Adipic Acid Production Processes PDF

ADIPIC ACID Adipic acid is a very important building block in the industrial chemistry, with a world production of 3,5 x 106 t/y. It’s used mainly in the synthesis of NYLON 6 and 6,6, some of the most important polymers for the fiber production, however it’s also widely used as plasticizer and in th...

ADIPIC ACID Adipic acid is a very important building block in the industrial chemistry, with a world production of 3,5 x 106 t/y. It’s used mainly in the synthesis of NYLON 6 and 6,6, some of the most important polymers for the fiber production, however it’s also widely used as plasticizer and in the polyurethane field. The POLYCONDENSATION PROCESS is quite simple, in terms of chemical reaction: However, to obtain HIGH QUALITY NYLON 6,6, it’s necessary to start with PURE MONOMERS, with a PERFECTLY DEFINED STOICHIOMETRY 1:1: when the two reagents are mixed, the formation of a SALT [-OOC (CH2)6COO-][+H3N(CH2)6NH3+] (Hexametylen Ammonium Adipate) is observed. This salt needs to be purified by crystallization. The CLASSIC COMMERCIAL ROUTE FOR THE ADIPIC ADIC SYNTHESIS can be schematize as: This reaction PRODUCES NOx, which can be dangerous and need to be disposed of properly. PHENOL STRATEGY Even if it requires 2 STEPS (transformation of benzene into phenol and then phenol into AA), it can be advantageous because of the LOW INVESTEMENTS COSTS. Moreover, it allows the production of a KA OIL MIXTURE with a HIGHER AMOUNTS OF KETONES, which CONSUME LESS H2 AND HNO3 in the successive oxidation step. SELECTIVE BENZENE HYDROGENATION TO CYCLOHEXENE Even though the TOTAL HYDROGENATION TO CYCLOHEXANE is thermodynamically more favorable, this process was developed by ASAHI, in 1990, trying to find a SIMPLER AND SAFER reaction. The solution was found in the HYDRATION, ON A ZEOLITE-BASED CATALYSTS, to produce CYCLOHEXANOL. This process also allows to reduce H2 consumption by one third The process is based on the use of a Pt OR Ru-BASED CATALYST POWDER, COATED WITH A LAYER OF AQUEOUS ZnSO4, to perform a PARTIAL HYDROGENATION: since the catalyst is surrounded by the aqueous phase, the ORGANIC MOLECULES that are BETTER SOLUBLE IN THE AQUEOUS PHASE are preferentially hydrogenated. Since cyclohexene is less soluble than benzene, it migrates to the organic phase, PREVENTING FURTHER HYDROGENATION. CYCLOHEXENE is obtained with 80% SELECTIVITY (about 20% of cyclohexane), at 70-75% benzene conversion. It is then SEPARATED FROM CYCLOHEXANE through an extractive distillation, probably with DIMETHYLACETHAMIDE (high boiling point to have an easier separation). As said before, the stream is then HYDRATED to generate pure CYCLOHEXANOL. CYCLOHEXANE OXIDATION The first and simplest way to produce the KA OIL MIXTURE, discovered by DUPONT in the ‘40s, was the TOTAL HYDROGENATION OF BENZENE TO CYCLOHEXANE, FOLLOWED BY ITS EXOTHERMIC OXYGENATION. The first step is carried out at 150-180 °C and 10-20 atm in the presence of a Co or Mn ORGANIC SALT (naphtalenate) CATALYSTS. It follows a RADICAL MECHANISM, which includes 2 SUCCESSIVE PASSAGES:  CYLOHEXANE HYDROPEROXIDE GENERATION. This is the rate limiting step, sometimes the use of a catalyst can be avoided.  CHHP DECOMPOSITION, carried out SEPARATELY with optimized catalyst amount. However, because of the mechanism itself, the consecutive reaction that leads to CARBOXYLIC ACIDS are favoured, causing a DECREASE IN SELECTIVITY: therefore, it’s necessary to LIMIT THE PER-PASS CONVERSION to 5-7%. In fact, the plant was built with 3 or 4 PARALLEL OXYDATION REACTORS, for the INTERNAL RECYCLE OF CYCLOHEXANE (and regeneration of the catalyst without process shut down), to keep the CONVERSION LOW AND CONTROLLED, even if it’s necessary to add a NEUTRALIZATION SECTION with a CAUSTIC WASHING to ensure the absence of acidic by-products. The KA mixture is then DISTILLED to separate it from unreacted compounds, in the column the pressure is reduced to facilitate the separation. An OPTIMIZATION TO INCREASE THE KA OIL SELECTIVITY, TOGETHER WITH THE PERMITTED CONVERSION PER PASSAGE (7%  12%), was the substitution of the catalyst in the initial series of oxidation reactors with ANHYDROUS META-BORIC ACID as a slurry, which react with CHHP to give a BORATE ESTER, that stabilizes the product and reduces its tendency to be oxidized further. The MAJOR DRAWBACK of the process is the NEED TO HYDROLIZE WITH HOT WATER the borate ester in order to recover the wanted CYCLOHEXANOL. This is an ENERGY-INTENSIVE STEP, so it requires to be properly OPTIMIZED to ensure profits. KA OIL OXIDATION in this step, the USE OF A Cu (II)-BASED CATALYST, favors the production of KEY INTERMEDIATES, Nitrosyl cyclohexanol and Adipomononitrolic acid, by limiting multiple Cyclohexanone Nitrosation and the formation of glutaric acid. The process can be summarized with the following mechanism: This step requires huge amounts of HNO3, with all the related HAZARDS: the reaction is EXOTHERMIC and can reach an AUTOCATALYTIC RUNAWAY STATE above 120°C, causing the EVAPORATION of the liquid and the consequent possible EXPLOSIONS. Therefore, this step is carried out at around 90°C in LOW VOLUME STIRRED TANK REACTORS, in which the KA FEED IS LIMITED in respect to a LARGE EXCESS OF NITRIC ACID with strict CONTROLS ON TEMPERATURE AND PRESSURE. A FINISHING REACTOR is sometimes employed to achieve improved PRODUCT QUALITY. NOx are removed from the product stream by BLEACHING, while the solution containing AA is cooled and the product CRYSTALLIZED. The CRUDE PRODUCT is removed via FILTRATION OR CENTRIFUGATION and the MOTHER LIQUOR is returned to the reactor after an evaporation step to achieve the right concentration of HNO3. Further REFINING is required to achieve POLYMER GRADE MATERIAL, usually by RECRYSTALLIZATION FROM WATER. The BLEACHER OFF-GAS, containing various NOX, is ABSORBED IN WATER, generating NITRIC ACID FOR RECYCLE. THE N2O present in the gaseous stream can’t be converted and requires FURTHER TREATMENTS. NITROUS OXIDE TREATMENTS As said before, the formation of adipic acid form KA oil mixture generates huge amounts of N2O (Almost 50 g/kg of Polyamide 6,6), which is 200 times MORE POTENT GREENHOUSE GAS THAN CO2 (It’s involved in ozone layer damage), and thus requires to be treated and transformed in something SAFER: METHANIZATION DECOMPOSITION CH4 + 4 N2O  4 N2 + CO2 + 2 H2O N2O + 0,5 O2  N2 + O2 The treatment of N2O with METHANE can be considered advantageous because of the STOICHIOMETRY of the reaction since it enables to treat huge amounts of N2O with relatively low quantities of methane. However, methane is a VALUABLE AND EXPENSIVE SOURCE, so this process is not widely diffused. The DECOMPOSITION PROCESS was the first developed and, until now, the most favorable treatment. This STRONGLY EXOTHERMIC reaction is carried out using CATALYTIC FIXED BED REACTOR, where the feed is DILUTED WITH AIR in order to avoid reaching the RUNAWAY STATE. This situation would cause the generation of high quantity of OXYGEN in an HOT ENVIRONMENT, dramatically increasing the RISK OF EXPLOSION. Different SOLUTIONS were studied to achieve a better CONTROL of the reaction TEMPERATURE: the RADICI PROCESS uses a CATALYTIC BED REACTOR divided in THREE SECTIONS. Each section is fed with a FRACTION OF THE N2O FEED (one third), enabling an easier to control the temperature while AVOIDING DILUTING THE FEED WITH AIR. ALTERNATIVE ROUTES Due to the huge number of ISSUES AND HAZARDS of the classical commercial route, many ALTERNATIVES were studied by industries, but only few of them could be developed as VALID AND COMPETITIVE LABORATORY-SCALE AND INDUSTRIAL PROCESSES. 