Process Control and Process Intensification
• References

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5.2 Green Chemistry and Catalysis
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Catalysis and Pollution Protection
The activation of new starting materials
Catalysis and Process Promotion
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Catalysis and Pollution Protection
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• Catalysis plays a important role in pollution
  protection
• Catalysis can decrease and eliminate the release
  of NOx in exhaust form cars and factories,and
  improve the air quality.

High Temperature, NOX.
Lower Temperature
Load catalyst
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• Catalysis plays a important role in pollution
  protection
• Catalysis can decrease the usages of volatile
  organic solvents
• Catalysis can substitute the synthesize methods
  and process composed of chlorine materials and
  intermediates, and decrease the formation of
  wastes.

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Catalysis plays a important role in newly
synthesize route without pollution
The chemical reaction may become more effective
and more selective over catalyst, which can
decrease the formation of by-products and other
wastes. Catalysts can improve the reaction
conditions, such as temperature, pressure and
energy consuming, and eliminate the usage of
toxic reaction medium. In short, the utilization
of catalyst can satisfy the requirements of Green
Chemistry, simulatously
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The activation of new starting materials
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The synthesis of catechol
Traditionally
Benzene is harmful, Too many steps, By-products
(ketene hydroquinone), SO2 is not safe chemicals,
continue
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Draths Frost glucose as starting martial
Avoid the usage of toxic and harmful chemicals
and sharply decrease the yield of by-products.
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• 3. Catalysis and Process Promotion

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• 1. The synthesis of acetic aldehyde

PdCl2 CuCl2???
CH2CH2
CH3CHO
O2
Disadvantages Consume large amount of
catalysts The consternation of Cl- is greater,
and can lead to the formation of chromate
by-products. Those by-products is harmful to
human healthy
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2. The synthesis of hydrochinone
Traditional Method
Disadvantage too many steps, a large mount
of by-product, corrosive chmicals (H2SO4,HCl)
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Environmental beigen method
Advantages Greener method Short reaction
chain, By-products only formatted in the final
step.
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The synthesis of carbonyl compounds
Varma et al the activation under microwave and
catalyst
cat. microwave
R1C(OH)R2
R1COR2
Traditionally Organic solvent CrO3, KMnO4 Slat
pollution
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Conversion of Biomass
New Catalysts
Small Molecules Such as CH3COOH, CH3OH etc

• Bamboos

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Chapter 5 Techniques in Green Chemistry

• 5.1 The performance of Catalysts in Chemical
  reaction
• 5.2 Green Chemistry and Catalysis
• 5.3 The Design of High Efficient and Safe
  Catalyst
• 5.4 Changing Starting Material for Chemical
  reaction
• 5.5 Changing Reagents
• 5.6 Changing the solvent of Chemical Reaction
• 5.7 Process Control and Process Intensification
• References

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5.3 The Design of High Efficient and Safe
Catalyst
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1Gross analysis

• 1. At first, analyses
• the possibility of reaction and the largest
  equilibrium yelid
• the optimized reaction condition
• the available martials
• atom economy of reaction in real reaction
• economy of catalysts
• economy of catalytic reactions

in order to underrated the reliability of real
catalysts
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Gross analysis

• 2. Several factors should be considered to design
  parameters of catalysts.
• activity, selectivity, stability, duration and
  toxicity, etc
• 3. According to the reaction routes,
• search the catalyst and possible starting
  materials, choose the most favorable catalysts,
  modify and optimize the reaction conditions.
• 4.Confirm the reaction possibility
  experimentally.
• If the experiments do not confirm the theoretical
  perdition, the process should be re-designed.

