Article

Vapor-Phase Oxidation of Benzyl Alcohol Using Manganese Oxide Octahedral Molecular Sieves (OMS-2)

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Abstract

Vapor-phase selective oxidation of benzyl alcohol has been accomplished using cryptomelane-type manganese oxide octahedral molecular sieve (OMS-2) catalysts. A conversion of 92% and a selectivity to benzaldehyde of 99% were achieved using OMS-2. The role played by the oxidant in this system was probed by studying the reaction in the absence of oxidant. The natures of framework transformations occurring during the oxidation reaction were fully studied using temperature-programmed techniques, as well as in situ X-ray diffraction under different atmospheres.

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... There were some recent reports that deal with partial oxidation of BzOH in vapor-phase, using air or oxygen as oxidant. [2,[30][31][32][33][34][35][36][37][38][39][40] These oxidants are hugely favoured as they generate only H 2 O as by-product, thus helping to achieve atom economy and a green process. The reported catalysts gave a wide range of conversions of BzOH between 58-100%, with BzH selectivity in the range of 95-100%. ...
... The reported catalysts gave a wide range of conversions of BzOH between 58-100%, with BzH selectivity in the range of 95-100%. [2,30,31,35,36,[38][39][40] In terms of atom economy and green chemistry point of view, the desired product selectivity is important compared to high substrate (BzOH) conversion. Ag and Au on various supports are active for the partial oxidation of BzOH in 240-380 8C temperature range. ...
... [38] Opembe et al. reported OMS-2 catalyst for vapor phase oxidation, achieved 92% BzOH conversion with 99% BzH selectivity at 210 8C. [30] However, long term stability of these catalysts was not reported. These studies indicate that catalysts containing Cu II and Mn III / IV can be good for partial oxidation of BzOH. ...
... Thus, the partial oxidation of alkyl benzene or benzyl alcohol (BzOH) using both homogeneous catalysts and heterogeneous systems has been developed, with numerous advanced methods for the selective production of benzaldehyde [1,2,5e8]. Since the homogeneous catalysts always yield a large amount of hazardous wastes, recent great efforts have currently been paid to the use of precious metals as efficient catalysts in both the vapor-and liquid-phase oxidation of benzyl alcohol to benzaldehyde [9,10]. Indeed, noble metals including Pt [11], Pd [12], Au [10,13], Ag [14], Ru [2,15] or bimetallic catalysts of MeePt [9,16,17] are preferentially designed for catalyst systems for the oxidation reaction of benzyl alcohol. ...
... Since the homogeneous catalysts always yield a large amount of hazardous wastes, recent great efforts have currently been paid to the use of precious metals as efficient catalysts in both the vapor-and liquid-phase oxidation of benzyl alcohol to benzaldehyde [9,10]. Indeed, noble metals including Pt [11], Pd [12], Au [10,13], Ag [14], Ru [2,15] or bimetallic catalysts of MeePt [9,16,17] are preferentially designed for catalyst systems for the oxidation reaction of benzyl alcohol. Nevertheless, noble-metal catalysts usually require high costs so that alternative heterogeneous catalysts using less expensive transition metal oxides for the oxidation processes have generated more attention in recent years. ...
... Meanwhile, Cr/sepiolite possesses smooth magnesium silicate nanorods (Fig. 1). Thus, it is suggested that MeO x can be finely distributed both on the external surface and in the tunnels of the sepiolite [9,18,19]. ...
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A series of MeOx/sepiolite (Me = Cu, Cr, Mn, Co, Ni) catalysts was prepared through the deposition–precipitation and characterized by means of XRD, EDS, SEM, BET, and H2-TPR. The structural characterization showed a fine distribution of MeOx metal oxides on magnesium silicate nanobars. The sepiolite-loaded-metal oxide catalysts have been screened for the partial oxidation of benzyl alcohol and shown a good conversion and a very high selectivity to benzaldehyde at mild conditions. Under similar experiments, the catalytic activity of the transition metal oxides on sepiolite deceased as the follows of Cr > Co > Cu > Mn > Ni. The productivity of benzaldehyde was associated with the behavior of transition metal ions and reaction temperatures.
... For instance, gas-phase selective oxidation of ethanol to acetaldehyde over OMS-2 catalysts is commonly carried out in reaction mixtures containing low concentrations of oxygen (1-3%) [8]. Oxygen-rich mixtures (air) can be used in gas-phase selective oxidation of aliphatic alcohols with low reaction ability (such as octanol-1) and aromatic alcohols (e.g., benzyl alcohol) [7,9]. Another way to increase the selectivity is to carry out liquid-phase oxidation reactions under mild conditions [10,11]. ...
... Moreover, the OMS-2 materials are environmentally http://dx.doi.org/10.1016/j.cattod.2016.05.058 0920-5861/© 2016 Elsevier B.V. All rights reserved. friendly catalysts for selective oxidation of alcohols to corresponding aldehydes [6,7,9,23], in particular, for liquid phase alcohol oxidation [10,24]. ...