2 PHASES CATALYTIC PROCESS This process takes advantage of the initial presence of 2 IMMISCIBLE PHASES. In the ORGANIC PHASE only CYCLOHEXENE is present. In the AQUEOUS PHASE the other various reactants are dissolved: the REACTION CATALYST (usually TUNGSTENATE ANIONS), the PHASE TRANSFER CATALYST (usually Aliquat 336) and the OXYDANT (usually H2O2). The MECHANISM of the reaction is dictated by the IONIC/NON-IONIC EQUILIBRIUM in water of the catalyst: the IONIC FORM interacts with H2O2, forming a COORDINATION BOND. Due to his properties, Aliquat 336 is able to transport the complex to the ORGANIC PHASE, where 4 OXYDATION STEPS occur to generate Adipic Acid: Even if this process can be considered GREEN, because of the ABSENCE of a SOLVENT, its application in an INDUSTRIAL SCALE is HIGLY UNLIKELY. This is due to the HAZARDS AND COSTS that the USE OF H2O2 ENTAILS. AA FROM GLUCOSE It’s also possible to generate adipic acid starting from BIORENEWABLE MATERIALS: it’s possible to take advantage of the FERMENTATION PATHWAY catalyzed by specific ENZYMES that able to convert GLUCOSE INTO CIS- CIS-MUCONIC ACID. This compound then requires only a HYDROGENATION STEP to be converted in Adipic Acid. Even if this technology is interesting in GREEN TERMS, it’s NOT COMPETITIVE with the traditional method in terms of COSTS: the CONTROL OF THE FERMENTATION BATCH AND THE SEPARATION STEPS entail very high costs. GREEN CHEMISTRY Green chemistry is a thinking current and a tool, whose mission is to PROMOTE INNOVATIVE CHEMICAL TECHNOLOGIES THAT REDUCE OR ELIMINATE THE USE OR THE GENERATION OF HAZAROUS SUBSTANCES in the design, manufacture and use of chemical products, leading to a MORE SUSTAINABLE CHEMISTRY. In fact, the INDUSTRIAL SUSTAINABILITY, namely the continuous innovation, improvement and use of CLEANER AND SAFER TECHNOLOGIES to reduce pollution levels, consumption of resources and improve safety and quality of work, while maintaining industrial competitiveness, has grown significantly from the ‘90s, driven by SOCIAL PRESSURE AND LEGISLATION. During the previous years, from the ‘60s to the ‘80s, a NEGATIVE IMAGE OF CHEMISTRY born, because of the LIMITED ATTENTION TO IMPACT ON THE ENVIRONMENT AND HUMAN HEALTH and the increase in size of chemical plants and their concentration in the same location, due to the SCALE ECONOMY, that lead to major CHEMICAL ACCIDENTS. Therefore, people became more aware about the POTENTIAL DAMAGES that an improper use of many substances or sources may cause, so, the 12 PRINCIPLES OF THE GREEN CHEMISTRY were born: Even though it may not seem like that, these principles are IN HARMONY WITH THE R&D ECONOMICAL INDUSTRIAL PHILOSOPHY, because they have the same DESIDERABLE CONDITIONS: Processes with high No toxicity and eco- yields and low compatibility of Direct conversion amounts of by- reactants, products, products solvents and waste Moderate T and Catalytic processes Pressure MEASURE OF SUSTAINABILITY In order to be able to COMPARE TWO PROCESSES from the “greeniness” point of view, different PARAMETERS have been developed: Atom Economy (AE) It is a simple calculation BASED ON THE STOICHIOMETRY, but does not account for solvents, reaction yield and reactant molar excess: AE (%) = [MWproduct/MWreactants] x 100 E-factor It takes into account the REACTION YIELD and include reagents, solvents losses, all process aids: E-factor = total waste (tons)/product (tons) Atom Efficiency = Yield x AE Effective Mass Yield EMY = [Desired products (tons) / All non-benign materials (tons)] x 100 However, this parameter can be considered ambiguous, because it's necessary to strictly define WHAT IS NON-BENIGN Carbon Efficiency It's the percentage of CARBON IN REACTANTS THAT REMAINS IN THE FINAL PRODUCT; it takes into account the yield and stoichiometry and is directly related to greenhouse gases Reaction Mass Efficiency (RME) It's the percentage of the MASS OF REACTANTS THAT REMAINS IN THE PRODUCTS; it takes into account the atom economy, yield and reactant stoichiometry A known example of the relevance of these parameters was the modification of the 1962 BOOTS process for the production of IBUPROFEN, a common painkiller, that, in 1992, increased its AE from 40% TO 77%. This was possible because BHC Company developed an efficient new technology, which involves ONLY 3 STEPS, INSTEAD OF SIX. Furthermore, the old process involved a FRIEDEL-CRAFTS ACETHYLATION, followed by a DARZENS CONDENSATION, to obtain an epoxyester, which then undergoes HYDRATION; the aldehyde thus formed reacts with an HYDROXYLAMINE, in order to get an OXIME, that, by a DEHYDRATION step generates a nitrile moiety and, then, the final acidic moiety. As it’s possible to understand, all these stoichiometric reactions INTRODUCE AND THEN REMOVE FUNCTIONAL GROUPS, ABSENT IN THE FINAL PRODUCT. Moreover, they produce HUGE AMOUNTS OF AQUEOUS AND NON-AQUEOUS WASTES. These problems were solved by replacing AlCl3 with ANHYDROUS HF, used both as catalyst and solvent, which can be RECYCLED. The process produces ACETIC ACID, that can be recovered and reconverted into ACETIC ANHYDRIDE. The successive step is the REDUCTION, of the ketone moiety, trough a selective catalyst (Ni- Raney). The FINAL CARBONILATION STEP, catalyzed on Pd, allows to obtain Ibuprofen. CATALYSIS Over 90% of the INDUSTRIAL CHEMICAL PROCESSES use one or more CATALYTIC STEPS, because the use of a catalyst enables a chemical reaction to proceed FASTER OR UNDER DIFFERENT, USUALLY MILDER, CONDITIONS. Furthermore, in complex reactions, it enables to maximize a specific reaction pathway providing a SELECTIVE SYNTHESIS, reducing the amount of by-products and wastes. Nowadays, the use of HOMOGENEOUS CATALYSIS is progressively decreasing, because, despite ITS HIGH SELECTIVITY, it has MANY DRAWBACKS THATINVOLVE HIGHER COSTS:  It requires the use of a SOLVENT: the reaction usually happens in the liquid phase, so the reactors need to be bigger  It's harder to properly RECOVER the CATALYST: the turnover per reactor volume is very low Therefore, it’s preferable to use HETEROGENEOUS CATALYSIS, which has fewer drawbacks. A known example was the modification of the CLASSICAL OXO PROCESS, namely the HYDROFORMYLATION PROCESS (Which consists in the transfer of one CO function to the double bond of an alkene, with the consequent generation of an aldehyde), from the COBALT-BASED HOMOGENEOUS CATALYSIS TO THE RODIUM- BASED BIPHASIC PROCESS. The major PROBLEMS OF THE OLD PROCESS were the RISK OF USING FLAMMABLE SOLVENTS AND THE DIFFICULTIES IN RECOVERING THE CATALYST. However, these issues were solved with the introduction of a BIPHASIC METHOD, which takes advantage on the use of a water soluble RODIUM-BASED CATALYST with a Triphenylphosphine- trisulfonate moiety (TPPTS). L= This specie is initially present in the water-based solution inside the reactor and the reaction can occur at the INTERFASE WITH THE BUBBLED ALKENE FEED, therefore, this olefin must be PARTIALLY SOLUBLE IN WATER, as PROPYLENE. After the reaction occurred, the SEPARATION OF THE TWO PHASES can be performed in a SIMPLE DECANTER, from which, it’s possible to COMPLETELY RECOVER AND RECYCLE THE CATALYST PRESENT IN THE WATER PHASE, because of the absence of organic traces. BIOCATALYSIS The use of SPECIFIC ENZYMES (Mainly hydrolases and redox types) in different reaction steps has grown significantly in recent years. In the PHARMACEUICAL AND FOOD INDUSTRY, the ADVANTAGES of this approach, commonly balance the DRAWBACKS of their use: Advantages Drawbacks Enzymes only catalyze one specific Problem of mixing (huge amount of reaction (in both directions usually) solvents) They show a very high selectivity Cost of separation: usually carried They work under mild conditions (pH out using membranes = 4-9 and 10-50 °C), decreasing the High sensitivity to denaturation with energetic requirements small changes in operational They can reduce pollution and waste parameters High production costs A well-known example was the modification of the CEPHALEXIN SYNTHESIS: the original process involved a FERMENTATION STEP, followed by 13 CHEMICAL REACTIONS. By using ENZYMES, it’s possible to complete the process in TWO STEPS, greatly improving the sustainability of the process as well as reducing