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2.design and develop the new type molecule oxygen
oxidative catalysts

• Traditional inorganic oxidants
• NaClO, NaBrO, HNO3, KHSO3,
• CrO3,KMnO4, KCr2O7 ,etc.
• The traditional inorganic oxidative can introduce
  a large amount of
• waste salts, hazardous gases and liquids
• heavy atoms

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Clean oxidative and their characters

• O2
• The cleanest oxidative chemical
• The limitation of its reaction conditions,
• Often companied by other auxiliary oxidants

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Clean oxidative and their characters

• H2O2

H2O2 contain more than 47 percent active oxygen,
and its oxidative products (water) is
environmental benign chemical.
H2O2 is more expensive than O2 and O3, and can
discorporate in room temperature
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Clean oxidative and their characters

• (O3)
• O3 is also the environmental benign chemical
  oxidative, and its oxidative products is oxygen
  molecule. But the usage of O3 often require some
  special method and equipments.

transformer
O3 tube
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Clean oxidative and their characters

• (N2O)
• its oxidative products is environmental benign
  product (N2)
• the synthesis of N2O is complex and the cost of
  N2O is very high.

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Design of oxidative catalyst based on the
reaction mechanism

• The reaction mechanism of different reaction
  system, including catalysts,may vary.
• Hence, the requirements for catalysts should also
  be different.
• The design of catalysts should be toughly
  considered the reaction mechanism to meet the
  requirement of reaction.

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• 3?The design of new-type metal complex catalysts

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Metal complexes

• Those metal-organics catalysts are widely used in
  homogenous catalytic reactions
• Chiral metal complexes have been used as
  homogenous catalyst,and can control the
  stereo-selectivity of the reaction.
• It is very important for high stereo-selectivity
  to search the suitable reaction conditions
  central me, proper central metal ions and chital
  groups.

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Samplethe synthesis of Naproxen
The yield of target product (S-Naproxen) reaches
97?
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Table 5-2 Some metal complexes in industry
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4, Designing of New Molecular Sieve Catalyst

• Molecular Sieve (???)

Molecular sieve refers to a kind of inorganic
polymer composed of aluminum silicate (silicon
aluminate), bearing open structure.
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Designing of New Molecular Sieve Catalyst

• Structurally, molecular sieve bears the tetra-XO4
  structure, in which one atom X shares O with
  other X atoms. X may be tri-(Al, B, or Ga), tetra
  (Ge, Si)-, or penta-(P) valent.

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Designing of New Molecular Sieve Catalyst

• The pore diameter of molecular sieve is dependant
  on the number of building units, and the
  molecular sieve is generally named macro-, meso-,
  or micro-molecular sieve corresponded
  respectively to the mean pore diameter of 0.75,
  0.67 or 0.43nm.

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Designing of New Molecular Sieve Catalyst
X

• Natural Molecular Sieve(Zeolite)is widely used in
  petrol refinery for its macropore structure.
• Synthesized zeolite is now commercialized and has
  become one of the most important catalyst in
  petrol industry.

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Designing of New Molecular Sieve Catalyst

• Natural Molecular Sieve(Zeolite)is also used in
  ion exchange process.

• Because Natural Molecular Sieve(Zeolite) often
  owns acid and base sites stimulatously.

• In catalysis, molecular sieve is widely used as a
  new acid-base catalyst in the related reactions
  such as the conversion of alkanes.

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The alkylattion of butene

• Traditional method
• HF and/or H2SO4 are used as the catalysts.
• Advantage
• high efficiency
• Disadvantages
• erosion of HF/H2SO4
• production of inorganic salts
• HF could be recycled, but H2SO4 could not and
  should be removed.

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• the use of solid molecular sieve
• acid catalyst
• The erosion of liquid acid is eliminated,
• No inorganic salts as wastes produced.

Solid acid catalyst
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Molecular Sieve could also be used as basic
catalysts or acidic-basic bifunctional catalyst

• already used for the production of fundamental
  chemicals but not as widely as acid catalysts
• It will undoubtedly play an important role in the
  production of fine chemicals and special
  chemicals. For example, Cs Molecular sieve is
  used in the synthesis of 4-methyl-thiazoline(4-???
  ?,one kind of anti-fungus) instead of Cl2 or CS2
  and NaOH.

