NEWER REACTIONS AND PROCEDURES: CATALYTIC AND NONCATALYTIC

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Chapter 12

NEWERREACTIONS AND PROCEDURES: CATALYTIC AND

NONCATALYTIC

M.Banu

Thefollowing chapter focuses mainly on some developments of industriallyimportant

reactions with and without catalyst.Briefly the contents of thischapter include:

Introduction

Biodieselproduction with new source by transesterification reaction with and

withoutcatalyst

Conversion of glycerol to valuable chemicals by heterogeneously catalysed

liquid-phaseoxidation

Catalytichydro-desulfurization

Catalytic and noncatalytic study of oxidativedehydrogenation reaction for ethane

conversion to ethylene as the one of theindustrially importantproduct

Noncatalyticsupercritical fluid methodfor the preparation of various

polyorganosiloxanes

1.0. INTRODUCTION

Thegeneral definition forcatalyst is "chemical marriagebrokers".

Fig.1(a)

Fig.1(b)

Fig.12.1 (a) General diagramfor catalytic reaction; (b)Energy profile diagramfor

catalyticreaction [1]

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NewerReactions and Procedures : Catalytic and Non catalytic

Thepresence of a catalyst facilitatesreactions that would be kinetically impossible or

veryslow without a catalyst. Thecatalyst does not alterthe overall thermodynamics of

thereaction.

1.1 IMPORTANCE OF CATALYST

Morethan 70 % of all existingprocesses on an industrial scalerely on catalysis.

Morethan 99 % of the worldgasoline production occursvia catalytic cracking of

oilfractions and other catalyticprocesses.

Morethan 90 % of all newindustrial processes arecatalytic.

Enzymesare catalysts thatfacilitate complex reactionswith 100 % selectivity at

extremelymild reaction conditions, i.e.in our bodies

Thechemical precision displayed in enzymatic reactions is a source of inspiration

forall catalysischemists.

In our life mainly one is depending on oil mostly fortransport, food,

pharmaceutical,industry and entire basis of modern life. So the demand for energy

sourcesmore in India compared to othercountries. Biodiesel is the one of the

importantenergy source. Biodiesel is a domestic, renewable fuel for diesel engine

derivedfrom natural oils. Biodiesel can be used in any concentrationwith petroleum

based diesel fuel in existing diesel engines with little or no modification.Biodiesel

production can be carried out by catalyticroute and also by noncatalyticroute.

Biodiesel is not the same thing as raw vegetable oil. It is produced by a chemical

processwhich removes the glycerinfrom the oil. Theglycerol is the one of the

byproducts in the biodiesel production. It can be converted in to valuable products by

environmentallyfriendly catalytic route.The industrial energysources like petrol and

diesel contain more amounts of sulphur and it leads to theformation of more

pollutionwhich is very harmful forhuman beings. For removal of sulphur content in

oil,hydrodesulfurisation is the importantprocess which can be carriedout by

catalyticroute with Mo, Ni, and Co loaded on various supports. Althoughcatalytic

route is feasible in industrially, some of the noncatalytic routes are also possible for

producingindustrially important productslike ethane and polyorganosiloxanes.

Ethene is the second majorcomponent of natural gas and it is also abundant in

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12.3

refinerygases. Polyorganosiloxanes arethe important material in chemical industries

which can be prepared by catalyst free super critical fluidmethod.

2.0BIODIESEL PRODUCTION BY TRANSESTERIFICATION REACTION

Biodiesel is a fuel composed of mono-alkylesters of long chain fattyacids derived from

variety of vegetable oils or animalfats. Biodiesel has becomemore attractiverecently

because of its environmental benefits and the fact that it is madefrom renewable

resources. Although there are manyways and procedures to convert vegetableoil into a

diesel like fuel, thetransesterification process [1] was found to be the most viableoil

modificationprocess.

2.1Transesterification Reaction

Transesterification is the process of using an alcohol (e.g. methanol,ethanol or butanol),

in the presence of a catalyst,such as sodium hydroxide or potassium hydroxide, to break

themolecule of the rawrenewable oil chemicallyinto methyl or ethyl esters of the

renewableoil, with glycerol as a by product.

Scheme.12.1. TransesterificationReaction

TheFats and oils are bigmolecules with a spinal of glycerol on which are boundthree

fatty acid rests as shown in Fig.12.2 (a). The fatty acid rest are removed fromthe

glycerol and it will form bond withmethanol by transesterification method.Further it

leads to the formation of one mole of glycerol and three moles of fatty acid methylester

which is shown in Figs. 12.2 (b) and 12.2 (c).

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NewerReactions and Procedures : Catalytic and Non catalytic

(a)

(b)

(c)

Fig.12.2 (a) Molecular structure of fatty acid rests withglycerol; (b) Formation of bond

withmethanol; (c) formation of glycerol and methylester[1].