the costs. Furthermore, because of their HIGH STEREO AND REGIOSELECTIVITY (ee > 99%), particular enzymes are able to catalyze the formation of STEREOCENTRES FROM PROCHIRAL SUBSTRATES, OR TO DISCRIMINATE BETWEEN ENANTIOMERS IN A RACEMIC MIXTURE. Therefore, taking advantage on this property, KINETIC RESOLUTION OF RACEMATES WITH ENZYMES to obtain ENANTIOMERICALLY PURE COMPOUNDS is a commonly used technique in PHARMACEUTICAL INDUSTRIAL APPLICATIONS. Another fundamental example was the FIRST EFFICIENT PROCESS FOR THE TRANSFORMATION OF A RACEMATE OF AN ALCOHOL INTO ENANTIOMERICALLY PURE PRODUCT, performed in 1997 by the Bäckvall group, which were able to overcome the LIMITATION OF CONVERTING ONLY ONE OF THE ENANTIOMERS. In this process, in fact, enzyme catalysis is combined with TRANSITION METAL CATALYSIS: the alcohol is, initially, exposed to a RACEMIZATION RUTHENIUM-BASED CATALYST, that interconverts the two alcohols during the enzymatic resolution, therefore, the enzyme, which recognizes only one of the enantiomers, is able to produce an enantiomerically pure product. OXIDATIONS CATALYTIC SELECTIVE OXIDATIONS play a central role in industrial chemistry, since a large portion of materials and commodities in daily use undergoes a selective oxidation process as a critical step in their production. However, oxidations are COMPLEX REACTIONS, with several TECHNOLOGICAL ISSUES:  Selectivity  Removal of reaction Heat  Efficient gas-liquid and gas-solid Contact  Safety Therefore, many STRATEGIES WERE DEVELOPED, TRYING TO IMPROVE THE SUISTAINABILITY of this kind of operations: New feedstocks New oxidants (i.e. substitution of (i.e. hydrogen alkenes with peroxide) alkanes) Development of Process new reactor design development One example could be the process developed for the production of VANILLIN using PHENOL AS FEEDSTOCK: MALEIC ANHYDRIDE Until the ‘60s, BENZENE was the UNIQUE STARTING MATERIAL for the production of MA: However, this process had several DRAWBACKS:  Benzene toxicity  Low benzene CONCENTRATIONS (1-1,5% molar) were allowed to stay BELOW FLAMMABLE LIMIT  Around 25% of the FEED WAS COMPLETELY OXIDIZED TO CO2, causing a huge increase in the E-factor, adding also the environmental problems related to the generation of such a big amount of CO2  Only a part of MA (50%) can be easily RECOVERED BY COOLING, the remaining must undergo a WASHING STEP, generating maleic acid, followed by DEHYDRATION AND EVAPORATION Therefore, in 1962, DENKA introduced the FIRST BUTENE OXIDATION-BASED PROCESS in order to generate MA; however, because of the RISE OF BUTENE PRICE, this innovation had short life, because the production went back to BENZENE. The huge turning point occurred during the OIL CRISIS, in ‘70s, in which a lot of companies converted their plant to BUTANE OXIDATION PLANTS, which was a SIMPLE AND ECONOMICALLY VIABLE, because of the use of the same chemistry: Furthermore, the use of BUTANE as starting material, introduced several ADVANTAGES:  Butane is CHEAPER AND NON-TOXIC  NO LOSS OF CARBONS by combustion to CO2  Better selectivity  LOWER FRAMMABLE LIMITS The plant remains VERY SIMILAR to the one which use benzene as starting material, with the difference that the MA generated cannot be recovered by condensation, due to the presence of water, but it must be ABSORBED WITH AN ORGANIC SOLVENT. The CRUDE MALEIC ACID obtained is CONCENTRATED, DEHYDRATED and finally the purified Maleic Anhydride is obtained THROUGH DISTILLATION. DAVY PROCESS The Davy process technology converts MALEIC ANHYDRIDE INTO 1,4-BUTANDIOL (BDO), TETRAHYDROFURAN (THF) AND GAMMA BUTYROLACTONE (GBL), useful compounds for many applications. This transformation id carried out using a 3-STAGE PROCESS. Initially, MA is MIXED WITH METHANOL and reacts exothermally to form MONO-METHYL MALEATE. Then, using an ACIDIC RESIN CATALYST, it’s converted to DIMETHYL MALEATE. This compound is then HYDROGENATED TO DIMETHYL SUCCINATE (DMS), and finally converted to GBL. GBL can be further treated to obtain BDO (useful for Polyurethanes polymerization) and THF. The CRUDE products can then be refined to market quality with different processes. During the refining stage it is usually possible to recover METHANOL and other UNREACTED COMPOUNDS (DMS and GBL), which can be RECYCLED to their respective reaction stages. This process represents a huge innovation because of its ADVANTAGES:  Possibility to MODIFY THE CONDITIONS to obtain a different ratio of the 3 products, depending on market demand.  LOW PRESSURE HYDROGENATION, use of a CHEAP CATALYST and STAINLESS-STEEL REACTORS (rather cheap) greatly reduce the investment and the operational costs.   Additional costs of ESTERIFICATION ARE COMPENSATED POLYURETHANES Polyurethanes are used for many everyday life applications, because of their unique properties: DEPENDING ON THE MONOMERS USED, the final polymer can be rigid, soft, with a particular elasticity or expanded (insulating properties). The formation of this polymers is due to a NUCLEOPHILIC REACTION of a bi-functional nucleophile (diol or diamine) with a molecule with 2 isocyanate moieties, forming A Urea or a Carbamate. The addition of small amounts of WATER plays a critical role in the polyurethane formation. Upon reaction, each molecule of water will generate a CARBAMMIC ADIC MOIETY. This moiety is unstable and tends to undergo DECARBOXYLATION, forming an AMINE TERMINAL and releasing a MOLECULE OF CO2, that can be useful for foam-based polymers. As said before, the final properties strictly depend on the monomers used. it’s thus necessary to understand the synthetic pathway for the generation of ISOCYANATES: Generally, the starting material is TOLUENE, which undergoes NITRATION, using the classical mixture of HNO3/H2SO4. This reaction produces the o,o- and the o,p-ISOMERS. The successive step is the HYDROGENATION OF THE -NO2 MOIETY TO AMINE, which is performed employing Ni-, Cu- or Cr-BASED CATALYSTS. The process can be executed in batch or continuous reactors, in fixed bed reactors at 400°C. It is possible to achieve a CONVERSION OF 97% WITH 99% SELECTIVITY. In the past, the reduction was performed with iron and aqueous HCl since one the co-products of the reaction were Iron Oxides, commonly used as premium pigments. ISOCYANATES are obtained by reacting the compounds obtained in the previous step with PHOSGENE, using o-dichlorobenxene as solvent. Through CARBOXYLATION, an instable specie is formed, that, when heated, releases HCl and forms the BIFUNCTIONAL ISOCYANATE MOLECULE. Due to the formation of huge amounts of HCl, an inert gas must be employed for its removal. SPANDEX AND GLYCEROL-BASED POLYMERS Spandex is the commercial name of widely used polymer with very vast and different fields of application. These numerous possible applications are due to its unique property of RETURNING TO ITS ORIGINAL SHAPE AFTER STRECHED. This peculiar property is due to the presence of FLEXIBLE ALIFATIC AND RIGID AROMATIC SEGMENTS in the polymer chain. This material is produced starting from a DIISOCYANATE that reacts with a OLIGO- TETRAMETHYLENE ETHER CHAIN with the desired MW. This Spandex precursor is obtained by the OPENING OF A TETRAHYDROFURAN RING, caused by FSO3H, and is subsequent reaction with other THF molecules. The reaction is stopped by quenching with water at the wanted MW. And so on A different polymer, with HIGH CROSS-LINKING, can be formed when a DISOCYANATE reacts with a POLYOLE, namely a compound with MORE THAN 2 HYDROXYLI FUNCTIONS. The degree of crosslinking depends on the NUMBER OF HYDROXYL FUNCTIONS, and this is strictly correlated to the STIFFNESS OF THE POLYURETHANE generated: the larger the number of crosslinks, the greater the difficulty of deforming the structure. One of the most used monomers for the generation of this kind of materials is GLYCEROL. This compound can be produced starting from TRIGLYCERIDES, or fatty acids, through the SAPONIFICATION REACTION. Glycerol is