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The silver-modified cryptomelane-type crystalline and silica-supported OMS-2 catalysts (Ag/OMS-2 and Ag/OMS-2/SiO2, respectively) were prepared by impregnation, co-precipitation, and consecutive impregnation methods and tested in selective oxidation of ethanol to acetaldehyde. Modifying OMS-2 and OMS-2/SiO2 catalysts by silver was shown to improve the redox properties of the catalysts and increase the ethanol oxidation rate. However, selectivity to acetaldehyde decreased with increased reaction temperature due to intensification of total oxidation on the Ag-MnOx sites. Preparation method of the catalyst affected the Ag localization and catalytic activity. Introducing silver into the OMS-2 channels provided a higher ethanol oxidation rate. The use of silica support for Ag/OMS-2 composites improved the selectivity towards acetaldehyde at higher temperatures and, therefore, increased the acetaldehyde yield in comparison with crystalline samples.
... Pd/H-OMS-2, Pd/C-OMS-2, and Pd/S-OMS-2 exhibit decreased binding energy of lattice oxygen. In general, the chemical states and molar ratios of surface oxygen and lattice oxygen have a great effect on the catalytic activity at low temperature in the process of the CO catalytic oxidation [12][13][14][15][16]. It can be inferred From Table 2 To our best knowledge, a relatively higher charged surface species might be preferred to enhance catalytic activity over palladium-based catalysts [32,33]. ...
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Manganese oxide octahedral molecular sieves (OMS-2) synthesized from hydrothermal (H-OMS-2), reflux (R-OMS-2), co-precipitation (C-OMS-2), and solid phase (S-OMS-2) methods were impregnated with palladium and used for CO catalytic oxidation. Preparation methods presented an obvious effect on the morphology and catalytic activity of Pd/OMS-2 catalysts for CO oxidation. The hydrothermal synthesized OMS-2 (Pd/H-OMS-2) exhibited more ordered nanorod structure and higher crystallinity than Pd/R-OMS-2, Pd/C-OMS-2, and Pd/S-OMS-2. Further surface analysis indicated that different preparation methods of synthesizing OMS-2 and the impregnation process followed have a significant effect on the chemical states of Mn and O over the final Pd/OMS-2 products. The kinetics studies showed the trend of apparent activation energy (Ea) over different catalysts: Pd/H-OMS-2 (18.19 kJ/mol) < Pd/R-OMS-2 (21.56 kJ/mol) < Pd/C-OMS-2 (22.57 kJ/mol) < Pd/S-OMS-2 (29.44 kJ/mol). Over 99 % of the CO conversion was obtained at 35 °C by the optimal Pd/H-OMS-2 catalyst.
... This noncatalytic method showed a severe drawback in that the oxidant was environmentally unfriendly; therefore, tremendous efforts have been devoted to the design of catalytic systems that use O 2 as the primary oxidant [7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Although there have been some achievements for the catalytic oxidation of benzyl alcohols to prepare benzaldehydes [21][22][23][24][25][26][27][28][29], the preparation of benzyl alcohols is still a problem. Benzyl alcohols are commonly obtained through the halogenation of benzyl compounds; it is still a stoichiometric reaction with a highly harmful process. ...
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The phase behaviour of Fe and Co particles, supported on CaCO3 was studied using mainly in situ PXRD and Mossbauer spectroscopy. This is of interest as these systems can be used as catalysts to produce multi-walled carbon nanotubes (MWCNTs) following appropriate heat treatment and reduction. The Fe-Co/CaCO3 samples were prepared by deposition of the metallic particles from solution onto the support. The study indicated that the CaCO3 support reacted in air or N-2 with the iron and cobalt present in all three samples studied (10% Fe/CaCO3, 10% Co/CaCO3 and 10% Fe-Co/CaCO3) to give a brownmillerite structure with the general formula Ca2M2O5 (M = Fe, Co or Fe + Co). Reduction of the bimetallic catalyst gave Fe2Co alloy and Co. This confirms earlier reports by other authors that implicated this alloy as the active phase in the synthesis of the MWCNTs. The bimetallic catalyst was easier to reduce than the monometallic catalysts.
Article
Manganese oxide octahedral molecular sieve (OMS) materials with well-defined pores have been extensively studied over two decades due to their intriguing chemical and physical properties. OMS-2, the synthetic cryptomelane form of manganese oxide, was synthesized by a modified reflux method and was found to be highly active for obtaining α,β-unsaturated esters (up to 95 % yield and with high diastereoselectivities) from a variety of benzyl, heteroaryl, allyl and alkyl alcohols via one-pot alcohol oxidation-Wittig reaction. The transformation utilizes air as the stoichiometric oxidant, and the inexpensive catalyst can be recovered and reused.
Article
Selective aerobic oxidation of thiols to disulfides without any over oxidized products is studied using cryptomelane type manganese oxides (K-OMS-2) with a tunnel structure as catalysts. Using K-OMS-2 prepared by different synthetic procedures, complete conversion was obtained under air atmosphere without generating any overoxidized products. K-OMS-2 prepared by solvent free method (K-OMS-2SF) with the highest surface area (155 ± 1 m2/g) gave complete conversion, while materials prepared using hydrothermal method (K-OMS-2HY) with the lowest surface area (44 ± 1 m2/g) gave only 18% conversion at room temperature. Selective poisoning of the acid sites suggests that Lewis acid sites are the dominating active site during the reaction. Effects of surface area of the catalyst, solvent polarity, substrate effect, catalyst recyclability and temperature were studied. The catalyst could be recovered in the active form after the reaction without significant structural changes. The characterization of the catalyst using XRD, SEM, TGA, BET, TEM, and FT-IR are reported. The process developed is environmentally benign and is indeed heterogeneous.