Thefollowing methods areemployed for the preparation of biodiesel

BatchBase Catalyzed

ContinuousBase Catalyzed

AcidCatalyzed Processes

Non-CatalyticProcesses

Feedstocksused in biodiesel productionmainly triacylglycerol or fats and oils (e.g.

100 kg soybean oil), primary alcohol(e.g. 10 kg methanol) and catalyst(e.g. 0.3 kg

sodiumhydroxide) and also the neutralizer(e.g. 0.25 kg) sulfuricacid. Thetriglycerids

sourcesare rendered from animalfats like beef tallow, lard, and vegetable oilslike

soybean, canola, palm, etc and chickenfat and also rendered greases likeyellow grease

(multiplesources).The recovered materials is browngrease, and soapstock, etc.

2.2BASE CATALYSED TRANSESTERIFICATION REACTION

Generally in the base catalysedtransesterification method thecatalyst is dissolved in

methanol by vigorous stirring in a smallreactor. The oil is transferred into thebiodiesel

reactor, and then, the catalyst/alcoholmixture is pumped into theoil. The final mixture is

stirredvigorously at particular temperature and ambient pressure. A successful

transesterificationreaction produces two liquidphases that is ester and crude glycerin.

Crudeglycerin, the heavierliquid, will collect at thebottom after several hours of

settling.Phase separation can be observed within 10 min and can be complete within 2 h

of settling. Complete settling can take as long as 20h. After settling is complete, water is

added at the rate of 5.5 % by volume of the methyl ester of oil and then stirred for 5 min,

and the glycerin is allowed to settle again. Washing theester is a two step process,which

is performed with extreme care. A water wash solution at the rate of 28 % by volume of

oil and 1 g of tannic acid per liter of water is added to the ester and gently agitated. Air is

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12.5

carefullyintroduced into the aqueouslayer while simultaneouslystirring gently. This

process is continued until the esterlayer becomes clear. Aftersettling, the aqueous

solution is drained, and water alone is added at 28 % by volume of oilfor the final

washing.The basic batch reactordiagram is given in Fig.12.3. Thetransesterification

can be carried out by withoutcatalyst also.

Water

Alcohol

Water

Ester

Biodiesel

Alcohol

catalyst

Dryer

Alcohol

Wash

Water

Acid

BatchReactor

Crude

Glycerol

Neutralized

Glycerol

Fig.12.3. Base Catalysed ReactorSystem [1]

2.3 ACID CATALYSED PROCESSES

Acidcatalyzed processes are usedfor direct esterification of free fatty acids in a high

FFAfeedstock, or to make esters from soapstock. The sensitivities of the acid catalysed

reaction is high FFA content requireswater removal duringreaction. In Acidcatalyzed

reactionthe ratio of alcohol: FFA is 40:1 and also this reactionrequires large amount (5

to 25 %) of catalyst.

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NewerReactions and Procedures : Catalytic and Non catalytic

Fig. 4. Acid catalysed FFA pretreatsystem [1]

2.4 NON-CATALYTIC SUPERCRITICAL METHANOL

TRANSESTERIFICAION REACTION

In this method,

thereaction is

performed by cylindrical autoclave maintained at

particulartemperature and pressure [2].The autoclave will be charged with a given

amount of vegetable oil and liquidmethanol with differentmolar ratios. After eachrun,

thegas is vented, and theautoclave is poured into a collecting vessel. All the rest of the

contentsare removed from theautoclave by washing withmethanol. Thevariables

affectingthe methyl ester yieldduring the transesterificationreaction, such as molarratio

of alcohol to vegetable oil and reaction temperature werealready investigated.The

viscosities of the methyl esters fromthe vegetable oils wereslightly higher than that of

diesel fuel. This method showsthat increase in temperatureespecially critical

temperaturehas a favorable influence of ester conversion. A typicalsupercritical

methanoltransesterification system is shown in Fig. 12.5.

Thetransesterification reaction of rapeseedoil in supercritical methanol was

investigatedwithout using any catalyst. In addition, it was found thatthis new

supercriticalmethanol process requires a shorter reaction time and a simpler purification

procedurebecause of that there is no catalyst.

Biodieselpreparation is also carried out by batch vs continuous flowmethod. Batch is

bettersuited to smaller plants(<1 million gallons/yr).This batch does notrequire 24/7

operation.This provides greater flexibility to tune process to feedstockvariations. The

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12.7

continuousoperation allows use of high-volume separation systems (centrifuges)which

greatlyincreases the throughput and also hybrid systems are possible.

Fig.12.5. Supercritical methanoltransesterification system. (1)Autoclave, (2)Electrical

furnace,(3) Temperature controlmonitor, (4) Pressurecontrol monitor, (5) Productexit

valve,(6) Condenser, (7) Productcollecting vessel [2].