an important chemical with several applications in many fields, from the PAINT INDUSTRY TO FOOD AND COSMETICS. EPOXY RESINS Another application for BISPHENOL A is the generation of epoxy resins, which are commonly used polymers for the preparation of ADHESIVES, COATINGS AND ANTICORROSION PAINTS (i.e.for offshore oil rigs). DIFFERENT KINDS of epoxy resins can be formed starting from a SINGLE MONOMER OR AN OLIGOMER as the main component (In order to get higher solvent and chemical resistant painting). Depending on the AMOUNT OF EPICHOLOHYDRIN that reacts with the main component it is possible to obtain a material (paints) with LOW VISCOSITY AND RESISTANCE (LARGE EXCESS OF ECH) or a material (coatings) with GOOD CHEMICAL RESISTANCE (SLIGHT EXCESS). To actually form the resin, a REACTION WITH A DIAMINE follows, in which a single NITROGEN ATOM can react with 4 EPOXYLIC FUNCTIONS of 4 different monomers, generating a HIGHLY CROSS-LINKED AND COMPLEX STRUCTURE, with terminal -OH functions able to interact with other materials. EPICHLOROHYDRIN SYNTHESIS It’s necessary to understand the synthetic pathway to generate the 2 building blocks of this starting monomer: BISPHENOL A is generated by the reaction of PHENOL AND ACETONE, coming from the CUMENE PROCESS, while EPICHLOROHYDRIN can be synthesize with several METHODS: CLASSICAL PROCESS In the oldest process, ALLYL CHLORIDE is treated with HYPOCHLOROUS ACID, in order to obtain the 2 regioisomer of DICHLORO PROPANOL, which lead to the formation of EPICHLOROHYDRIN when TREATED WITH A BASE. For this reaction, ALLYL CHLORIDE is needed. Theoretically, it’s possible to obtain it by INTRODUCTION OF Cl2 ON AN OLEFIN, however the COMPETITION BETWEEN ADDITION AND SUBSTITUTION IS A FACTOR. The addition is ENTROPICALLY DISFAVOURED and MORE EXOTHERMIC, the simplest way to limit the formation of dichloropropane is to work at HIGH TEMPERATURES (around 500 °C), FAVORING THE RADICAL SUBSTITUTION.  Therefore, the plant can be schemitize with the following block diagram: Propylene and chlorine streams are VAPORIZED AND SUPERHEATED to the reactor temperature in a FIRE HEATER; then, the mixed stream is sent to the REACTOR, in which ALLYL CHLORIDE AND BY-PRODUCTS are formed. The reactor effluent is sent to a SEPARATOR UNIT, in which DEIONIZED WATER is added, in order to dissolve HCl (previously generated) and UNREACTED Cl2. After this separation it’s possible to recover and RECYCLE UNREACTED PROPYLENE and ALMOST PURE ALLYL CHLORIDE, which is sent to the HYPOCHLORINATION UNIT. In this unit, ClOH is formed IN SITU, by mixing WATER AND Cl2, and reacts with the pure alllyl chloride, generating DICHLORO PROPANOL. This is sent to a DEHYDROCHLORINATION UNIT, in which the ADDITION OF AN ALKALINE BASED SOLUTION (Initially CALCIUM HYDROXIDE, then substituted with CAUSTIC SODA BRINE, which produces NaCl) leads to the to the generation of the DICHLORO PROPANATE SPECIE, which undergoes INTERNAL NUCLOPHILIC ADDITION, inducing the RING CLOSURE and generating the wanted EPICHLOROHYDRIN, that can be simply recovered through DISTILLATION. This process was used for a long time, however, it showed many DRAWBACKS, as HIGH WATER AND CHLORINE CONSUMPTION AND HIGH BY-PRODUCT FORMATION, therefore other processes were developed. SHOWA DENKO PROCESS This process was the first developed alternative to the classic process, and it takes advantage of the use of a PALLADIUM-BASED CATALYST, able to PROTECT THE PROPYLENE DOUBLE BOND FROM ACYCLATION WITH ACETIC ACID. The successive HYDROLYSIS of the generated ESTER leads to the formation of an ALLYL ALCOHOL and the REGENERATION OF ACETIC ACID, that can be recycled. The ALLYL ALCOHOL can be submitted to CHLORINATION, exploiting the possibility to use LOWER REACTION TEMPERATURE since it’s necessary to FAVOUR THE ADDITION OF THE CHLORINE. The product generated is DICHLORO PROPANOL, which is finally treated with NaOH to obtain the wanted product. This process represents a good improvement because of the LOWER WATER CONSUMPTION AND BY-PRODUCT FORMATION. However, the ENERGY CONSUMPTION remains too high and the CATALYST TOO EXPENSIVE. SOLVAY PROCESS In this process, the formation of ALLYL CHLORIDE is performed in the same way as the classical process. It is then DIRECTLY EPOXIDISED TO EPICHLOROHYDRIN, by exploiting a CATALYTIC OXIDATION WITH H2O2. This process can theoretically represent a good alternative to the classical process because it DIMISHES WATER and CHLORINE CONSUMPTION and the BY-PRODUCT FORMATION. It is however DIFFICULT to make it COMPETITIVE, due to the HAZARDS AND PRICE OF HYDROGEN PEROXYDE, as well as the HIGH ENERGY CONSUMPTION. SOLVAY EPICEROL PROCESS The last and better alternative route represented an improvement to the Solvay process. The company, in fact, decided to use GLYCEROL AS STARTING MATERIAL. This choice is SUISTAINABLE for different reasons:  Glycerol is simply generated by the SAPONIFICATION REACTION OF TRIGLYCERIDES,  It requires ONLY ONE CATALYTIC STEP (addition of HCl), to generate DICHLORO PROPANOL, After its formation, Dichloropropanol is treated with NaOH to obtain the wanted EPYCHLOROHYDRIN. ETHYLENE OXYDE Ethylene oxide (Boiling point = 13,5 °C) is quite HAZARDOUS, its production is necessary for the generation of many SECONDARY USEFUL PRODUCTS. It actually represents 15% of the GLOBAL ETHYLENE CONSUMPTION. CHLOROHYDRIN INTERMEDIATE PROCESS The FIRST INDUSTRIAL PROCESS (1931) used HUGE AMOUNTS OF CHLORINE AND WATER. The first reaction intermediate is CHLOROHYDRIN, which, after ALKALINE TREATMENT, generates EO and a HIGH AMOUNT OF UNUSEFUL CaCl2. This process was abandoned because of the IMPOSSIBILITY TO SELL THE FORMED SALT AND BECAUSE IT HAD A NEGATIVE ATOM ECONOMY: one chlorine molecule is consumed to generate one molecule of EO, which doesn’t contain chlorine. DIRECT OXYDATION PROCESS The DIRECT OXIDATION PROCESS was developed in 1937. This reaction was carried out with AIR OR OXYGEN, in the presence of a SILVER CATALYST (Ag 15% w/w) supported on LOW SURFACE AREA Al2O3: the noble metal has AN EXTREMELY HIGH SELECTIVITY FOR THIS REACTION, while Alumina with low SA presents bigger pores that release EO easily. Generally, catalytic supports with higher SA exhibit HIGHER ACTIVITIES, BUT MUCH LOWER SELECTIVITY (increased CO2 formation), since it is harder for EO to exit the smaller pores of the catalyst, thus favoring further oxidation. In some cases, INHIBITORS (Alkaline metals like Cs) are added in the material to prevent the complete ethylene oxidation and, furthermore, LOW VALUE OF CONVERSION (10%) are reached to improve the selectivity. The extremely high SELECTIVITY of this catalyst, which is influenced by the oxidant (AIR: 65-75%; OXYGEN: 70-80%), is due to the generation on its surface of an ATOMIC OXYRADICAL, which can react with the ETHYLENE DOUBLE BOND, generating EO. A less favored reaction (higher energy barrier) is the ABSTRACTION OF A H ATOM, producing a CARBON RADICAL that can react with the oxyradical and undergo COMPLETE OXIDATION. The difference in the energetic barrier can be overcame by HIGH TEMPERATURES. Since both reactions are HIGLY EXOTHERMIC, it’s fundamental to CONTROL THE TEMPERATURE to the optimal value. This is usually achieved with TUBULAR REACTORS IMMERSED IN WATER OR OIL, that can ENSURE AN EFFICIENT HEAT TRANSFER. The catalyst has a MEAN LIFE OF 2 TO 5 YEARS due to abrasion, carbon deposition and other processes. The INITIAL SELECTIVITY IS AROUND 80%, and as soon as it decreases the heat produced by the reactions increases. Therefore, the heat removal system must be sufficiently adaptable to cope with growing heat release in the reactor during the aging process of the catalyst. Temperatures on the coolant side (water or organic) of the catalyst tubes are restricted by the maximum design pressure, which USUALLY DOES NOT ALLOW TEMPERATURES HIGHER THAN 300°C. When the maximum temperature is reached the catalyst must be replaced. In