Article
Silver-containing hexagonal mesoporous silica (Ag-HMS) catalysts with different Si/Ag ratios were synthesized by one-pot hydrothermal method for gas-phase selective oxidation of benzyl alcohol to benzaldehyde. The samples were characterized by nitrogen adsorption, X-ray diffraction, scanning electron micrograph, transmission electron micrograph, X-ray photoelectron spectroscopy, and UV–vis diffuse reflectance spectra. It was found that the Ag-HMS catalysts with different Ag loadings (0.55–3.50 wt.%) and different Ag particle sizes (5–32 nm) showed a similar level of catalytic property because they possess a similar Ag surface area. The Ag-HMS catalyst with a Ag loading of 2.81 wt.% exhibited excellent catalytic properties at 583 K with a high benzyl alcohol conversion of near 100%, benzaldehyde selectivity of around 96.0%, and benzaldehyde yield of about 96.0%, superior to those of other M-HMS catalysts (M = Co, Ce, La, Cu, Sr, Cd, Ni, Mn, V, and Fe). The enhanced catalytic performance could be attributed to the presence of the Ag surface oxygen species generated via oxygen spillover process. The work would be helpful for the development of novel Ag catalysts for selective oxidation of benzyl alcohol to obtain high quality of benzaldehyde and understanding the catalytic mechanism.
Article
A new and efficient oxidation process of 2,3,6-trimethyl phenol to 2,3,6-trimethyl benzoquinone (TMQ) is reported forthwith using non-exchanged and H+-exchanged manganese oxide octahedral molecular sieves (K-OMS-2 and H–K-OMS-2) as benign catalysts. The oxidation reaction is efficiently carried out using TBHP as oxidant and with catalytic amounts of OMS-2 achieving >95% conversion with excellent selectivity (~99%) to TMQ in 30 min. Graphical Abstract
Article
A new soft-step chemistry method has been developed to prepare pure cryptomelane-type manganese oxide materials (OMS-2) with the smallest particle sizes ever reported. The synthetic procedure is based on the reduction of KMnO4 by H2O2 under acidic conditions followed by reflux. An acetate-containing buffer solution and HNO3 are used to control the pH of the reaction mixture. The formation process, particle size, crystallite size, crystal structure, and properties of these nanomaterials have been investigated by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), potentiometric titration, thermogravimetric analysis, and N2 sorption analyses. Both the concentration of H2O2 and the nature of the acid used affect the crystalline phase formation, microstructure, thermal stability, and the composition of the final product. HRTEM images reveal that the OMS-2 nanofibers are not oriented preferentially and present significant twinning, along with discontinuity in the growth of the tunnel. Catalytic studies of these OMS-2 nanomaterials for oxidation of benzyl alcohol and fluorene have been performed. These nanomaterials show a low performance for the oxidation of benzyl alcohol and a unique catalytic activity for the oxidation of fluorene compared to OMS-2 materials prepared by conventional methods.
Article
Oxidation of cyclohexane is an important industrial process, its oxidation products, cyclohexanol and cyclohexanone are raw materials for adipic acid and caprolactam synthesis. Acid exchanged cryptomelane type manganese oxide octahedral molecular sieves (H-K-OMS-2) was used for liquid phase oxidation of cyclohexane with t-butyl hydroperoxide as an oxidant. The H+ form of K-OMS-2 was synthesized by ion-exchange of K-OMS-2 with concentrated nitric acid. H-K-OMS-2 has been found to be an efficient catalyst for the oxidation of cyclohexane with a high conversion (∼60%). More than 90% selectivity for cyclohexanol and cyclohexanone was obtained. A turn over number (TON) of 73 was obtained in 24 h at 80 °C and if we consider only contributions of acidic manganese sites then the TON is 287. Effect of increase in number of acid sites of the K-OMS-2 was correlated with the increase in conversion. The reactions were carried out in a semi-batch reactor at 80 °C using acetonitrile as solvent. Effects of solvent, temperature, and amount of t-butyl hydroperoxide on conversion and selectivity were studied.
Article
A number of silver catalysts supported on SiO2, Al2O3, Celite, CeO2, kaolin, MgO, and activated carbon were screened for their catalytic activity in the selective liquid-phase oxidation of benzyl alcohol using a special screening approach. For this purpose 5–6 catalyst samples were mixed and tested simultaneously. When a high catalytic conversion (>30% over 2h) was found the number of catalyst components was reduced in the following tests. Thereby, a collaborative effect between a physical mixture of ceria nanoparticles and silver-impregnated silica (10wt.% Ag–SiO2) was found. The catalytic activity was highly dependent on the silver loading, the amount of ceria, and especially the calcination procedure. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray absorption spectroscopy (XANES, EXAFS) demonstrate that silver is mainly present as metallic particles. This is also supported by in situ XAS experiments. Oxygen species incorporated in the silver lattice appear to be important for the catalytic oxidation of the alcohol for which a preliminary mechanism is presented. The application of the catalyst was extended to the oxidation of a wide range of primary and secondary alcohols. Compared to palladium and gold catalysts, the new silver catalyst performed similarly or even superior in the presence of CeO2. In addition, the presence of ceria increased the catalytic activity of all investigated catalysts.
