Borate-catalyzed reactions of hydrogen peroxide: kinetics and mechanism of the oxidation of organic sulfides by peroxoborates

The kinetics of the oxidation of substituted phenyl methyl sulfides by hydrogen peroxide in borate/boric acid buffers were investigated as a function of pH, total peroxide concentration, and total boron concentration. Second-order rate constants at 25 degrees C for the reaction of methyl 4-nitrophenyl sulfide and H(2)O(2), monoperoxoborate, HOOB(OH)(3) (-), or diperoxoborate, (HOO)(2)B(OH)(2) (-), are 8.29 x 10(-5), 1.51 x 10(-2) and 1.06 x 10(-2) M(-1) s(-1), respectively. Peroxoboric acid, HOOB(OH)(2), is unreactive. The Hammett rho values for the reactions of a range of substituted phenyl methyl sulfides and hydrogen peroxide, monoperoxoborate or diperoxoborate are -1.50 +/- 0.1, -0.65 +/- 0.07 and -0.48 (two points only), respectively. The rho values for the peroxoborates are of significantly lower magnitude than expected from their reactivity compared to other peroxides. Nevertheless the negative rho values indicate positive charge development on the sulfur atom in the transition state consistent with nucleophilic attack by the organic sulfides on the peroxoborates as with the other peroxides. The kinetic parameters, including the lack of reactivity of peroxoboric acid, are discussed in terms of the differences in the transition state of reactions involving peroxoboron species with respect to those of other peroxides.     

Activity-stability parameterization of homogeneous green oxidation catalysts

Small-molecule synthetic homogeneous-oxidation catalysts are normally poorly protected from self-destruction under operating conditions. Achieving design control over both activity and half-life is important not only in advancing the utility of oxidation catalysts, but also in minimizing hazards associated with their use and disposal. Iron(III)-TAML (tetraamido-macrocyclic ligand) oxidant catalysts rapidly activate H(2)O(2) for numerous significant processes, exhibiting high and differing activity and varying half-lives depending upon the TAML design. A general approach is presented that allows for the simultaneous determination of the second-order rate constant for the oxidation of a targeted substrate by the active catalyst (k(II)) and the rate constant for the intramolecular self-inactivation of the active catalyst (k(i)). The approach is valid if the formation of the active catalyst from its resting state and the primary oxidizing agent, measured by the second-order rate constant k(I), is fast and the catalyst concentration is very low, such that bimolecular inactivation pathways can be neglected. If the oxidation process is monitored spectrophotometrically and is set up to be incomplete, the kinetic trace can be analyzed by using the equation ln(lnA(t))/A(infnity)=ln(k(II)/k(i)[Fe(III)](tot)-k(i)t, from which k(II) and k(i) can be determined. Here, A(t) and A(infinity) are absorbances at time t and at the end of reaction (t=infinity), respectively, and [Fe(III)](tot) is the total catalyst concentration. Several tools were applied to examine the validity of the approach by using a variety of different Fe(III)-TAML catalysts, H(2)O(2) and tBuOOH as oxidizing agents, and the dyes safranine O and orange II as target substrates. Learning how catalyst activities (k(II)) and catalyst half-lives (k(i)) can be controlled by ligand design is an important step in creating green catalysts that will not persist in the environment after they have achieved their purpose.     

Iron-molybdate deactivation during methanol to formaldehyde oxidation: effect of water

Stoichiometric iron-molybdate has been prepared by coprecipitation and submitted to a long activity test to check the role of water in its deactivation behavior during methanol to formaldehyde oxidation. Water retards the reoxidation of the catalyst which is responsible for the acceleration of its deactivation.     