Fig.12.6. Hybrid Batch/ContinuousBase Catalyzed Process[1]

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NewerReactions and Procedures : Catalytic and Non catalytic

2.5PROCESS ISSUES

Themain issue of the basecatalyzed transesterification process is free fatty acid in theoil

whichwhen reacted with alkalinecatalyst will form soaps and it will lead to loss of

catalyst and reduction in theoil.

K-OR + H2O

R-OH + KOH

Acid + KOH

Soap + water

Andwater formation is anotherissue in base catalysed transesterification reaction.Water

will deactivate the catalysts and also it requires the drying of oil.Water hydrolyses fats to

formfree fatty acids and thefree fatty acids react withalkali catalysts formingsoaps.

Soaps

semi solid mixture

glycerol separation

Triglycerids+water

Diglycerids + fatty acid

Anotherissue of the transesterificationreaction is the use of alcohol. Methanol is

commerciallyused. In methanolysis, emulsionforms and separated intolower glycerol

portion and upper ester portion.Reaction time is small. In ethanolysis, emulsionsare

stable and require more complicated separation and purification process and also reaction

time is large.

Thistransesterification reaction carriedout generally by the use of homogeneous and

heterogeneous catalysts. For homogeneouscatalysed reaction, thebasic catalyst used

wereNaOH, KOH, NaMeO and acid catalysts like H2SO4, PTSA, MSA, H3PO4, and

CaCO3. For heterogeneous reaction thecatalysts employed aresulfated zeolites and

clays,hetro-poly acids metalOxides, Sulfates compositematerials.

Basecatalyzed reaction is notsuitable for high FFAfeeds because of soapformation.

Most of the non-edible oilsavailable in India containhigh FFA (2-12%). In order to

decreasethe cost of biodiesel, it is imperative to utilize high FFA oil or fatty acids. So the

preferredmethod for high FFAcontent feedstock is acid catalysisfollowed by base

catalysis.

Themain barriers of homogeneous catalystare the focus on thesensitivity to FFA

and water content of the feedstocks, removal of catalyst, formation of soap with FFA

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12.9

feedstock. A large quantity of effluentwater is the main issue as a result of removal of

catalyst,which necessities pre-treatment of oil in case of high FFAcontent and also no

scopefor regeneration or re-utilization. In heterogeneous catalyst the mainutilization is

catalystregeneration, decrease of catalystcost, utilization of lowerquality feed stocks for

biodieselproduction, simplification of separation process, decrease of production cost

and decrease of wastewater and also development of environmental friendlyprocess.

2.6BIODIESEL FROM JATROPA PLANT

Thejatropa plant is a good source forproducing biodiesel. It is havingthe following main

advantages

· It thrives on any type of soil and it needs the minimalinputs or management.

· It has no insect pests and it is not browsed by cattle or sheep

· It can survive long periods of drought and the propagation by seed/cutting is

easy

· It is having the rapid growth and then it leads to givethe yield from the 2nd

yearonwards.

· Theyield from established plantations is 5 tonne per hactare.

· 30%oil from seeds by expelling and the seed meal is excellent organic

manure.

Fig.12.7. Jatropa Plant

TheEstimated biodiesel production per hectare = 3,000 litres/700Gal and the

potentialyields of 12 tonnes per hectare and 55%oil extraction are also attainable. The

literaturesurveys show that the 2500 trees per hectare produces the seed(6.9 tonnes),

seedcake(4.2 tonnes ), vegetable oil(2.7 tonnes ), glycerol(0.27 tonnes). It has someanti

erosiveproperty like it reduceswind and water erosion of soil and leads to improved

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NewerReactions and Procedures : Catalytic and Non catalytic

absorption of water by soil. Mainly theseedling preparation by 10×20 cm bag and

allowed to get germination by 3 days.

2.7CONCLUSION

Finally it is concluded that thebiodiesel is a renewable fuelfor diesel engines that can be

madevirtually from any oil or fat feedstock and it can provide hugerural employment

potential of 40 to 50 million families and transform the rural economy. It is used in the

remotevillage electrification and powerfor agriculture application.The technology

choice is a function of desired capacity,feedstock type and quality,alcohol recovery, and

catalystrecovery. The dominantfactor in biodiesel production is the feedstock cost which

is around 70%, with capital cost contributing only about 7 % of the product cost.

Thereforehigh FFA, lower qualityfeedstock should be promotedfor biodieselproduction

in India.

3.0CONVERSION OF GLYCEROL TO VALUABLE CHEMICALS BY

ENVIRONMENTALLY FRIENDLY PROCESS

As a renewable feedstock and due to itshigh functionality glycerol, is an attractive

reactantfor the production of a large number of valuablecompounds. Oxidation reactions

are of industrial importance forthe synthesis of finechemicals, eventhough

stoichiometricoxidizing agents (e.g. permanganate) or biotechnological processesare

used and a large number of by-productsare often formed whichdecrease theselectivity

to the desired oxidationproduct. An environmentally friendlyalternative is theoxidation

in the presence of a heterogeneous catalyst and oxygen [3].