Article
The present study focuses on the smooth oxidation of benzyl alcohol by air in a vapor phase process affording benzaldehyde with high selectivity and yield, using mono- and bimetallic gold–copper on silica catalyst. Particularly efficient was the catalyst 1% Au–Cu/SiO2 with the weight ratio Au/Cu = 4, leading to perfume-grade, acid-free benzaldehyde in over 98% yield.
Article
Cryptomelane-type manganese oxides octahedral molecular sieve (K-OMS-2) were used for the acid-catalyzed condensation of phenylhydroxylamine with aniline to produce 2-aminodiphenylamine. The H+-exchanged K-OMS-2 was found to be an efficient catalyst for this reaction. The reaction showed high selectivity (similar to 96%) for the ortho isomer of aminodiphenylamine compared with the para product. The effect of the amount of H+ exchange and temperature was investigated. (c) 2005 Elsevier Inc. All rights reserved.
Article
Octahedral molecular sieves (OMS), doped with Ag+, Co2+, and Cu2+, have been tested for their catalytic activity for carbon monoxide oxidation at low temperatures for long times on stream. Metal loaded OMS materials are highly active for this catalytic reaction and compare favorably with other catalysts such as Hopcalite-like CuMn2O4 catalysts, supported Ag catalysts, and supported noble-metal catalysts, especially with respect to resistance to deactivation in a long run. Co-doped OMS-2 has been tested for selective oxidation in the presence of a large surplus of hydrogen in the feed gas. This catalyst shows nearly exclusive oxidation of CO versus hydrogen with oxygen present in stoichiometric amounts with carbon monoxide. Its stability against reduction in CO or H2 containing gas is demonstrated from comparisons of X-ray diffraction patterns and X-ray photoelectron spectra before and after exposure to these gases. Average oxidation numbers and populations of Mn valence states were determined for these catalysts. Catalytic activity of doped OMS catalysts toward CO oxidation shows a correlation among average oxidation number of Mn and the position and nature of the doped cation. The structure of the active sites and the mechanism of the reaction are proposed.
Article
The oxidation activities of alkali-added Cu-Na-ZSM-5 zeolites in the gas-phase catalytic oxidation of benzyl alcohol were studied. The main products were benzaldehyde and carbon oxides (carbon dioxide + carbon monoxide). The partial oxidation activity was found to markedly increase when an alkali-metal-added Cu-Na-ZSM-5 was used, in comparison with that obtained when the Cu-Na-ZSM-5 zeolite itself was used. On the other hand, the increase in yield of deep oxidation products (CO2 + CO) catalyzed by the alkali-added Cu-Na-ZSM-5 was rather low. The alkali added to the Cu-Na-ZSM-5 zeolite was thus found to selectively promote catalytic activity for the partial oxidation of benzyl alcohol. Certain types of alkali salts were used as the additive with the results that Na+, K+, and Rb+, but not Li+, had similar behavior for benzyl alcohol oxidation. An alkali oxide such as Na2O, rather than the alkali salt itself, was suggested to substantially function as an active promoter for the oxidation of benzyl alcohol as judged from the pretreatment condition of the alkali-added Cu-Na-ZSM-5 zeolite and its similar catalytic activity in the oxidation of alkali salts with the same alkali cation but different counteranions. The added alkali was found to act more effectively as a promoter for oxidation when alkali and Cu ions were accessible to each other; therefore the alkali was suggested to interact directly with the Cu ion to promote the oxidation of benzyl alcohol, rather than this promotion being a separate function of the alkali itself. One of the roles of the alkali added to the Cu-Na-ZSM-5 zeolite was promoting the dissociation of the oxygen molecule, as evidenced by the results of the exchange reaction of the oxygen molecule. The alkali was also suggested to neutralize the acid sites in the Cu-Na-ZSM-5 zeolite and prevent its deactivation by retarding the formation of coke-like materials on the zeolite.
Article
A non-woven microfibrous thin-sheet structure with 5vol% 8μm diameter nickel fibers was built up using wet-lay papermaking and sintering process. Silver was then placed onto the sinter-locked Ni-fiber by the incipient wetness impregnation with AgNO3 aqueous solution. As-made Ag/Ni-fiber catalysts exhibited much higher activity/selectivity for the gas-phase selective oxidation of benzyl alcohol to benzaldehyde, and provided significant increase in the steady-state volumetric reaction rate with dramatic saving of Ag, in comparison with the electrolytic silver and Ag/α-Al2O3 catalysts. Conversion of 84% was achievable with benzaldehyde selectivity of 94% at 380°C using a high WHSV of 30h−1.