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Changing the selectivity of a chemical reaction
originated from the shape of molecular sieve by
chemical modification of molecular sieve

• The selectivity of chemical reactions based on
  the shape of the molecular sieve could be altered
  by chemical modification of the molecular sieve
  used as the catalysts, this provides wide
  applications of molecular sieve in controlling
  chemical reactions.

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For Example

• In the synthesis of 2,6-di-isopropyl naphthalene,
  a mixture of 2,6-, 2,7-, and 2,4-substituted
  naphthalene is obtained using ordinary methods.

2,6-di-isopropyl naphthalene
2,4-di-isopropyl naphthalene 2,7-di-isopropyl
naphthalene
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For Example

• The traditionally used catalyst SiO2/Al2O3 has
  large pores, and could not distinguish
  3-substituted-isopropyl naphthalene from
  4-substituted-isopropyl naphthalene, and the
  distinguish of 2,6-di-isopropyl naphthalene from
  2,7-di-isopropyl naphthalene could neither be
  realized.
• The separation of 2,6-di-isopropyl naphthalene
  and 2,7-di-isopropyl naphthalene by using special
  polymer liquid crystal is very troublesome and
  very expensive.

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For Example

• The use of small pore molecular sieve could
  inhibit the formation of 3-, or 4-, substituted
  products but the formation of equivalent amount
  of 2,6- and 2,7-substituted products could not be
  avoided.
• The formation of 3- and 4- substituted products
  could be eliminated, and a ratio of 2,6- to
  2,7-substituted products of 7/3 could be obtained
  by using Zeolite-C as the catalyst.
• Table 5-3 gives out the distribution of products
  by using different kinds of catalysts.

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Table5-3,The distribution of the products from
the alkylation of naphthalene by using different
kinds of catalysts
 
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Prospect for the research of molecular sieve
catalysts

• Molecular sieve catalysts may replace such
  substance as HF, H2SO4, etc., which are obviously
  dangerous to peoples health and the environment.
  Thus, molecular sieve catalyst is regarded as one
  kind of environmentally benign catalyst.
• Simultaneously, on account of the significant
  increase of the activity and selectivity due to
  the use of molecular sieve catalyst, the research
  of molecular sieve catalyst will undoubtedly
  become one of the most promising field in green
  chemistry.

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Chapter 5 Techniques in Green Chemistry

• 5.1 The performance of Catalysts in Chemical
  reaction
• 5.2 Green Chemistry and Catalysis
• 5.3 The Design of High Efficient and Safe
  Catalyst
• 5.4 Changing Starting Material for Chemical
  reaction
• 5.5 Changing Reagents
• 5.6 Changing the solvent of Chemical Reaction
• 5.7 Process Control and Process Intensification
• References

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5.4 Changing Starting Material for Chemical
Reaction
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Selection of starting materials

• The feedstock has great influence on the
  efficiency of the synthetic routes, on the
  environmental effects and the healthy of human
  beings.
• The hazard of feedstock must be considered by the
  producers, managers in the preservation and
  transportation, as well as the operators in the
  processing.
• For some bulk chemicals, the change of feedstock
  may change the market, for some substance are
  produced just to provide certain feedstock.

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1. Reducing hazardous properties

• (1). Certainly, a first level assessment of any
  starting material must be
• whether the substance itself is benign
• whether it poses a hazard for human beings and
  for the environments
• whether it poses a hazard in the form of either
  toxicity, accident potential, ecosystem
  destruction, or other form
• whether it is destructive for the ecological
  environment
• whether it poses other un-benign properties

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(2). Using preferable sources

• Currently, more than 90 organic starting
  materials are almost exclusively derived from
  non-renewable carbon feedstocks, such as coal or
  crude oil.
• Petrol-refinery is energy consuming. For example,
  in the U. S., the amount of energy consumed in
  petrol-refinery is about 15 of its energy
  consumption. The cost will augment for the
  quality of the crude oil is becoming bad.
• In the production of organic chemicals from oil,
  oxidation reactions are usually employed, and it
  is well known that oxidation reactions are
  seriously pollutant.