Heterogeneouslycatalyzed liquid-phase oxidation of glycerol performedunder

atmosphericpressure and at constant pH usingcarbon supported on gold as catalyst. The

aim of this work is to producehighly interesting chemicalslike glyceric acid fromthe

environmentallyfriendly oxidation of a biosustainable source with high yields.Oxidation

reaction of the glycerol can be explained in the following scheme.

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12.11

12.12

NewerReactions and Procedures : Catalytic and Non catalytic

catalystactivity is achieved by using(i) the gold-sol method and (ii) THPC as reducing

agent. To improve the catalystproperties in the glyceroloxidation by modifyinggold

with a second metal (Pt).Catalysts with 1 wt.% gold and 0.5 wt.% platinum on activated

carbons were prepared by thegold-sol method with THPC as reducing agent. The

presence of platinum in Au/BP catalystssignificantly increases theglycerol conversion

rate. The increase in activity can be maximized by promoting themonometallic gold

catalystswith platinum by theformation of Au/Pt alloyswith a platinum molefraction in

the range from 0.2 to 0.4, which corresponds to a Au0.8Pt0.2 composition.Also the

selectivity is affected by introducing a secondmetal in the Au/C catalysts. In fact, by

promotingthe gold catalyst withplatinum, it was possible to increasethe selectivity to

dihydroxyacetonefrom 26% to 36% at 50%conversion.

4.0 HYDRODESULFURIZATION REACTION

Thebasic operation of a refinery is the conversion of crude oilinto products such as

LPG,gasoline (boiling point<150°C), kerosene (boilingpoint150-250°C), diesel oil

(boilingpoint 250-370°C), fuel oil(>370°C), base oils forlubricants, bitumen, and

feedstocks for petrochemical industries.After separation of the crudeoil into different

fractions by atmospheric distillation, thesestreams are transformed intoproducts with a

highadditional value through a wide variety of catalyticallypromoted chemicalreactions

such as hydrogenation, isomerization,aromatization, alkylation, cracking and

hydrotreating.

Hydrotreatingrefers to a variety of catalytichydrogenation processes in whichsulfur,

nitrogen,oxygen and metal atoms areremoved and unsaturated hydrocarbonsare

saturated. Characteristic for hydrotreatment operations is that there is essentially no

change in molecular size distribution, this in contrast to, for instance,hydrocracking.

Whilehydrodesulfurization (HDS) is assuming an increasingly important role in view of

thetightening sulfur specifications,hydrodenitrogenation (HDN) is necessary to assure

theviability of subsequent upgradingprocesses [5].

4.1Importance of Sulphur Removal from Oil

Hydrodesulfurizationfirst came into practiceduring World War II in theproduction of

petroleum.Sulfur reduction in gasoline is prompted by severalfactors.

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12.13

Manycatalysts in reformer unitsare sensitive to the amount of sulfur in the feed.

In fact, some bimetallicreforming catalysts requirethe sulfur content to be

limited to the vicinity of 1ppm or less.

Airpollution control standardsrequire removal of sometimes up to 80% or more

of the sulfur that would be present in various fueloils.

Some of the sulfur in gas oilfed to a catalytic cracker is in the form of coke,

which is then hydrogenated and released as sulfur dioxide in thecombustion

gases.

This is not desired as thisproposes environmentalharms.

The

organosulfurcontent of the feed to thehydrocracker must be reduced to avoid

poisoning of the hydrocrackingcatalyst.

Thereduction of sulfur reducesthe amount of corrosion in the refining process,

improvesthe odour of the product, and reduces the amount of sulfur that can

poisonthe catalytic converter to an automobile.

One of the biggest movements in recentlegislation for reduction of sulfur in gasoline

products was started by a speech by Bill Clinton on May 1, 1999. He announced a new

EnvironmentalProtection Agency regulationcalling for a 90% reduction of sulfur

content in automobile gasoline in theUnited States by the year2004. Similar effortsare

underwayaround the world.

Thehydrodesulfurization process involvescatalytic treatment withhydrogen to

convertthe various sulfur compounds present to hydrogen sulfide. Thehydrogen sulfide

is then separated and converted to elemental sulfur by theClaus process. From thispoint,

some of the hydrogen sulfide is oxidized to sulfur dioxide by air and sulfur is formed by

theoverall reaction:

2H2S + SO2

3S(s) + 2H2O

Originallythe interest in hydrodesulfurization was initially stimulated to the

availability of hydrogen from catalyticreformers. However the demand for hydrogen for

hydrodesulfurization and hydrotreating is more thanthat can be generated by a refinery.