Article
A study on the oxidation activity of cobalt (Co) catalysts ion-exchanged and impregnated on NaY and NaUSY (ultra stable Y) zeolites and their alkali metal added counterpart, and the nature of the active supported Co species was carried out using the gas-phase catalytic oxidation of benzyl alcohol. The main oxidation products over the supported Co catalysts used in this study were benzaldehyde and CO2. The partial oxidation activity increased with increasing the amount of Co in the Co catalyst (Co/NaUSYimp) impregnated on NaUSY zeolite, although the degree of the increase in the benzaldehyde yield tended to decrease in the region of the higher amount of Co. The yield of benzaldehyde over the Co catalyst (Co/NaYimp) impregnated on NaY zeolite had a maximum at a comparatively lower amount of Co. The addition of an alkali metal to the impregnated and ion-exchanged Co catalysts with a low amount of Co (1wt.%) effectively promoted the partial oxidation activity. The redox property of the supported Co catalysts was investigated using UV–vis diffuse reflectance (DR) spectra. The role of the added alkali metal was inferred to be assistance for Co2+ to be oxidized to Co3+ to form Co3O4 [Co2+(Co3+)2O4] which was assumed to be active species for the partial oxidation of benzyl alcohol.
Article
The gas-phase catalytic oxidation of benzyl alcohol was carried out using Ag catalysts impregnated on a SiO2 or CaO support, as well as a potassium-added counterpart (K/Ag/SiO2), to clarify the surface state of the Ag species active in the oxidation reaction, the effect of the basic support and the added alkali metal. Benzaldehyde was obtained as the main oxidation product. The effects of the calcination temperature of the Ag/SiO2 catalyst and the reaction temperature on oxidation activity were investigated. The benzaldehyde yield markedly increased between 553 and 593 K. Increasing the calcination temperature from 773 to 1173 K also caused a significant increase in partial oxidation activity. Both a CaO support for the Ag species and potassium added to the Ag/SiO2 catalyst promoted partial oxidation at a comparatively low reaction temperature. X-ray diffraction and extended X-ray absorption fine structure demonstrated that a high calcination temperature, a basic support, and addition of an alkali metal led to a more ready reduction of the supported Ag to metallic Ag. Oxidized surface Ag species on the metallic Ag were inferred to be the active Ag components for the partial oxidation of benzyl alcohol based on the amount of O2 uptake and the diffuse reflectance spectra of the supported Ag catalysts.
Article
Manganese oxide octahedral molecular sieves with a cryptomelane structure (OMS-2) were used to catalyze the oxidation of cyclic olefins and benzylic double bonds with tertiary-butyl hydroperoxide (TBHP) as the oxidant. OMS-2 showed good catalytic activity with high selectivity under mild conditions (substrate:oxidant molar ratio of 1:1) for the oxidation of different substrates that were studied. Cyclooctene gets oxidized to cyclooctene epoxide selectively in the presence of OMS-2 among the different substrates used for oxidation. The order of reaction with respect to cyclooctene was determined to be pseudo-first order using OMS-2 and excess TBHP. The effects of time, reaction temperature, solvents, and amount of catalyst were investigated. Various reaction conditions and conversion of cyclooctene were maximized at 60°C using acetonitrile as the solvent.
Article
Nanowires of one-dimensional manganese oxide (cryptomelane) have been synthesized using reaction between Mn II (MnSO 4 ) and Mn VII (KMnO 4 ) in water by a low-temperature route at 60 ° C and 95 ° C. Characterization of the nanoparticles was carried out using powder X-ray diffraction, transmission electron microscopy, electron diffraction, and nitrogen adsorption - desorption. Two synthesis methods were developed depending whether the pH was initially adjusted or fixed all along the solid formation. Both methods exhibited variation of the nanowire morphology with diameter, length, and specific area varying in the 15 - 40 nm, 0.1 - 1.1 micrometer, and 35 - 110 m2/g ranges, respectively, depending on the growth conditions (acidity and temperature). The growth mechanism of the nanowires was proved to proceed by oriented attachment of primary 10 nm width nanorods for the lateral direction and dissolution - crystallization for the longitudinal direction. The diameter variation with acidic conditions was interpreted in terms of electrostatic interactions between primary nanorods.
Article
The complex structural transformations of Li 0.5 Ni 0.25 TiOPO 4 during electrochemical lithiation have been examined by in situ X-ray diffraction. During the first lithiation two structural changes take place: first a transition to a second monoclinic phase (a = 9.085(4), b =8.414(5), c =6.886(5), β = 99.85(4)) and secondly a transition to a third phase with limited long-range order. The third phase is held together by a network of corner sharing Ti–O octahedra and phosphate ions with disordered Ni–Li channels. During delithiation the third phase is partially transformed back to a slightly disordered original phase, Li 0.5 Ni 0.25 TiOPO 4 without formation of the second intermediate phase. These phase transitions correspond well to the different voltage plateaus that this material shows during electrochemical cycling.