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using preferable sources

• Considering the use up of oil, natural gas and
  coal, we must reduce our dependence on these
  fossil resources.
• Agriculture resources and bio-resources are good
  alternative. Recent studies show that, many
  agriculture resources, such as corn, potato,
  soybean, and so on could be converted to textiles
  or nylon. Agriculture waste, biomass containing
  cellulose and lignin could also be converted to
  chemicals.

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• 2. Advantages and disadvantages of biomass as a
  chemical feedstock
• (1). Advantages

advantages

• biomass can be broke down into a huge array of
  structurally diverse materials, frequently
  stereochemically and enantiomerically defined,
  giving the user a wide range of new structural
  features to exploit in synthesis.

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advantages

• The structural complexity of the building blocks
  available from biomass is frequently higher when
  compared to building blocks derived from
  petrochemicals. This property could lead to a
  reduction of reaction side products, and hence, a
  reduction of the amount of waste material
  produced in chemical processes if methodology
  were available to incorporate this complexity
  into final products.

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advantages

• Building blocks isolated from crude oil are not
  oxygenated, yet many of the final products of the
  chemical industry are. There are few ways to add
  oxygen to hydrocarbons, and many of them require
  the use of toxic reagents (chromium, lead, etc.)
  in stoichiometric amounts resulting in severe
  waste disposal problems. Biomass derived
  materials are often highly oxygenated.

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advantages

• Increased use of biomass would extend the
  lifetime of the available crude oil supplies, and
  then make contribution to sustainable development
  and make sure the production of certain chemicals
  that could only be synthesized from oil.
• The use of biomass has been suggested as a way to
  mitigate the buildup of greenhouse CO2 in the
  atmosphere. Since biomass uses CO2 for growth
  through photosynthesis, the use of biomass as a
  feedstock results in no net increase in
  atmospheric CO2 content when the products break
  down in the environment.

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advantages

• A chemicals industry incorporating a significant
  percentage of renewable materials is secure
  because the feedstock supplies are domestic,
  leading to a lessened dependence on international
  hot spots .

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advantages

• Biomass is a more flexible feedstock than is
  crude oil. Crude oil is formed and its
  composition set by geological forces. The
  diversity of building blocks from biomass offers
  a great opportunity for the production of a range
  of chemicals as wide as that available from
  non-renewables. With the advent of genetic
  engineering, the tailoring of certain plants to
  produce high levels of specific chemicals is also
  possible.

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disadvantages

• (2)?disadvantages
• Many of the reported disadvantages are related to
  current economic circumstances. The petrochemical
  industry is huge and highly efficient, from the
  initial removal of crude oil, to the extraction
  of the simpler building blocks that comprise the
  crude oil, through the final transformation of
  these building blocks into their many
  intermediates and chemical products. Moreover,
  the petrochemical industry is well established.
  Much of its capital investment is paid off. The
  mechanisms and operation of its processes are
  well understood and give single products of high
  purity. The biomass industry is still developing
  processes that possess these features.

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disadvantages

• Many of the biomass sources being considered as
  chemical feedstocks have traditionally been used
  as sources of food, and the justification for
  diverting part of this resource to chemical
  production has been questioned. The issue becomes
  more acute when biomass is considered as a
  feedstock for fuel as well as chemical
  production. Biomass also requires space to grow,
  and the environmental impact of large scale
  biomass plantations has been examined.

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disadvantages

• Traditional sources of chemical feedstocks have
  been referred to as three dimensional because
  the structures in which they are found have depth
  as well as length and width. The presence of the
  third dimension allows much more feedstock to be
  concentrated in a smaller area. In contrast,
  biomass feedstocks are two dimensional
  feedstocks, and require proportionally more space
  for the same amount of material.

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disadvantages

• Biomass is necessarily seasonal. The crop is
  planted in one part of the year, and harvested in
  another. This leads to peaks and valleys in the
  supply of feedstock yet the chemical producer
  planning to use biomass needs a regular day to
  day supply, and needs to be assured that the
  material used at the beginning of the year will
  be of the same quality as that used at the end of
  the year.