Because of this, most refineriesrecycle the hydrogen formedfrom sidedehydrogenation

reactions back to the inlet. Since hydrogen is so expensive to manufacture, it is very

12.14

NewerReactions and Procedures : Catalytic and Non catalytic

important to run all hydrodesulfurization and hydrotreating processes at theiroptimum to

reduce costs.

Thesupported molybdenum sulfidecatalyst containing cobalt is operated underpressures

of 150-160 psi hydrogen at 300-400°C.The sulfur content in oil of 1-5% is reduced to

0.1% in gasoline and future sulfurlimits may be reduced to as little as 0.003-0.04%. For

lowpoint and middle boilingpoint distillates, typicalHDS reaction conditions areabout

300 to 400°C and 0.7 to 5 MPahydrogen pressure. The higherthe boiling point of the

feedstock is, the higher thesulfur content. More severeoperating conditions areneeded

forhigher fraction boilingpoints. Then high pressure and low temperaturecombinations

areused to reduce the hydrogenconsumption and correspondingcosts.

HDSreactions are exothermic.Most reactors are adiabatic fixedbeds and may be

multistage.Adding additional hydrogenbetween the stages usuallydoes cooling; the

term"cold-shot cooling" is used to describe this process. If the feed for thereaction

conditions is a mixed vapour and liquid,the liquid is normallycaused to flowcounter-

currentlydownward through a fixed bedcatalyst, or "trickle-bedreactor".

Thesulfur is present largely in theform of thiols, sulfides, and various thiophenes and

thiophenederivatives.

Mercaptans and sulfides react to form hydrogensulfide and

hydrocarbons.

RSSR' + H2

RH + R'H + H2S

RSH + H2

RH + H2S

RSR' + 2H2

RH + R'H + H2S

R and R' are various hydrocarbongroups.

H2S +C4H8 (mixedomers)

+2H2

Scheme.12.3. Reaction pathway of thiophene

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12.15

Studies have indicated that thehydrodesulfurization and subsequent hydrogenation

reactions occur on separate sites. Thethiophene ring is nothydrogenated before sulfur is

removed,although the first stepmay involve an essentiallysimultaneous removal of a

sulfuratom and donation of twohydrogen atoms to themolecule.

Forbenzothiophene, substituted or unsubstituted, the thoiphenering is hydrogenated

to the thiophane derivativebefore the sulfur atom is removed, in contrast to thebehavior

of thiophene. The reactionpathways for dibenzothiopheneare as follows:

Scheme. 4. Reaction pathway of dibenzothiophene

4.2Preparation of Catalyst

Catalystsused in industry are derivedfrom oxides of such elementlike Mo, W, Co, Ni

supported on different compounds, althoughthe most commonly used is alumina. The

catalystused in HDS is almost alwaysCoMo/Al2O3, and sometimes NiMo/ Al2O3. The

ratio of molybdenum to cobalt is alwaysconsiderably greater than 1.

Themolybdenum sulfide catalyst is prepared by impregnation of γ- Al2O3 with an

aqueoussolution of ammonium molybdate and cobalt nitrate or nickelnitrate. This

precursor is dried and calcined, whichconverts the molybdenum to MoO3. This is then

treated with a mixture of H2S and H2 or a feed containing sulfurcompounds and H2. The

resultingmolybdenum catalyst is almostcompletely sulfided.

If the catalyst is not

completelysulfided, then there is thepossibility, that it will not be acting as a active

catalyst.

4.3Conclusion

Since the mechanism for thehydrodesulfurization of thiophenes is notcompletely

understood,there has been extensivework to try and develop themechanism and kinetics

forthe reactions in order to develop better catalysts.Nickel treated compounds have had

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NewerReactions and Procedures : Catalytic and Non catalytic

somesuccess, and while thenickel containing catalystsappear to be better at sulfur

removal,the Co-containing catalystsgive slightly more oilyield. In the end, it may be a

simplematter of economics that determines which catalyst is used.

5.0

CATALYTIC

AND

NONCATALYTIC

STUDY

OXIDATIVE

DEHYDROGENATION REACTION

Ethene is one of the most basic feedstocks in chemical industry and its demand is steadily

increasing.The main commercial routesfor production of ethene aresteam thermal

cracking and FCC (fluid catalytic cracking)processes. The drawback of these methods is

highenergy input required by thehighly endothermic reactions,high operation costs due

to coke deposition on catalyst and reactor, and generation of low molecularweight

alkanes. The reserves of rawmaterials for theseprocesses are becomingincreasingly

limited.Consequently, alternative processeswith higher efficiency,which utilizes more

abundant and economic sources for ethene production, are becomingincreasingly

necessary. Ethane is the second majorcomponent of natural gas and is also abundant in

refinerygas.