Article
Microwave heating as an alternative to conventional heating for K-OMS catalyzed oxidation of tetralin under atmospheric pressure conditions has been described. The conversions and selectivities obtained by microwave and conventional heating are compared for K-OMS-2 catalysts. Significant differences were found in conversion and product distribution when comparing the conventional and microwave heated experimental results performed under the same reaction conditions. Conversions up to 88% and turnover frequencies up to 20 were achieved under microwave irradiation. The major products of oxidation of tetralin were tetralone and tetralol (>75%). In addition, the effects of using different solvent systems in the reactions were studied. The characterization of catalysts before and after the reactions using XRD, SEM, and Raman are reported.Graphical abstractOxidation of tetralin with OMS-2 catalysts under conventional and microwave conditions are compared. Significant differences were found in conversion and product distribution when experiments performed at the same reaction conditions. The characterization of catalysts before and after reactions using XRD, SEM and Raman are reported.
Article
Volatile organic compounds (VOCs) continue to be the major source of direct and indirect air pollution. Here, cryptomelane-type octahedral molecular sieve (OMS-2) manganese oxide, amorphous manganese oxide (AMO), and mixed copper manganese oxide (CuO/Mn 2 O 3) nanomaterials were synthesized and, together with commercial MnO 2 , characterized by various techniques. These catalysts were investigated for gas-phase total oxidation of six VOCs under air atmosphere. Using OMS-2 at 250 °C, the average conversions for toluene, benzene, ethylbenzene, p-xylene, m-xylene, and o-xylene were 75%, 61%, 45%, 23%, 13%, and 8%, respectively, whereas using CuO/Mn 2 O 3 , 72%, 44%, 37%, 29%, 27%, and 26%, respectively, were obtained. Generally, the conversion of VOCs to CO 2 using the synthesized catalysts increased in the order: o-xylene ≈ m-xylene < p-xylene < ethylbenzene < benzene < toluene. However, using commercial MnO 2 , benzene (44% conversion) was more reactive than toluene (37%), and the xylenes showed similar reactivities (13−20%). Differences in reactivity among VOCs were rationalized in terms of degree of substrate adsorption and structural effects. For example, the reactivity of xylenes was dictated by the shape-selectivity of stable OMS-2. The higher oxidative activities exhibited by OMS-2, AMO, and CuO/Mn 2 O 3 as compared to commercial MnO 2 were attributed to a combination of factors including structure, morphology, hydrophobicity, and redox properties. The mobility and reactivity of active oxygen species were strongly correlated with catalytic activities. Lattice oxygen was involved in the VOC oxidation, suggesting that the reaction could proceed via the Mars−van Krevelen mechanism.
Article
The effect and the role of alkaline earth metals added to Ag/SiO2 catalyst were studied in the gas-phase catalytic oxidation of benzyl alcohol. The main oxidation product was benzaldehyde, together with only small amounts of CO2, benzene, and toluene, depending on the reaction conditions. The influence of the method regarding how to add the alkaline earth metal to the supported Ag catalyst, and the amount of the added alkaline earth metal on the partial oxidation activity, was investigated, along with the analyses of these alkaline earth metal added Ag/SiO2 catalysts by diffuse reflectance (DR) spectroscopy, XRD, thermal gravimetric−differential thermal analyses (TG−DTA), and extended X-ray absorption fine structure (EXAFS). The addition of alkaline earth metals such as Ca, Sr, and Ba to the Ag/SiO2 catalyst by a coimpregnation method significantly promoted the catalytic activity of the partial oxidation of benzyl alcohol. The surface oxygen species on the supported Ag played an important role in the partial oxidation of benzyl alcohol on the basis of the results of the transient response experiment and the O2 adsorption on the prereduced Ag/SiO2 catalysts with and without alkaline earth metal. The alkaline earth metal added to the Ag/SiO2 catalyst was suggested to inhibit the formation of carbonaceous material, make the metallic Ag disperse, and facilitate the adsorption of oxygen on the Ag surface to form an oxygenated Ag surface, which was thought to be responsible for the partial oxidation of benzyl alcohol.
Article
Hollandite and cryptomelane materials have been prepared using two different methods. Octahedral molecular siev (OMS) having the 2x2 hollandite structure with a one-dimensional pore diameter of 4.6 â« Synthetic cryptomelane or OMS-2 can be formed by refluxing or autoclaving an acidic solution of KMnOâ and Mn{sup 2+}. Temperature, pH, and countercation are important synthetic parameters. The hollandite formed shows thermal stability up to 600 °C. OMS-2 formed by oxidation of Mn{sup 2+} under basic conditions and calcined at higher temperature (200-800 °C) is thermally stable up to 800 °C. X-ray powder diffraction and electron diffraction patterns have been simulated that show good agreement with experimental data supporting a tetragonal crystal system in the I4/m space group. Hollandites were also prepared in the presence of other transition-metal ions such as Cu{sup 2+} and Fe{sup 3+}. Electron paramagnetic resonance (EPR) data show that OMS-2 materials synthesized in the presence of Cu{sup 2+} and Fe{sup 3+} contain nonexchangeable Mn{sup 2+}. EPR data for Cu-OMS-2 showed a characteristic six-line pattern with a g value of 2.0 and A value of 85 G indicative of octahedral Mn{sup 2+} coordination. The Mn{sup 2+} EPR peaks in Fe-OMS-2 showed similar g and A values. EPR spectra, ion-exchange data, X-ray diffraction patterns, and the theoretical simulations of diffraction data suggest that Cu{sup 2+} and Fe{sup 3+} are located in the tunnels of OMS-2. 29 refs., 8 figs., 3 tabs.