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disadvantages

• The wide range of materials that comprise biomass
  could be a detriment especially if new processes
  need to be developed for each feedstock.
  Moreover, the building blocks extracted from
  biomass are foreign to traditional chemical
  producers and must be demonstrated to function in
  a manner similar to existing building blocks
  without undue manipulation.

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Chapter 5 Techniques in Green Chemistry

• 5.1 The performance of Catalysts in Chemical
  reaction
• 5.2 Green Chemistry and Catalysis
• 5.3 The Design of High Efficient and Safe
  Catalyst
• 5.4 Changing Starting Material for Chemical
  reaction
• 5.5 Changing Reagents
• 5.6 Changing the solvent of Chemical Reaction
• 5.7 Process Control and Process Intensification
• References

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5.5 Changing Reagents
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Selection of reagents
High synthetic efficiency practicable benign to
humans health and the environment
Many progress have been achieved in this aspect
on green chemistry For example Using light
instead of some reagents Using recoverable
catalyst as it is possible Loading the
reagents on the support to realize the reactions
(using oxidative agents, reductive agents to
realize the loading)
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Selection of reagents
High synthetic efficiency practicable benign to
humans health and the environment
For example, for the oxidation of tertiary
hydrocarbons to ketones, the traditional method
involves the reaction of copper acetate and
hydrogen peroxide in the aqueous solution,
whereas, this reaction can also be well realized
by supporting nitrate of copper onto the hydrogen
peroxide impregnated K10 clay.
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5.6 Changing reaction solvent
Is solvent necessary for the reaction?
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• Aqueous solution system
• Ionic liquid
• Immobilization of the solventSolution of
  polymer
• Carrying out polymerization reactions using the
  solvent as one of the monomer to obtain
  polymerized-solvent-derivates that bear the
  property of the solvent. Since this solvent is
  anchored on the polymer, thus the separation of
  the products from the solvent is eliminated and
  pollution from the volatile solvent is also
  eliminated.

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Solvent-free reaction

• Bandger et al combine the use of environmentally
  benign catalyst and microwave to synthesis
  3-carbinyl-coumarin from di-methyoxybenzaldehyde
  and Meldrum acid without using solvent.The
  combination of microwave and catalyst instead of
  solvent is effective in such processes as group
  protection, deprotection, oxidation, reduction,
  rearrangement reaction.

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????
Supercritical
Region
Liquid
Solid
Pressure
Vapour
Temperature
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Formation of SCF CO2
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Formation of SCF CO2
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Transmission characters of SCF

• SCF
• Density Similar to liquid
• Viscosity 1/100 than liquid
• Liquidity much better than liquid
• Reynolds number much better than liquid (same
  current velocity) ?
• Transfer coefficient much better than liquid

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Viscosity
Newton Formula µtyx/dµx/dy
With temperature increasing, for gas Viscosity
increases for liquid Viscosity decreases.
SCF Its viscosity is not equal to that of
liquid or gas. But it is liable to that of liquid
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Partial molar volume

• In SCF,the partial molar volume of infinite
  dilution solute is negative
• Near the critical region, it will further become
  more negative (about -100016000ml/mol)

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Advantages of SCF in chemical reaction solvent

• 1. It is convenient to adjust the prosperity of
  SCF from like gas-like phase to liquid-like phase
  in term of controlling pressure. That is to say,
  the control of pressure can alter the prosperity
  of SCF, which makes the reaction become more
  effective.
• 2.The control of pressure can adjust the density
  of SCF, and can also adjust other properties
  related with density, such as dialectic constant
  and viscosity, which promote the possibilities to
  control reaction and to increases the reactive
  selectivity.
• 3. SCF also own characteristics like some gases,
  such as low viscosity, large diffusion
  coefficient, which is much important to
  accelerate the reaction rate, especially to those
  reactions including gaseous reactants.