TEMPO/HCl/NaNO2 catalyst: a transition-metal-free approach to efficient aerobic oxidation of alcohols to aldehydes and ketones under mild conditions

Hydrochloric acid, a very inexpensive and readily available inorganic acid, has been found to cooperate exquisitely with NaNO(2)/TEMPO in catalyzing the molecular-oxygen-driven oxidation of a broad range of alcohol substrates to the corresponding aldehydes and ketones. This transition-metal-free catalytic oxidative conversion is novel and represents an interesting alternative route to the corresponding carbonyl compounds to the metal-catalyzed aerobic oxidation of alcohols. The reaction is highly selective with respect to the desired product when carried out at room temperature in air at atmospheric pressure. Notably, the use of very inexpensive NaNO(2) and HCl in combination with TEMPO for this highly selective aerobic oxidation of alcohols in air at ambient temperature makes the reaction operationally and economically very attractive. The results of mechanistic studies, performed with the aid of electrospray ionization mass spectrometry (ESI-MS), are presented and discussed. TEMPO, TEMPOH, and TEMPO(+) were observed in the redox cycle by means of ESI-MS. On the basis of these observations, a mechanism is proposed that may provide an insight into the newly developed aerobic alcohol oxidation.     

Supercritical fluids as reaction media for molecular catalysis

Chemical transformations in supercritical fluids (SCFs) have enormous potential advantages. The possibilities of rate enhancement and adjustable selectivities will motivate the research of molecular catalysis in SCFs. Recent progress in the organometallic catalysis under supercritical conditions is reviewed with emphasis on the benefits of utilization of supercritical carbon dioxide (scCO₂) both as a reaction medium and reactant.     

A "green route" to propene through selective hydrogen oxidation

The pros and cons of oxidative dehydrogenation of propane are outlined and a new catalytic system based on metal-doped cerianite catalysts is introduced. These novel materials catalyze the selective combustion of hydrogen from a mixture of hydrogen, propane, and propene at 550 degrees C. This gives three key advantages: energy is supplied directly where needed, product separation is made easier, and the dehydrogenation equilibrium is shifted to the desired products. A set of eighteen doped cerianites was synthesized in parallel, characterized, and screened for activity, selectivity, and stability in a cyclic redox system. The best results were obtained with Ce(0.89)Cr(0.02)Fe(0.09)O(2), Ce(0.98)Sn(0.02)O(2), and Ce(0.96)Cu(0.02)Zn(0.02)O(2), which gave 98 %, 91 %, and 98 % selectivity, respectively. Ce(0.89)Cr(0.02)Fe(0.09)O(2) also shows excellent stability in over 120 cycles (66 h on stream at 550 degrees C). Importantly, these doped cerias are monophasic crystalline materials. The dopants are incorporated as solid solutions throughout the fluorite lattice. This means that these catalysts are very stable (they do not sinter during reduction) as opposed to traditional supported metal oxides. The results show that both activity and selectivity towards hydrogen combustion can be tuned (increased or decreased) by selecting the appropriate dopant. Furthermore, the trends in selectivity differ from those measured on supported oxides of the same elements, which indicates that these novel materials indeed contain unique active sites. The factors governing selectivity towards hydrogen oxidation and the nature of the active site are discussed.     

Homogeneous catalysis: Kinetics and mechanism of oxidation of Ru(II) sensitizers by inorganic peroxides

The present research work involves the nature of interaction between the Ru(II) complex and inorganic peroxides in the absence of excitation by light photons. We synthesized a photosensitizer, ([Ru(dcbpy)₂(biq)]Cl₂·3H₂O) and studied its thermal degradation in the presence of inorganic peroxides (hydrogen peroxide, peroxomonosulfate and peroxodisulfate). Progress of the thermal reaction between the Ru(II) complex and each peroxide in 0.5 M H⁺ solutions was followed spectrophotometrically by monitoring the disappearance of the Ru(II) complex at its maximum absorbance (λ = 514 nm). The reactivities of the peroxides were compared and suitable mechanisms proposed.     