the AIR PLANT, PURIFIED AIR IS MIXED WITH ETHYLENE (with a precise ratio in order to stay away from the flammability limits). The mixture is sent to the REACTOR, whose TYPICAL CONDITIONS are 200-300 °C and 10-30 atm, with a PER-PASS ETHYLENE CONVERSION of around 20-30%, to ensure high selectivity (70/80%). The process stream exiting the reactor may contain 1-3 mol% of EO, which can be ABSORBED USING WATER. The product can then be recovered almost pure using a system of DESORBER (EO is desorbed from water) and STRIPPER that removes the impurities (lighter gases, CO2 and N2). On the other hand, the GAS EXITING THE PRIMARY ABSORBER still contains ETHYLENE and is split in two streams. Two thirds are recycled in the primary reactor while one third is sent to a PURGE REACTOR, with the stream rich of CO2, N2 AND TRACES OF EO coming from the purge absorber. In the purge reactor the reaction is continued at higher temperature (lower selectivity). The purge is needed in order to KEEP A CONSTANT N2 CONCENTRATION in the gas recycled to the reactor. The successive PURGE ABSORBER is useful to recover ADDICTIONAL EO (which is sent to the desorber/stripper unit) and to REMOVE CO2, increasing the catalyst life. The OXYGEN PLANT shows many SIMILARITIES, indeed the REACTOR CONDITIONS, THE WATER- BASED ABSORBER AND THE FINAL RECOVER SYSTEM ARE IDENTICAL. There are, however, important DIFFERENCES which make the process ECONOMICALLY ADVANTAGEOUS, mostly due to the fact that it is possible to AVOID THE PURGE ZONE, which usually contributes largely to the ETHYLENE LOSS (in an oxygen plant less ethylene is lost compared to an air plant). A vent stream (i.e. VaporSep) to remove the impurities that can build up in the reactor (mainly Ar, up to 30-40mol%) is still needed, but it has much lower volumes that the one needed in the air plant so HIGHER ETHYLENE CONCENTRATION in the reactor can be used, improving the CATALYST SELECTIVITY. The VaporSep unit consists of a single-stage membrane system that separates Ethylene vapors from Argon, recovering more than 90% of vent Ethylene that would otherwise be flared. The ethylene-enriched permeate is returned to the reactor, while the argon- enriched residue is flared. However, the presence of CO2 adversely influences catalyst activity, therefore it’s necessary to add a CO2 ABSORBER UNIT WITH BASIC WASHING, in order to remove it. MAJOR INDUSTRIAL APPLICATIONS OF EO Ethylene oxide, despite its dangerous FLAMMABILITY LIMIT, is generally FURTHER PROCESSED, WITH RING OPENING REACTIONS, TO OBTAIN SEVERAL CHEMICALS. Most EO is used for the production of glycols, principally ETHYLENE GLYCOL. ETHYLENE GLYCOLS ETHYLENE GLYCOL is a fundamental MONOMER for PET PRODUCTION, a polyester with EXCELLENT THERMAL PROPERTIES. EG is reacted with TEREPTHALIC ACID, which comes from petroleum refineries, or VANILLIN, which can be produced from the BIORENEWABLE LIGNIN. EG can also be used for the production of AUTOMOTIVE ANTIFREEZING MIXTURE. The reaction performed is a simple ring opening with a HIGH SELECTIVITY (90%), however, because THE GENERATED PRODUCT IS BETTER NUCLEOPHILE THAN WATER, it’s necessary to work WITH A H2O EXCESS, at 60°C and in the presence of a SMALL AMOUNT OF ACID CATALYST (H2SO4 1%), to limit the generation of OLIGOMERS. Alternatively, it is possible to WORK AT 140- 230°C AND 20-40ATM WITHOUT A CATALYST. WATER AND EO are mixed (1) and sent to the EG REACTOR (2). In this reactor no catalyst is needed, but the OPERATING PRESSURE is strictly controlled (to avoid vaporization of ethylene oxide from the aqueous solution) and sufficient RESIDENCE TIME to reach complete conversion is provided. The water-glycol mixture from the reactor is fed to the first stage (3) of a MULTIPLE STAGE EVAPORATOR, which uses HIGH PRESSURE STEAM, that is successively LOWERED UNTIL THE FINAL STAGE (5), which usually works UNDER VACUUM CONDITIONS. The EVAPORATED WATER is RECOVERED as condensate and RECYCLED TO THE MIXING SECTION. The concentrated crude glycol solution from the final evaporation stage is then STRIPPED, in order to eliminate the remaining water and LIGHT ENDS in the column (6). Then, the water-free glycol mixture is FRACTIONATED in a series of VACUUM DISTILLATION TOWERS (7,8), to produce PURIFIED POLYMER-GRADE ETHYLENE GLYCOL (EG), diethylene glycol (DEG) and triethylene glycol (TEG), which are the main by-products of the reaction. Even with 20:1 MOLAR EXCESS OF WATER HIGHER ALCOHOLS ARE OBTAINED. However, HIGH YIELDS IN ETHYLENE GLYCOL (> 90%) can be achieved. GLYCOL ETHERS EO + ALCOHOL  Mono-GE + By-Products A portion of EO is added to a pre-heated circulating stream of the DESIRED ALCOHOL, AT A SPECIFIC RATIO, which controls the GE product distribution. The reaction mixture is non-aqueous, and CATALYST is needed to accelerate the reaction. The process is performed in a SERIES OF ADIABATIC REACTORS, in each of which A PORTION OF EO IS ADDED, in order to ensure an optimal control of the temperature. The reaction effluent is COOLED in an intercooler, positioned between each step, before proceeding to the successive reactor. The effluent from the final reactor is sent to an ALCOHOL COLUMN (2), where the un-reacted alcohol is removed and RECYCLED TO THE REACTOR. Instead, the product stream is sent to 2 FRACTIONATOR TOWERS, working under vacuum conditions, which are able to recover the WANTED MONO-GLYCOL ETHER and the successive OLIGOMERS. ETHANOLAMINES EA are another product that can be obtained by NUCLEOPHILIC ADDITION OF NH3 TO EO, with a LARGE EXCESS OF THE NUCLEOPHILE (Usually in a 10/1 ratio) and LOW T and P, to avoid oligomers formation. NON-IONIC SURFACTANTS ALKYLPHENOLS obtained from long chain alcohols (triglycerides hydrolysis) are REACTED WITH HUGE EXCESS OF EO (10-30 molar equiv.) to generate products losing the hydrophobic nature of the starting materials. The hydrophilicity can be regulated with the Oxyethylenic (-CH2CH2O) unit number. The reaction is promoted by a BASIC CATALYST (NaOAC or NaOH) AT 120-220°C. These compounds are used as low foam detergent (for the dishwasher), emulsifiers and dispersant. However, they are being investigated because SUSPECTED OF INTERFERING WITH NORMAL HUMAN HORMONES FUNCTION. PROPYLENE OXIDE PO is mainly consumed for the synthesis of POLYETHER POLYOLS, which are used for the successive generation of POLYURETHANES and for the production of PROPENE GLYCOLS. It shows peculiar reactivity due to resonance stabilization of carbocations and radicals CHLOROHYDRIN INTERMEDIATE PROCESS One of the oldest processes takes advantage of THE SAME CHEMISTRY used for the generation of ethylene oxide: it relies on the formation of CHLOROHYDRIN as intermediate molecule, derived from the addition of HUGE AMOUNT OF CHLORINE AND WATER. The Chlorohydrin, after a STRIPPER SEPARATION WITH STEAM, it’s submitted to an ALKALINE-TREATMENT (Ca(OH)2), which leads to the generation of PO, HIGH AMOUNT OF UNUSEFUL CaCl2 AND 1,2- DICHLOROPROPANE AS BY PRODUCT. This process isn’t competitive nowadays because of the IMPOSSIBILITY TO SELL THE FORMED SALT, THE AMOUNT OF BY-PRODUCTS and WASTEWATER, AND ITS NEGATIVE ATOM ECONOMY: one chlorine molecule is consumed to generate one molecule of PO, which doesn’t contain chlorine. Nonetheless, already existing plants that are technologically up-to-date and integrated with Cl2 production can still be competitive in some cases. HYDROPEROXIDE INTERMEDIATE PROCESS Another possible route involves the GENERATION OF AN HYDROPEROXIDE moiety from ISOBUTANE (Conv. 50%, Sel. 60%) OR EHTYL BENZENE (Conv. 15%, Sel. 85%). The hydroperoxide can, aided by a CATATYST, ACT AS OXIDIZER FOR THE PROPYLENE. This process avoids the use of huge amount of chlorine, solving the ATOM ECONOMY ISSUE. However, the reaction leads to the generation of an ALCOHOL AS CO-PRODUCT (in quantities 3 times larger than the PO), whose presence is UNAVOIDABLE. This can represent an ADVANTAGE because the alcohol formed can