5.1Oxidative Dehydrogenation(ODH)

Production of ethene via oxidative dehydrogenation(ODH) of ethane [6] has received

increasingattention, owing to itspotential advantages, such as exothermicreaction heat

and less coke deposition. Thismethod can be carried out at relatively low temperatures in

thepresence of properly selectedcatalysts. Until now,numerous catalysts wereemployed

forthe ODH of ethane, such as composite oxides betweenalkaline earths and rare earths,

halogen(particularly F and Cl) and/oralkali ion-promoted, as well as some transition

metal(Mo, V, Bi, etc.) oxide-based catalysts.

As an alternative to the heterogeneous route, a hetero-homogeneous processfor the

ODH of ethane at temperatures higher than 900◦C, the so-called auto-thermaloxidative

dehydrogenation, was also employed. The followingreaction carried out by vanadium

magnesiumcatalyst and withoutcatalyst.

5.2Catalyst Preparation

Meso-VMgcatalysts were prepared by using vanadium source like V2O5 and the

magnesium source like magnesium chloride(MgCl2·6H2O).Surfactants such as

cetyltrimethylammoniumbromide (CTAB), sodiumdodecylbenzene sulfonate(SDBS),

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12.17

benzyltrimethylammoniumbromide (BTAB), and thetemplate is hexadecylamine(HDA)

wereused for the synthesis of the mesoporous materials.

MgCl2·6H2O and thetemplate were dissolved into an aqueous solution of

hydrochloric acid. The vanadium source was dispersedhomogeneously intodistilled

waterwith vigorous stirring. Thenthe solution containingvanadium was addedslowly

intothat containing magnesium,with vigorous stirring at room temperature. The pH of

themixture was adjusted to 4.0 or 10.0. After stirring at roomtemperature for 24 h, the

mixture was allowed to age statically at room temperature for 2 days.The solid formed

was recovered by filtration, washed withdistilled water, and dried at 100◦C for 12 h. To

removethe surfactant, the preparedspecimens were heated in a flow of argon fromroom

temperature up to 750◦C at a rate of 10◦Cmin-1 and kept at that temperature for 4 h.

Meso-V was prepared using the sameprocedure, except that no magnesium was

introduced and the pH of the mixture was adjusted to 7.0.

TheMix-VMg catalysts wereprepared via a solid-statereaction. Powders of

vanadium and magnesium source were mixedtogether and ground thoroughly in a

mortar, and the mixture obtained was calcined at 750◦C for 2 h after it was heatedfrom

roomtemperature at a rate of 4 ◦Cmin-1.

5.3Reaction Set Up

Thecatalytic performance was determined at atmospheric pressure in a tubularfixed-bed

quartzmicroreactor (internal diameter = 5 mm, operation length = 30 cm). The reactor

was packed as the middle of thereactor was plugged withquartz wool and a catalyst

(about0.25 g) was located over it.The space of the reactor above the catalyst bed was

filledwith quartz granules. Thereactor was placed into a tubularfurnace with thecatalyst

bedlocated in the constanttemperature zone.

In addition, four other reactorconfigurations free of catalystwere employed in the

study of the noncatalytic conversion of ethane, namely, ET (the empty tube), FQ (the

reactorfilled with quartz granules up to a height of 1 cm), HFQ(the upper half of the

reactor was filled with quartzgranules and the rest of thereactor was empty), and FFQ

(theentire reactor was filledwith quartz granules). Two thermocouples were employed to

monitor and control the temperature.One of them was embedded in the furnace, and the

other one was located in the center of the catalyst bed and tightlycontacted theexternal

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NewerReactions and Procedures : Catalytic and Non catalytic

surface of the quartz reactor. The temperatures measured by these twothermocouples

werealmost the same. Thecompositions of reactant mixtures (N2, O2, and C2H6) and

gaseouseffluents from reactor weredetermined by on-line gaschromatography withFID

and TCD detector

5.4Results

Theresults are shown in Fig12.8.

At low temperatures, thedifferences betweenthe

yields of ethene for the noncatalytic and catalytic thermolysis and ODH of ethane are

smallwhen the conversions of ethane are low. The highestyield of ethene forthe

noncatalyticODH occurs for a conversion of about 7%; however, theyield to ethene for

thecatalytic ODH can be higher,being larger for theMeso-VMg catalysts than the Mix-

VMg ones and the highest forthe Meso-VMg-3catalyst.

