Alcohols bimetallic catalysts

Alcohol oxidation by enzymes, 610-613 Alloy/bimetallic catalysts, 6-7, 70-71, 245-266, 317-337 Anderson-Newns Hamiltonian, 33-34 Anion adsorption effects, 143, 174-175, 208-239, 254, 281-283, 336, 525, 535-536... 

Only scare data is available in the literature on the application of rhenium containing mono- or bimetallic catalysts in the hydrogenolysis of esters to alcohols. Decades ago Broadbent and co-workers studied the hydrogenation of organic carbonyl compounds (aldehydes, ketones, esters, anhydrides, acids,... 

A bimetallic catalyst can be obtained by the reaction of tetrabutyltin with Rh/Si02 catalyst. The partial hydrogenolysis leads to the Rhs[Sn(n-C4H9)2]/ Si02 surface organometallic complexes, which proved to be fully selective in the hydrogenation of unsaturated aldehydes into the corresponding unsaturated alcohols.318... 

Ffirai and Toshima have published several reports on the synthesis of transition-metal nanoparticles by alcoholic reduction of metal salts in the presence of a polymer such as polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP). This simple and reproducible process can be applied for the preparation of monometallic [32, 33] or bimetallic [34—39] nanoparticles. In this series of articles, the nanoparticles are characterized by different techniques such as transmission electronic microscopy (TEM), UV-visible spectroscopy, electron diffraction (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) or extended X-ray absorption fine structure (EXAFS, bimetallic systems). The great majority of the particles have a uniform size between 1 and 3 nm. These nanomaterials are efficient catalysts for olefin or diene hydrogenation under mild conditions (30°C, Ph2 = 1 bar)- In the case of bimetallic catalysts, the catalytic activity was seen to depend on their metal composition, and this may also have an influence on the selectivity of the partial hydrogenation of dienes. 

Partial hydrogenation of acetylenic compounds bearing a functional group such as a double bond has also been studied in relation to the preparation of important vitamins and fragrances. For example, selective hydrogenation of the triple bond of acetylenic alcohols and the double bond of olefin alcohols (linalol, isophytol) was performed with Pd colloids, as well as with bimetallic nanoparticles Pd/Au, Pd/Pt or Pd/Zn stabilized by a block copolymer (polystyrene-poly-4-vinylpyridine) (Scheme 9.8). The best activity (TOF 49.2 s 1) and selectivity (>99.5%) were obtained in toluene with Pd/Pt bimetallic catalyst due to the influence of the modifying metal [87, 88]. 

A major improvement in the selectivity towards crotyl alcohol by the hydrogenation of crotonaldehyde has been attained by Margitfalvi et al. [91] through the modificahon of Pt/Si02 by Sn addition via SnEfi, which was then reduced at 573 K. For Sn/Pb = 1.2, both the overall activity of the catalyst and its selectivity towards the formahon of crotyl alcohol were strongly increased. On this bimetallic catalyst, the selechvity of the formation of crotyl alcohol was over 70%. 

In CO hydrogenation, the achvity and selechvity to C1-C5 oxygenates over the bimetallic samples are higher than those of the monometallic counterparts [187-190]. Bimetallic catalysts also showed improved activity in the hydroformylation of ethylene compared to either of the monometallic catalysts [191]. The promotion for higher alcohol production is proposed to be associated with the adjacent Ru-Co sites. However, the lack of an exhaustive characterization of catalysts does not allow a clear correlation to be established between the characteristics of the active sites and the catalytic behavior. A formyl species bonded to a Ru-Co bimetallic site has been proposed to be the intermediate in the alcohol synthesis in these systems. A subsequent reaction with an alkyl-surface group would lead to the C2-oxygenate production [187]. 

Carboxylic acids are one of the most difficult of all functional groups to hydrogenate. They can be hydrogenated to the alcohol, however, over ruthenium 34 or rhenium heptoxide 35,136 t 150°-200°C and 300-400 atmospheres pressure. Activated Cu-Ba-CrO is also effective at 250°-300°C and 300-400 atmospheres (Eqn. 18.31). 3 A Re-Os bimetallic catalyst has been used to hydrogenate carboxylic acids at 100 -120°C and 20-100 atmospheres of hydrogen. High conversions to the alcohol were reported. 38... 

However, only alkyl formates are formed in the conventional reactions of alcohols, CO2 and H2 using transition metal complexes, because intermediary hydride complexes generally react with CO2 to give formate complexes. On the other hand, we have found that mthenium cluster anions effectively catalyze the hydrogenation of CO2 to CO, methanol, and methane without forming formate derivatives [2-4]. Ethanol was also directly formed from CO2 and H2 with ruthenium-cobalt bimetallic catalyst [5]. In this paper, we report that this bimetallic catalytic... 

In this chapter, an overview is presented of studies that deal with the electronic and chemical properties of Pd in bimetallic systems. We will focus on palladium for three main reasons. First, bimetallic catalysts that contain Pd or other Group-10 metals have many uses isomerization of hydrocarbons, olefin hydrogenation, CO oxidation, alcohol synthesis, acetylene trimerization, etc. [8,10,19-21]. Second, palladium is very sensitive to the formation of bimetallic bonds [22-24]. And third, there is a vast number of experimental and theoretical articles in the literature that examine the properties of Pd in bimetallic systems [14,15,19-23,25-44]. From this large volume of work, one can get a general idea of how deep is our knowledge about the basic nature of bimetallic bonding and how it affects the properties of a metal. 