Article
Dealloyed forces: Nanoporous gold, made by selective etching of Ag–Au alloys, exhibits extraordinary catalytic activity towards the gas-phase oxidation of benzyl alcohol to form benzaldehyde with selectivities up to 98.2 %, using molecular oxygen as an oxidant under relatively low-temperature conditions.
Article
Self-assembled multidoped cryptomelane hollow microspheres with ultrafine particles in the size range of 4–6 nm, and with a very high surface area of 380 m2 g−1 have been synthesized. The particle size, morphology, and the surface area of these materials are readily controlled via multiple framework substitutions. The X-ray diffraction and transmission electron microscopy (TEM) results indicate that the as-synthesized multidoped OMS-2 materials are pristine and crystalline, with no segregated metal oxide impurities. These results are corroborated by infrared (IR) and Raman spectroscopy data, which show no segregated amorphous and/or crystalline metal impurities. The field-emission scanning electron microscopy (FESEM) studies confirm the homogeneous morphology consisting of microspheres that are hollow and constructed by the self-assembly of pseudo-flakes, whereas energy-dispersive X-ray (EDX) analyses imply that all four metal cations are incorporated into the OMS-2 structure. On the other hand, thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) demonstrate that the as-synthesized multidoped OMS-2 hollow microspheres are more thermally unstable than their single-doped and undoped counterparts. However, the in-situ XRD studies show that the cryptomelane phase of the multidoped OMS-2 hollow microspheres is stable up to about 450°C in air. The catalytic activity of these microspheres towards the oxidation of diphenylmethanol is excellent compared to that of undoped OMS-2 materials.
Article
Mixed-valent manganese octahedral molecular sieves K-OMS-2 and H-K-OMS-2 are used to oxidize a wide range of alcohols with 100 % selectivity and 90-100 % conversion in most cases. The reaction is aerobic, catalytic, mild, efficient, stable, inexpensive, selective, and environmentally friendly.
Chapter
ChemoselectivityRegioselectivityDiastereoselectivityEnantioselectivityAtom EconomyConclusion References
Article
Cryptomelane-type manganese oxides (OMS-2) were synthesized in the presence of microwave heating at different microwave frequencies and also using variable-frequency heating. The materials were prepared using a two-step hydrothermal procedure. Catalytic activity of the materials was tested for the oxidation of 2-thiophenemethanol. OMS-2 prepared at a high-frequency limit of 5.5 GHz showed the highest conversion (50%) to the 2-thiophenecarboxaldehyde among all of the tested OMS-2 samples. The OMS-2 precursor showed remarkable conversion (89%) in the oxidation reaction. In addition, the OMS-2 materials and the precursor showed differences in oxygen evolution based on the thermal decomposition experiments, as well as differences in porosity.
Article
Catalytic oxidation of volatile organic compounds (VOCs) has been studied with cryptomelane-type manganese oxide octahedral molecular sieves (OMS-2) synthesized in buffer solutions. Characterization of OMS-2 has been done using X-ray diffraction (XRD), infrared (IR), thermogravimetric analysis–differential scanning calorimetry (TGA–DSC), combined temperature programmed decomposition and mass spectrometry (TPD–MS), adsorption and adsorbate IR techniques. OMS-2 was found to possess excellent hydrophobicity and strong affinity for VOCs. Excellent activity and stability of OMS-2 in oxidation of VOCs at relatively low temperatures (100% conversion of 0.9% C6H6 in air or 2.5% ethanol in air at 20 000 h−1 GHSV below 300°C) are due to these hydrophobic properties and to facile evolution of lattice oxygen without destroying the structure (120–650°C).
Article
Manganese octahedral molecular sieve (OMS) catalysts prepared by different methods have been employed for terminal ring opening of epoxides. Conversions ranging from 14% to 80% were obtained depending on the properties of the OMS catalysts. The catalysts act as Lewis acids in the reaction to facilitate nucleophilic attack on styrene oxide. Doping with other transition metals such as V, W, and Mo may alter the Lewis acidity of the materials and hence, leads to significant enhancement in conversions (100%) and selectivities for the ring opening. Effects of different solvents and nucleophiles were also studied in the reaction. This process using OMS catalysts is environmentally friendly and the catalysts can be reused without loss of activity.
Article
Reactions between MnSO(4) and KMnO(4) in the presence of carboxylic acids provide a facile, one-pot route to nanostructured manganese oxides with high surface areas. Acetic and propionic acid induce formation of hierarchical nanosphere morphologies whereas butyric acid promotes assembly of hollow spheres. The materials are active catalysts for CO oxidation.
Article
We report a new synchrotron based in situ X-ray diffraction (XRD) technique to study the chemical delithiation of LiFePO(4). This technique provides a new powerful tool to study chemical reactions with excellent time-resolving power for dynamic studies.