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Advantages of SCF in chemical reaction solvent

• Another advantage of non-oxidizability for SCF
  CO2 makes it become an ideal reaction solvent.
• The high concentration of CO2 in SCF CO2 make it
  liable to react in its SCF condition, which
  accelerate the reaction rate and make some
  reaction to occur.

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Chapter 5 Techniques in Green Chemistry

• 5.1 The performance of Catalysts in Chemical
  reaction
• 5.2 Green Chemistry and Catalysis
• 5.3 The Design of High Efficient and Safe
  Catalyst
• 5.4 Changing Starting Material for Chemical
  reaction
• 5.5 Changing Reagents
• 5.6 Changing the solvent of Chemical Reaction
• 5.7 Process Control and Process Intensification
• References

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5.7 Process Control and Process
Intensification
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The monitoring And controlling of Chemical
Process
Process intensification

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1. The monitoring and controlling of Chemical
Process
If small amount of a dangerous pollutant (X) will
form in the process of a reaction as a
side-product, and its formation is facilitated
under high pressure and at high temperature, in
situ monitoring of the formation of X could be
applied to detect continuously production of X,
and if its concentration surpasses a certain
threshold, the reaction conditions (temperature
and pressure) will be changed immediately to
reduce its production.

• Other reaction parameters, such as the ratio
  of the feed and so on could also be controlled in
  situ to facilitate or inhibit the formation of
  certain product.

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2.Process intensification
Definition

• a strategy for making dramatic reductions in the
  size of a chemical plant so as to reach a given
  production objective.

• via improvement of technical methods

• via improvement of technical methods

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2.Process intensification Process
intensification via improvement of equipment

• These reductions can come from
• shrinking the size of individual pieces of
  equipment
• cutting the number of unit operations or
  apparatus

• Process intensification via improvement of
  equipment
• Static Mixer Reactor
• Monolithic Catalyst
• Microreactors

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(1)Static-mixer-reactor (SMR)

• The technology of stirring has been greatly
  intensified during the last 30 years.
• Surprisingly, this was achieved not by improving
  mechanical mixer but quite the opposite by
  abandoning them in favor of static mixer.
• These devices are fine examples of
  process-intensifying equipment.
• They offer a more size- and energy-efficient
  method for mixing or contact fluid.

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Sulzer SMR static-mixer-reactor, which has
mixing elements made of heat-transfer tubes, can
successfully be applied in processes in which
simultaneous mixing and intensive heat removal or
supply are necessary, such as in nitration or
neutralization reactions.
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Sulzer SMR static-mixer-reactor

• One of the more important disadvantages of
  static-mixing-reactor is their relatively high
  sensitivity to clogging by solids. Therefore,
  their utility for reactions involving slurry
  catalysts is limited.
• Sulzer solved this problem (at least partially)
  by developing structured packing that has good
  static-mixing properties and that simultaneously
  can be used as the support for catalytic material.

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(2). Monolithic catalyst

• Materials used in the preparation of monolithic
  catalysts
• Metallic or Non-metallic substrates
• Which could provide a multitude of straight
  narrow channels of defined uniform
  cross-sectional shapes.
• To ensure sufficient porosity and enhance the
  catalytically active surface, the inner walls of
  the monolithic channels are usually covered with
  a thin layer of washcoat, which acts as the
  support for the catalytically active species.

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The characteristics of monolithic catalysts

• very low pressure drop in the single and two
  phase flow, one to two orders of magnitude lower
  than that of conventional packed systems
• high geometrical areas per reactor volume,
  typically 1.5-4 times more than in the reactors
  with particulate catalysts
• high catalytic efficiency, practically 100 due
  to very short diffusion paths in the thin
  washcoat layer
• exceptionally good performance in processes in
  which selectivity is hampered by mass-transfer.

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(3). Micro-reactors

• Micro-reactors are chemical reactors of extremely
  small dimensions that usually have a
  sandwich-like structure consisting of a number of
  slices with micro-machined channels.
• The layers perform various functions, from mixing
  to catalytic reaction, heat exchange, or
  separation.