Two procedures for metal introduction in chitosan base were used impregnation and coprecipitation. According to the first procedure the metal deposition on chitosan micro beads was carried out from aqueous and alcohol solutions of NazPdCU, HzPdCU, RhCb, Rh2(CH3COO)4, ZnS04 and Pb(CH3COO)2. Pd and Pb/Zn in bimetallic catalysts was deposited by subsequent precipitation. Pd-Pb (Zn) atomic ratios were 1/1. Metal contents in the resulting samples were 0.5 - 4%. 

Unfortunatly, oxidation of 2-methylphenol with the previous catalysts (Pd-Sn/C and Pd/C) only gives small amounts of 2-hydroxybenzoic acid and heavies (table 2, entry 10). These heavies are polyethers probably obtained by reaction of o-cresol itself with 2-hydroxybenzylacetate or 2-hydroxybenzylalcohol. Apparently, palladium catalysts activate the benzylic C-H bond of o-cresol, but the oxidation of the intermediates seems less rapid than side reactions. On the other hand, we have check that platinum catalysts, which are known to be excellent catalysts for the oxidation of 2-hydroxybenzylalcohol into 2-hydroxybenzaldehyde in basic aqueous medium (ref. 3), is unable to activate efficiently the benzylic C-H bond of cresols. We synthesized bimetallic catalysts, Pd-Pt / C, with the hope that palladium would activate benzylic C-H bond and platinum would accelerate the oxidation of intermediate alcohols. Effectively, this new catalyst allows to recover 2-hydroxybenzaldehyde with 14 % selectivity at 70% conversion (Table 2, entries 11-12). Addition of bismuth salts are known to improve the aldehyde yield in the saligenin process. With such additives, the selectivity of the aldehyde can reached 60% for a total cresol conversion. Of course Pd-Pt / C can also oxidize 4-methylphenol but it does not bring significant improvement compared to initial catalysts. 

A patent from Monsanto [100] disclosed the oxidation of alkaline solutions of amino alcohols and poly(ethylene glycol) in the presence of bimetallic catalysts prepared by deposition of copper on the surface of platinum. High yields to the corresponding carboxylates were reported. 

These results compare well with those reported by Mallat (7) in his studies on the oxidation of cinnamyl alcohol to cinnamaldehyde using a 5% Pt, 3% BPC catalyst in an aqueous solvent. Careful control of the system pH was critical to achieve high selectivities to the aldehyde. In our system using organic solvents rather than the water/base solvent no such control of the pH is necessary and a simpler monometallic catalyst can be used rather than the Bi promoted bimetallic catalyst. 

In the oxidation of several secondary alcohols an optimum in the catalyst composition Bi/Pts rf 0-50 was found. The formation of a two-dimensional alloy on the surface efficiently sup, sses the by-product formation and increases the conversion by a factor of 3 to 66. Some examples on the beneficial influence of Bi promotion are shown in Table 1. A three-component Degussa catalyst was used as a reference. This catalyst is, to our knowledge, the only commercially available alcohol oxidation catalyst, which has been developed for the selective oxidation of glucose to gluconic acid [20]. We suppose that the superior behavior of our bimetallic catalyst in the reactions studied is due to the more homogeneous distribution of Bi on Pt. 

The present studies indicate that the measurement of the "real" potential of oxide-supported bimetallic catalysts during the aqueous phase oxidation of alcohols is possible. The catalyst potential provides information concerning the oxidation state of the catalyst, which is essential determining the role of promoters and the nature of the deactivation processes. The in-situ electrochemical measurements were found to be helpful in the optimization of the process variables and in controlling the rate of oxygen supply. 

Nevertheless only scare data is available in the recent literature on the application of Group VIII noble metal (M) or rhenium-based mono- and Re-M bimetallic catalysts, in the hydrogenolysis of esters or hydrogenation of acids to alcohols. Recently a few publications, - and patents. have been reported on the transformation of different carbonyl compounds (saturated and unsaturated esters, acids and carboxamides) over rhenium-containing catalysts. In the bimetallic catalysts used for the hydrogenation of carbonyl compounds the rhenium was combined with Pd, or Rh. In the case of catalysts used for the hydrogenation of unsaturated carbonyl compounds the rhenium is usually modified with tin. ... 

Reductions. Polymer-bound Rhg(CO)jj catalyzes the reduction of conjugated carbonyl compounds leading to allylic alcohols. On the other hand, the bimetallic catalyst Rhg(CO),g -RejfCOjjg converts amides to amines. ... 

In Table 3.1 C is the initial acetylene alcohol concentration, Q is the catalyst concentration, S is the selectivity (%), A is the acetylene alcohol conversion (%), turnover frequency (TOP) is the mole of substrate converted over a mole (Pd) of the catalyst per second, Xt is the relative concentration Xi= Q /Q (where Q is the current concentration of the substrate at i= 1 and product at i=2). Strictly speaking TOP should be calculated per Pd atoms participating in the catalytic reaction (available surface atoms), but for the sake of comparison with hterature data, in this chapter we will use the TOP definition given above. To find the kinetic relationships, we have studied the reaction kinetics at different substrate-to-catalyst ratio SCR=Co /Q. Kinetic curves for DHL hydrogenation with Pd and bimetallic catalysts are presented in Pig. 3.4. 

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