Adipic acid can be also obtained by oxidation of cyclohexene by
hydrogen peroxide in presence of a catalyst (Scheme 1).Hydrogen peroxide (H2O2) is widely accepted as a green oxidant because it is relatively non-toxic and its decomposition leads to benign by-products like water and oxygen.5,6 H2O2 has several advantages as an oxidant for industrial processes: easy handling and storage, high active oxygen content and low cost.7 However, H2O2 alone is rarely sufficient to achieve the conversion and catalysts are essential.8
Oxidation of cyclohexene by H2O2 typically involves epoxidation
of the double bond, its opening and transformation to a diol and
then a BaeyereVilliger oxidation and multiple hydrolysis steps
resulting in adipic acid (Scheme 2).9 The key step for this reaction in
an aqueous medium is to bring all the reagents into close contact.
Deng and co-workers5 described the synthesis of adipic acid in
a water emulsion by hydrogen peroxide oxidation of cyclohexene
without surfactant and under strong stirring. In this system, addition
of an organic acid as ligand (co-catalyst) and severe reaction
conditions (20 h at 94 C following to 0 C overnight) were required
to obtain a good yield of pure adipic acid. Noyori and co-workers10
described the use of a phase transfer agent (methyltrioctylammonium
hydrogenosulphate) in water to produce adipic acid
with a good yield, but reuse of the system required an additional amount of phase transfer agent to be effective. More recently, the
potential of transition metal-mesoporous materials as heterogeneous
systems have been described11 in water or organic solvent
but the long reaction time and/or the low adipic acid production
yield are not advantageous for an industrial process. Also, the use of
surfactant-type peroxotungstates or peroxomolybdates has been
studied but the reuse of these catalysts for a new reactional cycle
(in the perspective of an industrial process) is never described.
12The main difficulty inwater or hydrogen peroxide is the weak
contact between the hydrophobic and hydrophilic reagents. The
use of microemulsions as aqueous media for organic reactions is
a way to improve the compatibility between the different reagents13
and to overcome the use of polluting organic solvent. In
this paper, we present, for the first time, the use of hydrogen peroxide
microemulsions as reaction media for the oxidation of
cyclohexene to adipic acid with hydrogen peroxide. The establishment
of the microemulsions zone, the reactivity in these media and
the reuse of the system (to provide a first approach for an industrial
process) will be presented and discussed.
Microemulsions are transparent and thermodynamically stable
dispersions of oil and water or non aqueous structured polar solvent
(formamide, glycerol and their mixture with water)14 spontaneously
formed by the addition of an amphiphile: a surfactant
and a co-surfactant (an amphiphile with a short chain like an alcohol,
an acid or an amine).15 There are two types of microemulsions
according to the nature of the continuous phase (Fig. 1).
Their thermodynamic stability and their nanostructure are
two important characteristics that distinguish them from emulsions.
Direct microemulsions are usually expressed as oil-inwater
microemulsions in which nanoscopic oil droplets
(10e50 nm in diameter) are dispersed in water with the help of
a surfactant and a co-surfactant. These microemulsions are very
suitable as reaction media for a sustainable development because
of their numerous properties;16 they allow organic synthesis in
water, specific localization of the reactants, interfacial orientation
of the molecules and increase of local concentrations and reaction
rate. Many chemical reactions have been studied in
microemulsion media: oligomerization of norbornene,17 photoamidation
of olefin,18 catalytic hydrolysis of phosphate esters,19
olefin oxidation,20 halogen nucleophilic substitution,21 mustard
gas oxidation,22 DielseAlder reaction23 etc. Holmberg13 has
demonstrated that microemulsions can be considered as an alternative
to biphasic systems with added phase transfer reagent.
Generally, microemulsions are described as versatile reaction media for many organic reactions but there is still no industrial
process based on microemulsions, essentially because of the
difficulty to recover the products and to reuse the system.