be simply DEHYDRATED TO ISOBUTENE OR STYRENE AND SOLD, but this is true only if a market for this product is present. So, another solution to improve the economy of this process was found in the ADDITION OF A FURTHER HYDROGENATION UNIT, ABLE TO REGENERATE THE STARTING ALKANE. The plant is composed by an INITIAL PEROXYDATION REACTOR, in which the OXYGEN (or AIR) is bubbled inside the ALKANE at 130°C and 30 atm. The formed HYDROPEROXYDE is mixed with a METAL-BASED CATALYST AND PROPENE and the final solution is sent to a successive EPOXIDATION REACTOR, in which the reaction is carried out at around 100°C and 30atm. The mixture of PO, α-METHYLBENZYL ALCOHOL AND CATALYST is then SEPARATED: the first column is required to RECYCLE UNREACTED PROPENE, the second to separate the PO from the alcoholic solution containing the SPENT CATALYST and other impurities. The third column is used to separate the spent catalyst from the rest. The final stream, which contains the alcohol, undergoes a DEHYDRATION STEP, that leads to the recovering of STYRENE, that can be further HYDROGENATED to reform the starting EB. SUMITOMO PROCESS In 2003, another HYDROPEROXIDATION-BASED PROCESS was developed: in this case, CUMENE was chosen as starting alkane molecule, since its RADICAL STABILITY was well known. At the beginning of the process, a HYDROPEROXYDATION STEP IN THE PRESENCE OF AIR occurs, in order to generate CUMENE HYDROPEROXIDE. This compound, in respect to ethylbenzene hydroperoxide, can be formed with HIGHER SELECTIVITY and at HIGHER CONCENTRATION due to its SAFER HANDLING (the equipment is SMALLER AND LESS EXPENSIVE). In this step, it is fundamental the presence of an ALKALINE SOLUTION, to avoid possible DECOMPOSITION of the hydroperoxide, due to the presence of acidic compounds. After a SEPARATION STEP (N2 and the alkaline solution are removed), the CHP stream is sent to an EPOXIDATION REACTOR, in which is carried out the oxidation of an EXCESS OF PROPYLENE (10/1), at 60°C and in the presence of a CATALYST (TITANIUM GRATED ONTO SILICA). The reactor is structured with MULTIPLE FIXED-BED LAYERS, and heat exchangers are present in between each of them, in order to PREVENT AN EXCESSIVE INCREASE IN TEMPERATURE, that would cause thermal decomposition of hydroperoxide and SIDE-REACTIONS of PO. The produced PO is RECOVERED THROUGH A SERIES OF SEPARATION STEPS. In this process, CUMENE ALCOHOL is an UNAVOIDABLE CO-PRODUCT, therefore, two unit are necessary to DEHYDRATE CUMENE ALCOHOL TO AMS AND HYDROGENATE IT, obtaining CUMENE, thus generating a closed plant theoretically able of self-sustainment. HYDROGEN PEROXYDE PROPENE OXIDE (HPPO) PROCESS The necessity to have a market for the generated co-product or to add several units to the process are uneconomic for an industrial plant; therefore, one of the recent solutions to these problems for the generation of EO is the use of HYDROGEN PEROXIDE AS DIRECT PROPYLENE OXIDANT, TAKING ADVANTAGE ON THE ENICHEM TECHNOLOGY. TS-1, a TITANIUM SILICATE catalyst, can SELECTIVELY EPOXIDIZE PROPENE, using DILUTED HYDROGEN PEROXIDE IN METHANOL. This mixture is also used as solvent to increase the reaction rate, to decrease the glycols formation and to facilitate the successive separation from PO. The plant is composed by a FIRST REACTOR, in which the reaction is carried out under MILD CONDITIONS (50°C and 10-20atm), in presence of TS-1 catalyst and a mixture of methanol/water as solvent. The reaction mixture is then sent to a SECOND REACTOR, in order to ensure HIGH CONVERSIONS. This reactor is followed by a FRACTIONATORY SYSTEM, with several distillation columns to separate the BY-PRODUCTS (OFF-GAS, METHANOL, GLYCOLS AND WATER), from the CRUDE PO. This new process shows several ADVANTAGES: with a direct oxidizer it’s possible to DECREASE THE ENERGY CONSUMPTION, THE WASTEWATER AND BY-PRODUCTS FORMATION AND THE CAPITAL COST OF THE PLANT. Despite all these improvements, the HIGH COST AND HAZARDS OF HYDROGEN PEROXIDE have made the use of this technology uneconomical. However, the development of an INTEGRATED ANTRQUINONE-HP GENERATION PROCESS, with which it’s possible to produce HP directly in the plant, greatly improve the competitivity of the process. GAS-PHASE HYDRO-OXIDATION OF PROPENE WITH O2 AND H2 A POTENTIAL FUTURE SOLUTION, studied in order to further decrease the production costs of PO, is to make a DIRECT HYDRO-OXIDATION OF PROPENE, using GOLD NANOPARTICLES supported ON TiO2 OR MICROPOROUS TS-1. The study of this technique is at the beginning, but it shows encouraging results: MIXING HYDROGEN AND OXYGEN, trying to mimic the in-situ formation of HYDROGEN PEROXIDE, has demonstrated an EXCELLENT SELECTIVITY, with, however, LOW CONVERSION. Will it be possible to achieve a significant increase of the conversion, this strategy could represent the best way to generate PO. MECHANISM OF ETHENE/PROPENE EPOXIDATION AND COMBUSTION Here presented a comprehensive oxyradical mechanism to explain the differences in reactivity of ethene and propene in both the epoxidation and the complete combustion. This mechanism is based on valence-bond calculations on the thermodynamics of the different reaction steps and is verified by experimental results. The active species is assumed to be a surface atomic oxyradical, which is adsorbed atomic oxygen with its unpaired electron pointing away from the catalyst surface. This is in contrast to the di-sigma-oxide-type oxygen, which has both electrons bonded to the silver surface. The thermodynamic calculations suggest that the oxyradical type oxygen will only form at higher oxygen surface coverages. This is consistent with reported higher epoxidation selectivities at higher oxygen surface coverages. This increasing selectivity of the silver catalyst at an increasing oxygen coverage was explained by a decreasing valence charge density. The most common promoter for the reaction, chlorine, has a similar electronic effect on the adsorbed atomic oxygen: chlorine (or another halogen) adsorbed on silver decreases the valence charge on oxygen ads atoms, thereby favoring O- insertion into the C=C bond, rather than C-H cleavage followed by combustion. When ethene reacts with oxygen, the reaction can proceed in two ways. The first possibility is the C-H cleavage, causing the abstraction of a H atom. This reaction has a relatively high activation energy (184 kJ/mol), and once this reaction step has occurred, the only possible subsequent reaction steps result in the complete combustion of ethene. On the other hand, O-insertion into the C=C bond, caused by an oxyradical attack on the double bond, is able to proceed without a significant activation to produce an oxygenated reaction intermediate. This intermediate can subsequently produce adsorbed ethene oxide, with the desorption of the epoxide only having a small barrier energy (42 kJ/mol). This scheme is consistent with a decrease in epoxidation selectivity at higher temperatures, because the initial barrier for the combustion precursor will then be less of a problem. For propene, the direct abstraction of a H atom is more favorable than that for ethene, because propene is able to produce an allylic intermediate. This allylic intermediate results in a significantly lower barrier energy (67 kJ/mol) for the direct combustion step, which can partly explain the lower epoxidation selectivity for propene. The initial reaction of the oxyradical with propene to produce the epoxide precursor is thermodynamically somewhat more favorable for propene than for ethene (21 kJ/mol for ethene, 29 kJ/mol for propene), which could partly compensate for this decrease in selectivity. A bigger problem is the propene epoxide precursor. It has an alternative reaction pathway available in addition to the epoxidation reaction, as in the case for ethene epoxide precursor. The -H atoms of the propene oxide reaction intermediate extend 0.27 nm across the surface from the location where the