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(3). Micro-reactors

• Integration of these various function within a
  single unit is one of the most important
  advantages of micro-reactors. The very high
  heat-transfer rates achievable in micro-reactors
  allow for operation highly exothermic processes
  isothermally, which is particularly important in
  carry out kinetic studies.
• Very low reaction-volume/surface area ratios make
  micro-reactors potentially attractive for
  processes involving toxic or explosive reactants.

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• Process intensification via improvement of
  technical methods
• Multifunction rector
• Membrane reactor
• Integration of separation techniques
• Alternative energy sources

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(1). Multi-functional reactors

• These can be described as reactors that, to
  enhance the chemical conversion taking place and
  to achieve a higher degree of integration,
  combine at least one more function that
  conventionally would be performed in a separate
  piece of equipment.

For example Reverse-flow reactor

• To date, reverse-flow reactors have been used in
  three industrial processes, SO2 oxidation, total
  oxidation of hydrocarbons in off-gases, and NOx
  reduction.

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• It integrated the function of heat transfer and
  that of chemical reaction.
• For exothermic processes, the periodic flow
  reversal in such units allows for almost perfect
  utilization of the heat of reaction by keeping it
  within the catalyst bed and, after reversion of
  the flow direction, using it for preheating the
  cold reactant gases.

107
(2). Membrane reactors

• Membrane reactor is one kind of multiple function
  reactor integrating the function of chemical
  reaction and separation.
• It can be used for selective in situ separation
  of reaction products, thus providing an
  advantageous equilibrium shift, enhancing the
  conversion of the reactants and the yield of the
  target products.
• It also can be applied for controlled distributed
  feed of some of the reacting species.

108
(2). Membrane reactors

• The membrane can enable in situ separation of
  catalyst particles from the reaction products.
• Practically, no large-scale industrial
  application of membrane reactors have been
  reported so far. The primary reason for this most
  definitely is the relatively high price of
  membrane units, although other factors, such as
  low permeability, as well as mechanical and
  thermal fragileness also play an important role.

109
The advantages of membrane distillation

• 100 rejection of ions, macro-molecules, colloid,
  cells and other non-volatiles
• lower operating pressure across the membrane than
  in the pressure driven processes
• less membrane fouling, due to large pore size
• potentially lower operating temperatures than in
  conventional evaporation or distillation.

110
(3). Hybrid separations

• Many of the developments in this area involve
  integration of membranes with another separation
  technique.
• In membrane absorption and stripping, the
  membrane serves as a permeable barrier between
  the gas and liquid phases.
• Membrane distillation is the best known hybrid,
  and is being investigated worldwide. It basically
  consists of bringing volatile component of a
  liquid feed stream through a porous membrane as a
  vapor and condensing it on the other side into a
  permeate liquid.

111
(4). The use of alternative forms and sources of
energy

• Several unconventional processing techniques that
  rely on alternative forms and sources of energy
  are of importance for process intensification.

Among other techniques, research on
sono-chemistry appears to be the most advanced.
Formation of micro-bubbles in the liquid reaction
medium via the action of ultra-sound waves has
opened new possibilities for chemical synthesis.
These cavities can be thought of as high energy
micro-reactors. Their collapse creates
microimplosions with very high local energy
release.
112
(4). The use of alternative forms and sources of
energy

• This may have various effects on the reacting
  species, from homolytic bond breakage with free
  radical formation, fragmentation of polymer
  chains by the shock wave in the liquid
  surrounding the collapsing bubble.
• For solid catalyzed systems, the collapsing
  cavities additionally can affect the catalyst
  surface----this, for example, can be used for in
  situ catalyst cleaning/rejuvenation.

113
(4). The use of alternative forms and sources of
energy

• Solar energy also may play a role in chemical
  processing. A novel high temperature reactor in
  which solar energy is absorbed by a cloud of
  reacting particles to supply heat directly to the
  reaction site has been studied.
• Microwave heating can make some organic synthesis
  proceed up to 1.24 times faster than by
  conventional techniques.

114
? ? ? ?
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