The Impact of Surfactants on FeIII–TAML‐Catalyzed Oxidations by Peroxides: Accelerations,Decelerations, and Loss of Activity

Iron(III) complexes of tetraamidato macrocyclic ligands (TAMLs), [Fe{4‐XC₆H₃‐1,2‐(NCOCMe₂NCO)₂CR₂}(OH₂)]^?, 1 ( 1 a : X=H, R=Me; 1 b : X=COOH, R=Me); 1 c : X=CONH(CH₂)₂COOH, R=Me; 1 d : CONH(CH₂)₂NMe₂, R=Me; 1 e : X=CONH(CH₂)₂NMe₃⁺, R=Me; 1 f : X=H, R=F), have been tested as catalysts for the oxidative decolorization of Orange II and Sudan III dyes by hydrogen peroxide and tert‐butyl hydroperoxide in the presence of micelles that are neutral (Triton X‐100), positively charged (cetyltrimethylammonium bromide, CTAB), and negatively charged (sodium dodecyl sulfate, SDS). The previously reported mechanism of catalysis involves the formation of an oxidized intermediate from 1 and ROOH (k_I) followed by dye bleaching (k_II). The micellar effects on k_I and k_II have been separately studied and analyzed by using the Berezin pseudophase model of micellar catalysis. The largest micellar acceleration in terms of k_I occurs for the 1 a ? tBuOOH? CTAB system. At pH 9.0–10.5 the rate constant k_I increased by approximately five times with increasing CTAB concentration and then gradually decreased. There was no acceleration at higher pH, presumably owing to the deprotonation of the axial water ligand of 1 a in this pH range. The k_I value was only slightly affected by SDS (in the oxidation of Orange II), but was strongly decelerated by Triton X‐100. No oxidation of the water‐insoluble, hydrophobic dye Sudan III was observed in the presence of the SDS micelles. The k_II value was accelerated by cationic CTAB micelles when the hydrophobic primary oxidant tert‐butyl hydroperoxide was used. It is hypothesized that tBuOOH may affect the CTAB micelles and increase the binding of the oxidized catalysts. The tBuOOH? CTAB combination accelerated both of the catalysis steps k_I and k_II.     

Catalysis for Oleochemical Platforms

The present review articles the most recent efforts, made in the Department of Chemical Sciences of the University of Naples Federico II, in the catalytic treatment of biomasses derived from vegetable oils. The review is focused on several technologies aimed at the production of either biofuels or valuable chemicals: (i) biodiesel production; (ii) esterification to obtain high-added value products; (iii) epoxidation of vegetable oils; (iv) glycerol ketalization; (v) oxidative cleavage of unsaturated fatty acids. The results are critically summarized to highlight the scientific activities and the main results in the field of biorefinery concept.     

合成气催化转化的绿色化学

将Ru催化剂加工到2 4 nm就可以使费托合成的反应温度降低100 ℃而活性却达到传统Ru催化剂(200 ℃)的2～5倍. 这一成果迅速被国内能源公司买断,显示了原创的绿色能源技术在中国生机无限. 本文围绕甲醇、乙醇合成和铁基催化剂上的费托合成等讨论了发展液相低温的合成气转化新化学.     

Titanium Salan Catalysts for the Asymmetric Epoxidation of Alkenes: Steric and Electronic Factors Governing the Activity and Enantioselectivity

A new insight into the highly enantioselective (up to >99.5 % ee) epoxidation of olefins in the presence of chiral titanium(IV) salan complexes is reported. A series of 14 chiral ligands with varying steric and electronic properties have been designed, and it was found that electronic effects modulate the catalytic activity (without affecting the enantioselectivity), whereas the steric properties account for the enantioselectivity of the epoxidation. Competitive oxidations of p‐substituted styrenes reveal the electrophilic nature of the oxygen‐transferring active species, with a Hammett ρ value of ?0.51; the enantioselectivity is unaffected by the electron‐donating (or withdrawing) ability of the p‐substituents. Mechanistic studies provide evidence in favor of a stepwise reaction mechanism: in the first (rate‐determining) stage, olefin most probably coordinates to the active species, followed by intramolecular enantioselective oxygen transfer. The enantioselectivity increases with decreasing temperature. The modified Eyring plots for the epoxidation of styrene and (Z)‐β‐methylstyrene are linear, indicating a single, enthalpy‐controlled mechanism of stereoselectivity, with ΔΔH^≠=?6.6 kJ mol^?1 and ?5.4 kJ mol^?1, respectively.