reaction with the first O atom occurred. The distance between the different O atoms on the surface is 0.29 nm, which is close enough for the -H atoms to reach. The abstraction of one of these H atoms is favorable and the intermediate formed can only result in a complete combustion of the molecule. PHASE TRANSFER CATALYSIS (PTC) This catalytic technique is used to carry out reactions between two or more reagents, located in TWO OR MORE CONTIGUOUS PHASES, in systems where the reaction is inhibited because the reagents cannot easily come in contact because of their DIFFERENT SOLUBILITIES. Therefore, the MAIN ROLE of the PT catalyst is to TRANSPORT ONE REACTANT FROM A PHASE TO ANOTHER, where a rapid reaction can take place. Generally, this transport is performed taking advantage on the so-called ANION PROMOTED REACTIONS, namely reactions in which an ANIONIC REAGENT (as is or generated through deprotonation) IS TRANSFERRED INTO THE ORGANIC PHASE AS ION PAIR WITH THE CATION CATALYST, through a continuous ionic exchange at the aqueous/organic interphase. The use of this kind of technology has been widely exploited for SEVERAL INDUSTRIAL SYNTHESIS, as nucleophilic substitution, additions, eliminations, oxidations, reductions…, in almost all industrial fields, because of the remarkable PRACTICAL ADVANTAGES, over traditional methods: Increase in productivity: Elimination of Mild conditions, yields, selectivity and hazardous, toxic and enhancing the safety reduce the units expensive reagents of the process and solvents Reduction in generation Minimal consumption Simplified separation of of wastes and of energy reaction products environmental issues PRATICAL PROBLEMS OF ANION PROMOTED REACTIONS However, the PTC reactions have also shown to have several ASPECTS TO CONSIDER AND SOLVE: SOLUBILITY ANION ACTIVATION To find a common To activate the anion, in solvent for solubilizing order to have RAPID both the BASIC SOURCE REACTION, under OF THE ANIONIC relatively MILD REAGENT AND THE CONDITIONS SUBSTRATE Moreover, the choice of the BEST REACTION MEDIUM is always a crucial step for anion promoted reactions: before the two active species can come in contact in solution, the SOLVATATION SHELL AROUND THE TWO REACTANTS MUST BE PARTIALLY DISRUPTED. Therefore, the REACTION RATE in solution is determined primarily by the amount of the ENERGY NEEDED TO DESTROY THE SOLVATION SHELL, which IN PROTIC AQUEOUS SOLUTIONS of inorganic salts, is EXTENDED AND IT TENDS TO COMPLETELY SHIELD AN ION. So, in general, the use of DIPOLAR APROTIC SOLVENTS is favored because, when solvation increases, ionic bonding decreases, as well as the rate of the reaction. Another fundamental aspect to take into account is the STRENGTH OF THE INTERACTION BETWEEN THE SUBSTRATE AND THE PTC. The weaker is the interaction the higher the reaction rate will be. According to Coulomb’s law (E = e2 N / ε r), CATION-ANION INTERACTIONS CAN BE MINIMIZED, BY INCREASING THE DIELECTRIC CONSTANT OF THE MEDIUM, OR THE DISTANCE BETWEEN THE IONS. The effect of DIELECTRIC CONSTANT is relevant only when the IONIC RADIUS IS SMALL, whereas the effect of the IONIC RADIUS is prevalent in solvents with a low dielectric constant, where ion pairs or ion pair aggregates are present. In general, the influence of the DISTANCE BETWEEN THE PAIR, which depends on their radius, is GREATER ON THE REACTION RATE. Therefore, a HIGLY REACTIVE ION PAIR may be obtained by using a BIG CATIONS SOLUBLE IN LOW DIELECTRIC CONSTANT ORGANIC SOLVENTS, as in the case of TETRABUTYL AMMONIUM SALTS, QUATERNARY PHOSPHONIUM SALTS, CROWN ETHERS OR CRIPTANDS. Therefore, summarizing, the OPTIMAL CONDITIONS FOR AN ION PROMOTED REACTIONS are: Virtually Unsolvated Low Interactions with Anions Cations Solved by the use of DIPOLAR APROTIC Solved with the use of BIG SOLVENTS STRUCTURED CATIONS MOLECULES KINDS OF PT LIQUID-LIQUID (LL-) PTS SOLID-LIQUID (SL-) PTC This kind of PTC was the fisrt developed This kind of PTC takes advantage of and was based on the contact between the possibility to use a SOLID PHASE, 2 INSOLUBLE LIQUIDS, usually an instead of the aqueous phase, from ORGANIC and an AQUEOUS SOLVENT, which the anion is extracted. This with different affinities for the 2 solution can be employed in order to reactants, generating, in this way, a FACILITATE THE SEPARATION of the partitioned system 2 phases by SIMPLE FILTRATION RAPPRESENTATION OF PTC REACTIONS AND PTC MATRIX From a simple schematization of the process, it’s possible to observe that a PTC reaction involves, at least, 2 STEPS, KINETICALLY DRIVEN BY 2 DIFFERENT CONSTANTS. The catalyst Q+ must transport the REACTANT ANION SPECIE X- from the aqueous or solid phase into the organic one (Being soluble in both phases, if it's small, or by exchanging the anion at the interphase), namely the TRANSFER STEP KT Once transferred, catalyst Q+ must make X- available in an ACTIVATED FORM in the organic phase, so that the INTRINSIC REACTION STEP KI with the substrate can proceed at a good rate The catalyst Q+ must transport the LEAVING GROUP Y- from the organic phase into the aqueous or solid one, so that the CATALYTIC CYCLE CAN BEGIN AGAIN (The presence of this step is mandatory) Therefore, the CHOICE OF A CATALYST PERFORMANT IN BOTH STEPS is fundamental to ensure a good reaction rate. However, this SELECTION OF THE CATALYST AND THE APPROPRIATE REACTION CONDITIONS is often complex because of the presence of UNCLEAR INTERACTIONS BETWEEN THE STEPS. The use of the PTC MATRIX can help to sort out important issues: this matrix brings the 2 PTC steps together in a plot of INTRINSIC RATE VS TRANSFER RATE, arbitrarily divided in 4 QUADRANTS, which allow to see DIFFERENT REACTION PATTERNS, depending on the relative rates of the 2 steps.   FAST REGION: In the upper right quadrant both transfer and intrinsic reactions rates are fast, in fact, such PTC processes are easy to run since ALMOST ANY KIND OF CATALYST CAN BE USED and it is easy to find reaction conditions under which the reaction will go well.  TRANSFER RATE LIMITED REGION: In the upper-left quadrant, transfer is slow, but intrinsic rate is high, indicating that to increase the overall rate, it’s necessary to increase the rate at which anion is delivered to the organic phase (i.e. by INCREASING THE STIRRING).  INTRINSIC RATE LIMITED REGION: This kind of reactions has a fast transfer rate but slow intrinsic rate, therefore, it’s necessary to select reactivity factors that enhance the rate of the intrinsic reaction, such a CATALYTIC STRUCTURE, TEMPERATURE, ORGANIC SOLVENTS etc.  SLOW REGION: both steps of the PTC process are slow and different strategies can be used to overcome this problem. Carefully CHOOSING ONE OR EVEN TWO CATALYSTS (one to assist the transfer and the other to speed up the reaction), operating at the HIGHEST POSSIBLE TEMPERATURE with the LEAST AMOUNT OF WATER and with HIGH DEGREE OF STIRRING are all viable alternatives. Each PARAMETER can AFFECT THE TRANSFER OR THE INCTRINSIC RATE to different extents: CATALYST STRUCTURE The catalytic structure is the most impacting factors on the rates, in fact, it’s also the most studied. The cation and the anion must be EASILY PARTITIONED INTO THE ORGANIC PHASE, therefore, LARGE QUATERNARY SALTS, such as tetrahexyl- or tetrabutyl ammonium salts, are generally used to transfer MONOVALENT ANIONS into most organic phases. Furthermore, these cations provide the MOST ACTIVATION FOR ANIONS, in fact, they are the best for reactions that tend to have SLOW INTRINSIC ORGANIC PHASE REACTIONS and require a highly activated anion. Quaternary cations that are relatively OPEN-FACED OR ACCESSIBLE, such as didodecyldimethyl- or benzyltriethylammonium, EASILY OCCUPY INTERFACIAL REGIONS, enabling them to INCREASE THE INTERFACIAL AREA and, thus, the TRANFER RATE of the anion to the organic phase. Accessible quaternary salts, in fact, are best for reactions where rates are limited due to SLOW ANION TRANSFER. When