Fig.8(a). Yield of ethene as a function of conversion of ethane for noncatalytic and

catalyticthermolysis and ODH of ethane at 550 ◦C (The thermolysis cases aremarked by

-T.) [6]

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12.19

Fig. 8 (b) Yield of ethene as a function of conversion of ethane for noncatalytic and

catalyticthermolysis and ODH of ethane at 700◦C. (The thermolysis cases aremarked by

-T.) [6]

Thehigher performance of themeso-VMg for ODH of ethane than the Mix-VMgones

may be due to the V2O3 phase containing highlydispersed magnesium species and

possessinglarge specific surface area in the former case. The Mg species probably

moderate the redox capability of V2O3, thuscontrolling the activations of ethane and

oxygen.The activation mechanism of ethane over these catalysts is dependent on

temperature and the heterogeneous processes occur at low temperatures, whereas

heterogeneoushomogeneousones account for thebehavior of the catalysts at high

temperatures.

6.0 PREPARATION OF POLYORGANOSILOXANES BY SUPERCRITICAL

FLUID METHOD

Polyorganosiloxanes or silicones are the mostpopular silicon-based polymericmaterials

in which the backbone is composed of repeating SiO linkages. It has good thermal

stability,low-temperature stability, weatherability, transparency, and electricinsulation.

Thematerials are used in almostall industries includingautomobile, construction,

electronics,personal and household care, and chemical industries.Silicones

manufactured by sequential hydrolysis and polycondensation reactions of chlorosilanes

with or without using organicsolvents.

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NewerReactions and Procedures : Catalytic and Non catalytic

Thehydrolysis and polycondensation processes in an organic solvent areconducted

in the presence of an acid catalystbecause the hydrolysis rate of alkoxysilanes is smaller

thanthat of chlorosilanes. Theneutralization process is also necessary to obtain final

productseven though alkoxysilaneswere used as the substrates. In addition, a large

amount of organic solvent waste is generated in both cases. According to these facts, the

currentsilicone manufacturing processescannot be regarded as an environmentally

benignprocess. Typical examplesfor syntheses of poly(phenylsilsesquioxane)starting

fromphenyltrialkoxysilanes and fromphenyltrichlorosilane, both of whichare performed

in a significant amount of an organicsolvent for more than 10 h, have been reported

6.1Supercritical FluidMethod

Thistechnology has attractedsignificant interest for thelast two decades from an

environmentalviewpoint. The mostattractive aspect of thistechnology is to reduce or

eliminatethe use of organic solvents in process. Supercritical carbondioxide (scCO2) or

supercriticalwater (scH2O), is used as a solvent. Manyindustrial applications areunder

development of several which have beenglobally been commercializedbasically due to

reduction of hazardous material waste. SeveralSCF-based technologies havebeen

proposed to synthesize silicones and otherSi-containing materials withoutusing organic

solventslike Polysiloxane synthesis by a catalytic polymerization of siloxanecyclics and

silanol-terminatedsiloxane oligomers, a hydrosilylationprocess yieldingfunctional

polysiloxanes, and silica aerogel production by sc-CO2 drying were reported as scCO2-

assistedtechnologies. On the otherhand, scH2O-assistedtechnologies include i) recycling

of waste silicone elastomers via treatmentwith MeOH/H2O mixture at hightemperature,

ii)silicone particle formation by degradation of electrophotographic developercarriers,

iii)silicon nanotubes formation, and iv) silicon oxide nanowiresformation. These

technologiesare not tailor-made siliconesynthetic processes. Thefollowing method is

thefirst example of a catalyst-freesilicone synthesis viasequential hydrolysis and

polycondensationreactions of alkoxysilanes [7].Organic solvent-solublenon-linear

siliconessuch as silsesquioxanes were selected as target materials due to theirhigh

potentialfor development of value-addedproducts.

Synthetic Strategies in Chemistry

12.21

6.2Equipment

Thismethod need the followingequipments

A reactor and narrow tubingfor the high T&P processwere made of ½"and1/8"

stainless steel tubing.

Theinternal volumes of thereactor are 10 and thetubing were 2 mL.

A pressure gauge was a KH15pressure transmitter and a sandbath was used as a

heat source in which sand is circulated by a compressor to maintain the

temperaturedeviation within ±3◦C.

Productsare analyzed by GC-MS

6.3Synthesis of Poly(phenylsilsesquioxane)(PPSQ)

PTMS and deionized water were loaded in a one-end capped reactor and sealed

with a connector attached to narrow tubing(stainless steel coil withthe diameter of 1/8";

length: 90 cm; internal volume: 2 mL) as a trap and a pressuregauge. The reactor was

placed in a preheated sand bath at 300◦C to start the reaction. Duringheating, pressure

insidethe reactor was monitoredperiodically. After thepressure value becameconstant,

thereactor was pulled out of the sand bath and pouredinto a water bath to terminate the

reaction.After the work-up similar to that for the trap-freesystem, a solid product was

obtained.

12.22

NewerReactions and Procedures : Catalytic and Non catalytic

of alkoxysilanes. In this study,alkoxysilanes were used as substrates since chlorosilanes

arenot appropriate due to evolving corrosivegas of HCl.