Article
(Graph Presented) A newly developed mesoporous mixed metal oxide (K-Cu-TiO2) catalyst is capable of highly selective, gas-phase benzyl alcoholbenzaldehyde transformation at excellent yields (>99%) under surprisingly low temperatures (203°C, bp of benzyl alcohol). The low-temperature reaction conditions and integration of K and Cu(I) components into the TiO2 matrix are of vital importance for the stabilization of an active Cu(I) oxidation state and resultant stable, excellent catalytic performance.
Article
The kinetics of the alpha-PcCu --> beta-PcCu conversion in air has been followed, under isothermal conditions, in situ real-time in the 423-443 K temperature range. Data have been fitted following the JMAK model. The reaction order of the kinetics at 423 K is consistent with a diffusion controlled, deceleratory nucleation rate process for 2D laminar particles, whereas at higher temperatures it is consistent with a phase boundary controlled, deceleratory nucleation rate process for 2D laminar particles. At 423 K, the overall transformation mechanism implies three steps: growth of the alpha-PcCu phase, disordering of adjacent columns of molecules of phthalocyanine, and nucleation and growth of the beta-PcCu phase. The calculated empirical activation energy is of 187 kJ/mol significantly greater than that for the alpha-PcCo --> beta-PcCo conversion. This fact seems to support the reported different structures of alpha-PcCo and alpha-PcCu. Investigation of the thermal behavior of beta-PcCu indicates a strongly anisotropic thermal expansion that follows the alpha(c) > alpha(a) approximately = alpha(b) trend. Moreover, the beta angle decreases with increasing temperature. Such anisotropy is consistent with the geometry of the very weak N3...H3 hydrogen bond which acts mainly along the c axis.
Article
A magnetic route has been applied for measurement of the average oxidation state (AOS) of mixed-valent manganese in manganese oxide octahedral molecular sieves (OMS). The method gives AOS measurement results in good agreement with titration methods. A maximum analysis deviation error of +/-7% is obtained from 10 sample measurements. The magnetic method is able to (1) confirm the presence of mixed-valent manganese and (2) evaluate AOS and the spin states of d electrons of both single oxidation state and mixed-valent state Mn in manganese oxides. In addition, the magnetic method may be extended to (1) determine AOS of Mn in manganese oxide OMS with dopant "diamagnetic" ions, such as reducible V5+ (3d0) ions, which is inappropriate for the titration method due to interference of redox reactions between these dopant ions and titration reagents, such as KMnO4, (2) evaluate the dopant "paramagnetic" ions that are present as clusters or in the OMS framework, and (3) determine AOS of other mixed-valent/single oxidation state ion systems, such as Mo3+(3d3)-Mo4+(3d2) systems and Fe3+ in FeCl3.
Article
Cryptomelane-type manganese oxide octahedral molecular (OMS) sieve three-dimensional (3D) nanostructures were synthesized via facile temple-free low-temperature hydrothermal reactions. Morphologies of the cryptomelane-type OMS-2 nanoparticles with tunnel dimension of 4.6 x 4.6 A can be tuned by varying reaction temperatures. At low temperature (120 degrees C), OMS-2 dendritic nanoclusters composed of uniform single-crystal nanotetragonal prisms with square cross-sections were formed. At high temperature (180 degrees C), the morphologies of OMS-2 became spherical dandelion-like microspheres composed of uniform single-crystal OMS-2 nanoneedles. Slow oxidation of Mn(2+) by Cr(2)O(7)(-) under hydrothermal conditions is critical for the formation of the hierarchically ordered OMS-2 3D nanostructures.
Article
Cryptomelane-type manganese oxides have been synthesized, characterized, and tested in the total oxidation of volatile organic compounds and CO oxidation. The structural, compositional, morphological, acid-base, physisorptive-chemisorptive, and thermal stability properties (especially the reversible evolution of lattice oxygen) have been studied in detail using ICP-AES (inductively coupled plasma-atomic emission spectroscopy), HRSEM (high-resolution scanning electronic microscope), XRD (X-ray diffraction), IR (infrared) and adsorbate-IR, N2 and CO2 physisorption at 77 and 273 K, respectively, TPD-MS (temperature-programmed decomposition-mass spectroscopy), and TGA-DSC (thermogravimetric analysis-differential scanning calorimetry) techniques. Kinetic and mechanistic studies for the catalytic function have been conducted and related to the characterization results. Cryptomelane has shown to be highly microporous, by using CO2 physisorption, and highly hydrophobic, possessing both Brönsted and Lewis acid sites. A part of the lattice oxygen atoms can be reversibly removed from the framework and recovered at elevated temperature without changing the framework structure. These lattice oxygen atoms can react with CO even at room temperature and are active sites for the oxidation of benzene. The consumed lattice oxygen atoms are replenished by gaseous oxygen to complete a catalytic cycle. The ease of reversible evolution of lattice oxygen, together with the high porosity, hydrophobicity, and acidity, leads to the excellent oxidation properties of OMS-2.
Catalytic, Aerobic Oxidation of Alcohols with Octahedral Molecular Sieves Efficient Oxidation of 2,3,6-Trimethyl Phenol Using Non-Exchanged and H + Exchanged Manganese Oxide Octahedral Molecular Sieves (K-OMS-2 and H−K-OMS-2) as Catalysts
  • A R Howell
  • S L Suib
  • C K Efficient
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