BOTH the transfer step and the intrinsic organic reaction step are SLOW, it may be advisable to use TWO PHASE-TRANFER CATALYST, one to increase transfer rates and the other to activate transferred anions. When BOTH steps are FAST, almost ANY catalyst will perform satisfactorily. Therefore, the best catalysts for each quadrant can be summarized, as in the following table: STIRRING Transfer of anions from the aqueous to the organic phase requires agitation, because WITHOUT IT, PTC reactions are almost always TOO SLOW TO BE USEFUL. In fact, stirring INCREASES INTERFACIAL AREA between the organic and the aqueous phases, speeding up the transfer of reactive species, and therefore ACCELERATING TRANSFER RATES. Reactions LIMITED BY A SLOW TRANSFER RATE become faster as the level of agitation is increased. If the transfer rate substantially exceeds intrinsic reaction rate, the PTC process becomes INDIPENDENT OF AGITATION RATE. TYPE OF INORGANIC REAGENTS The RATE OF TRANSFER of anions from an aqueous environment to an organic environment depends markedly on the KIND OF ANIONS TO BE TRANSFERRED AND THEIR CONCENTRATION. Harder Easier AMOUNT OF WATER The optimal water concentration for a given PTC system needs to be EVALUETED on a CASE-BY- CASE basis, but, from the standpoint of reactivity, optimal water concentration is USUALLY LOW, because of the negative effect of the hydration of the reacting anion under LL-PTC conditions. This effect is emphasized when transferring and/or reacting HARD ANIONS (HO–, F–, Cl- CN-). On the other hand, for very fast PTC reactions it may be desirable to add water TO MODERATE AND CONTROL THE REACTION. AMOUNT AND TYPE OF ORGANIC SOLVENT One of the outstanding features of PTC reactions is the FREQUENT OPPORTUNITY TO CONDUCT REACTIONS WITHOUT ORGANIC SOLVENT, if the starting reactant or product are liquids, improving yields, rates, purity and avoiding solvent environmental and recovery problems. However, sometimes, the presence of a solvent is MANDATORY, TO INCREASE THE TRANSFER OR THE INTRINSIC REACTION RATE, or simply because REAGENTS OR PRODUCT ARE SOLID AT REACTION TEMPERATURE. In these cases, it’s necessary to study the DIELECTRIC CONSTANT OF THE MEDIUM AND THE SOLVATATION OF THE ANION, which affects both the transfer and the activation of the anion. TEMPERATURE The temperature is the first parameter MODIFIED, TRYING TO INCREASE SLOW INTRINSIC REACTION RATES; however, it’s fundamental to take into account the THERMAL DECOMPOSITION OF THE CATALYST. The most commonly used salts (quaternary ammonium etc.) DECOMPOSE AT HIGH TEMPERATURE (120- 200°C). Moreover, but they are sensitive to the presence of concentrated bases, which reduce this value to 50-70°C. On the contrary, POLYETHYLENE GLYCOLES AND CROWN ETHERS are more resistant to thermal decomposition under basic conditions, but sensitive to acidic ones. CO-CATALYSIS Co-catalyst may be added to increase either the transfer rate or the intrinsic organic reaction rate, in example, the addition of ALCOHOLS, particularly diols, significantly increases the EASE OF HYDROXIDE ANION TRANFER. COMPETITIVE PARTITIONING OF ANIONS KSel As well as the rate of the 2 steps, another critical parameter to take into account for the development of a successful PTC process is the RELATIVE PARTITIONING EQUILIBRIUM OF THE REACTANT AND PRODUCT ANIONS between the organic and the aqueous phase. Considering the previous general example of the ALIPHATIC NUCLEOPHILIC SUBSTITUTION of alkyl halides R-Y in an aqueous-organic two phases system in the presence of catalytic amounts of a quaternary salt Q+Y– and an excess of a metal salt M+X–; the catalyst transfers the reacting anion X– into the organic phase as unsolvated and, therefore, very reactive, ion pair Q +X–: This process is regulated by several equilibria, but among all, the most important is the EXTRACTION EQUILIBRIA, related to the CAPABILITY OF AQUEOUS ANIONS X- AND Y- TO BE EXTRACTED INTO THE ORGANIC PHASE AS ION PAIRS [Q +X-]Org AND [Q+Y-]Org: Therefore, the summarized equilibrium can be written and it’s defined by the SELECTIVITY CONSTANT KXYSel, where the X- is substituted by Y- as ion pair: The value of this constant is related to the COMPETITION BETWEEN THE REACTANT X- AND LEAVING ANIONS Y- FOR THE ASSOCIATION WITH THE QUATERNARY CATION Q+ AND, ALSO, THEIR PRESENCE IN THE ORGANIC PHASE; therefore, it must have an HIGH VALUE for a feasible PTC reaction. Therefore, in order to understand the feasible reactions and the KXYSel value, it’s necessary to firstly consider the MAIN FACTORS that determine PARTITIONING OF ANIONS BETWEEN THE 2 PHASES:  Charge-to-Volume Ratio (Polarizability)  Electronegativity  Hydration Energy  Structure So, knowing these parameters, it’s possible to state that those anions which are MORE POLARIZABLE, LESS ELECTRONEGATIVE, WEAKLY HYDRATED AND WITH AN ORGANIC STRUCTURE ARE MORE FAVORABLY PARTITIONED INTO THE ORGANIC PHASE. Furthermore, these same anions HAVE A GREATER AFFINITY FOR ASSOCIATION WITH Q +, and, therefore, it’s possible to CREATE A LIST, in which the anions at the LEFT HAVE A GREATER AFFINITY THAN THOSE AT THE RIGHT, that, therefore, CAN BE SUBSTITUTED BY THE LEFT ONES. However, it’s fundamental to consider that those anions present at the EXTREME LEFT MAY ACT AS CATALYST POISONS, because they have such a HIGH AFFINITY, that they tend to remain PREFERABLY BONDED TO THE QUATERNARY CATION IN THE ORGANIC PHASE, thus first DECELERATING and, then, STOPPING THE PTC REACTION. HYDRATION AND ANIONS REACTIVITY Anions are hydrated to DIFFERENT EXTENTS DEPENDING ON THE CHARGE-TO-VOLUME RATIO OF THE ANION. The more strongly the anion is hydrated, the more strongly it will be attracted to the aqueous phase, and the MORE DIFFICULT it will be to TRANSFER TO THE ORGANIC PHASE, as in the case of OH –, F –, Cl – anions. Part of this water hydration ACCOMPANIES THE ANION when it is transferred into the organic phase, where it exists as (Q+X– n H2O)Org under LL-PTC conditions. The value of n depends mainly on the NATURE OF THE ANION: in general, THE LARGER THE CHARGE-TO-VOLUME RATIO OF THE ANION THE GREATER THE NUMBER OF WATER OF HYDRATION. The specific solvation of anions by a number n of water molecules REDUCES, even noticeably, THE RATE OF INTRINSIC REACTION LATE LIMITED PTC processes. Furthermore, HIGHLY CONCENTRATED AQUEOUS ALKALINE SOLUTIONS (50% aqueous NaOH, 60% aqueous KOH), where the water activity (αH2O) tends to zero, proved to be VERY EFFICIENT SYSTEMS FOR DEHYDRATING THE ANIONS. Using 50% aqueous NaOH solutions instead of water, the nucleophiles X– ARE EXTRACTED INTO THE ORGANIC PHASE LARGELY DEHYDRATED; thus, THEIR REACTIVITY BECOMES SIMILAR WITH THAT OF THE ANHYDROUS HOMOGENEOUS PHASE. FACTORS AFFECTING THE INTRINSIC REACTION RATE STRUCTURE OF THE SUBSTRATE: reflected in the tendency to undergo nucleophilic substitution (PhCH2 > Allyl > Primary Alkyl > Secondary Alkyl) LEAVING GROUP ACTIVITY: the leaving group can’t be a strong nucleophile, otherwise it could act as a catalyst poison (MeSO3- > Br- > Cl-) NUCLEOPHILICITY OF THE DISPLACING GROUP: the stronger nucleophile the displacing group is, the faster the reaction (RS- > SCN-, I- > CN- > Cl- > F-) QUATERNARY AMMONIUM COMPOUNDS – KEY FACTORS There are 2 FUNDAMENTAL PARAMETERS that must be introduced in order to UNDERSTAND AND PREDICT THE RATE OF THE REACTIONS: C# VALUE Q-VALUE It represents the ACCESSIBILITY It's the number of CARBON of the positive charge in the ATOMS, which is an index of catalyst and it's calculated by the solubility of the catalyst in adding the RECIPROCALS OF the aqueous phase or, on the THE NUMBER OF CARBON contrary, of its ATOMS on each of the four ORGANOPHILICITY alkyl chains Both this parameters INFLUCE, in different ways, THE TRANSFER RATE AND THE INTRINSIC RATE OF A REACTION: Transfer Rate Limited Intrinsic Reaction Rate Limited C# VALUE May vary, Q more Influential 16-32 Q-VALUE 1.0 - 2.5 (preferably 1.5 – 2.0)