Adipic acid can be also obtained by oxidation of cyclohexene byhydrogen peroxide in presence of a catalyst (Scheme 1).Hydrogen peroxide (H2O2) is widely accepted as a green oxidant because it is relatively non-toxic and its decomposition leads to benign by-products like water and oxygen.5,6 H2O2 has several advantages as an oxidant for industrial processes: easy handling and storage, high active oxygen content and low cost.7 However, H2O2 alone is rarely sufficient to achieve the conversion and catalysts are essential.8Oxidation of cyclohexene by H2O2 typically involves epoxidationof the double bond, its opening and transformation to a diol andthen a BaeyereVilliger oxidation and multiple hydrolysis stepsresulting in adipic acid (Scheme 2).9 The key step for this reaction inan aqueous medium is to bring all the reagents into close contact.Deng and co-workers5 described the synthesis of adipic acid ina water emulsion by hydrogen peroxide oxidation of cyclohexenewithout surfactant and under strong stirring. In this system, additionof an organic acid as ligand (co-catalyst) and severe reactionconditions (20 h at 94 C following to 0 C overnight) were requiredto obtain a good yield of pure adipic acid. Noyori and co-workers10described the use of a phase transfer agent (methyltrioctylammoniumhydrogenosulphate) in water to produce adipic acidwith a good yield, but reuse of the system required an additional amount of phase transfer agent to be effective. More recently, theศักยภาพของวัสดุโลหะเปลี่ยนตัวเป็นแตกต่างกันระบบได้รับ described11 ในน้ำหรือตัวทำละลายอินทรีย์แต่นานปฏิกิริยา/ การผลิตกรด adipic ต่ำผลผลิตประโยชน์ในกระบวนการอุตสาหกรรมไม่ได้ ยัง การใช้ได้รับ surfactant ชนิด peroxotungstates หรือ peroxomolybdatesศึกษา แต่นำของเหล่านี้สิ่งที่ส่งเสริมสำหรับใหม่ reactional วน(ในมุมมองของกระบวนการอุตสาหกรรม) เคยอธิบายไว้12The ปัญหาหลัก inwater หรือไฮโดรเจนเปอร์ออกไซด์จะอ่อนแอติดต่อระหว่าง reagents hydrophobic และ hydrophilic ที่ใช้เป็นสื่ออควีสำหรับปฏิกิริยาอินทรีย์ microemulsionsวิธีการปรับปรุงความเข้ากันได้ระหว่าง reagents13 แตกต่างกันและการใช้ตัวทำละลายอินทรีย์ polluting เอาชนะ ในกระดาษนี้ เสนอ ครั้งแรก การใช้ไฮโดรเจนเปอร์ออกไซด์microemulsions เป็นปฏิกิริยาการเกิดออกซิเดชันของcyclohexene กับกรด adipic ด้วยไฮโดรเจนเปอร์ออกไซด์ การจัดตั้งโซน microemulsions เกิดปฏิกิริยาในสื่อเหล่านี้ และนำระบบ (เพื่อให้เป็นแนวทางแรกสำหรับอุตสาหกรรมกระบวนการ) จะถูกนำเสนอ และกล่าวถึงMicroemulsions ใจโปร่งใสมั่นคง thermodynamicallydispersions ของน้ำมัน และน้ำ หรือตัวทำละลายขั้วโลกมีโครงสร้างไม่ใช่อควี(formamide กลีเซอร และส่วนที่ผสมกับน้ำ) 14 ธรรมชาติก่อตั้งขึ้น โดยเพิ่มการ amphiphile: surfactant เป็นและ surfactant ร่วม (การ amphiphile ด้วยโซ่สั้น ๆ เช่นมีแอลกอฮอล์an acid or an amine).15 There are two types of microemulsionsaccording to the nature of the continuous phase (Fig. 1).Their thermodynamic stability and their nanostructure aretwo important characteristics that distinguish them from emulsions.Direct microemulsions are usually expressed as oil-inwatermicroemulsions in which nanoscopic oil droplets(10e50 nm in diameter) are dispersed in water with the help ofa surfactant and a co-surfactant. These microemulsions are verysuitable as reaction media for a sustainable development becauseof their numerous properties;16 they allow organic synthesis inwater, specific localization of the reactants, interfacial orientationof the molecules and increase of local concentrations and reactionrate. Many chemical reactions have been studied inmicroemulsion media: oligomerization of norbornene,17 photoamidationof olefin,18 catalytic hydrolysis of phosphate esters,19olefin oxidation,20 halogen nucleophilic substitution,21 mustardgas oxidation,22 DielseAlder reaction23 etc. Holmberg13 hasdemonstrated that microemulsions can be considered as an alternativeto biphasic systems with added phase transfer reagent.Generally, microemulsions are described as versatile reaction media for many organic reactions but there is still no industrialprocess based on microemulsions, essentially because of thedifficulty to recover the products and to reuse the system.
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