Synthesis of PPSQ via hydrolysis and subsequent polycondensation reactions of PTMS

was selected as the firstprocess because hydrolysis of phenyl-based alkoxysilanes at high

temperatures proved to be more controllablethan that of methyl-basedalkoxysilanes. The

overallreaction intended is as follows:

C6H5Si(OCH3)3 +3/2H2O

C6H5SiO3/2 +3CH3OH

Theinitial study was made by a batch process using a stainless tube reactor withthe

diameter of ½" and the volume of 10 ml.The reactor containing PTMS and excess

amount of water was heated at 300◦C. During this reactionmethanol formed above 200◦C

as a byproduct.

Fig.12.9. Reaction equipment for a trap-attached system in whichthe narrow tubing

betweenthe reactor and the pressuregauge acts as the trap.The sizes of the reactor and

thetrap are 10 and 2 ml,respectively [7]

As the amount of substratesincreases, the molecularweight of PPSQ increases to

thetop value and then decreasesindicating that the reaction is strongly depending on the

pressure. This is because the reaction is an equilibrium reaction in whichthree molar

methanol as a byproduct generates from one molar phenyltrimethoxysilane. Thepressure

range at which the PPSQ's molecularweight becomes the maximum is between

Synthetic Strategies in Chemistry

12.23

approximately 3 and 7. It is notable that themolecular weight of PPSQincreased by

introducingnarrow tubing as a trap in the reaction system. This is because trapping of

methanol

shifts

the

equilibration

the

product-side.

The

process

yields

polyorganosiloxaneswith relatively high content of SiOH and SiOMegroups.

Synthesis of polysiloxanes with otherstructural units by usingvarious alkoxysilanes is

also prepared by the same method. It can explain in the followingmanner.

6.4 STRUCTURAL UNITS OF POLYORGANOSILOXANES

Fig.12.10. Structural units of polyorganosiloxanes. R: methyl, phenyl,vinyl, H, and

otherreactive groups such as aminopropyl and glycidoxypropyl.

Q unit: SiO4/2

Tetramethoxysilane(TMOS) yielded an insolublesolid product based on Q units with

significantamount of OH/OR groups. This can be as a part of the alkoxysilanes was

trapped into the tubing by evaporation as water is and then undergoes hydrolysis to yield

insolublesolid in the tubing.

T unit: MeSiO3/2

Methyltrimethoxysilane(MTMS) was used as a T source. Thisreaction carried out by

withouttrap because of the highvolatility of this compound and it leads to theformation

of highly reactive hydrolysatewith high content of silanol/methoxy that turnedinto an

insolublematerial.

D unit: Me2SiO2/2

Polycondensationbetween a silanol-terminatedoligodimethylsiloxane as a D source

and other alkoxysilanes such as PTMS, MTMS, and methylphenyldimethoxysilane

(MPDMS)proved to be possible. This reactionshows that this oligomer is stable at 300◦C

in the presence of excess waterwithout both of dehydrativeself-condensation as shown

in the following Schemeand siloxane bond cleavage. These resultsindicate that the

polycondensation observed here proceeds by a de-methanolreaction.

12.24

NewerReactions and Procedures : Catalytic and Non catalytic

Scheme.12.6. Dehydrative condensation of silanol-terminatedpolydimethylsiloxanes

6.5CONCLUSION

It can be concluded that polysiloxanescomposed of any D, T, and Q unitsare easily

synthesized.The volatility of thesubstrates is a critical factor to select the reaction

systemwith narrow tubing as a methanol trap. Since siloxanebond cleavage hardlytakes

place, the present system is advantageous forthe material design by whichthe structure

of siloxane-containing staring materialremains in the polysiloxaneproduct. The largest

advantage of this new process is no contamination of volatile organiccompounds in the

productbecause the product is obtained as a solvent-free form. In addition, as the present

syntheticmethod is simplified due to skip of the neutralization processbeing necessary

for a conventional solution-based process,this is also advantageous from theeconomical

standpoint.

7.0REFERENCES

1. http://biofuels.coop/pdfs/4_commercial.pdf

2. Ayhan Demirbas, EnergyConversion and Management, 44 (2003)2093.

3. S. Demirel, K. Lehnert, M. Lucas, P. Claus, Appl. Catal. B: Environmental 70 (2007)

637.

4. R. Garcia, M. Besson, P. Gallezot,Appl. Catal. A: Gen. 127 (1995) 165.

5. http://www.che.lsu.edu/COURSES/4205/2000/Mattson/HDS.htm

6. Zi-Sheng Chao and Eli Ruckenstein,Journal of Catalysis 222 (2004)17.

7. Takuya Ogawa, Jun Watanabe,Yoshito Oshima, Journal of Supercritical Fluids 45

(2008)80.

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