MC catalyst

Mesoporous carbon materials were prepared using ordered silica templates. The Pt catalysts supported on mesoporous carbons were prepared by an impregnation method for use in the methanol electro-oxidation. The Pt/MC catalysts retained highly dispersed Pt particles on the supports. In the methanol electro-oxidation, the Pt/MC catalysts exhibited better catalytic performance than the Pt/Vulcan catalyst. The enhanced catalytic performance of Pt/MC catalysts resulted from large active metal surface areas. The catalytic performance was in the following order Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It was also revealed that CMK-1 with 3-dimensional pore structure was more favorable for metal dispersion than CMK-3 with 2-dimensional pore arrangement. It is eoncluded that the metal dispersion was a critical factor determining the catalytic performance in the methanol electro-oxidation. 

Fig. 14.15 Linear sweep voltammo-grams of (50 wt%) Pt/Carbon black, (30 wt%) Pt-NH3/MC, and (30wt%) Ptac/MC (catalyst loading 14 pgcm 2). Operating conditions 1600 rpm rotating speed and 20 mVs 1 sweep rate (Reprinted from [145] with permission from Elsevier).

Transition metal carbides and phosphides have shown potential as highly active catalysts. In these compounds, the C and P sites cannot be considered as simple spectators. They moderate the reactivity of the metal centers and provide bonding sites for adsorbates. The reactivity of the C centers in MC(OOl) surfaces varies in a complex way with the position of the metal in the Periodic Table and the filling of the carbide valence band. M Cj metcars should display a catalytic performance even better than that of the well-known Mo C or MC catalysts. By introducing six pairs of groups in the structure, the system is stabilized, while the presence of four low-coordinated M sites allows a reasonably high chemical reactivity. 

Figure 11.7 EPMA map on the Ru/MC catalyst (magnifi sion image of the catalyst structure (b) Ru La emission image.

The oxidation reaction in Eq. (4.1) is actually very complex. There are four major intermediates that are made during the sequential oxidations of methyl groups of pX (Figure 4.1). The conversion of each intermediate requires intervention of the MC catalyst. 

From a mechanistic point of view, the MC catalyst, that is, Co-Mn-Br, is essentially a cobalt-bromide catalyst (Co-Br) promoted by Mn ions. The reason for this definition is that the Co-Br catalyst exhibits all properties of the Co-Mn-Br catalyst, while the Mn-Br catalyst is much less active and has significant mechanistic differences from the Co-Br catalyst. For this reason, we first discuss the nature of the Co-Br catalysis mechanism and then the mechanism of the Co-Mn-Br system. 

Use of Br-containing catalysts poses challenges such as high rates of corrosion. As such, commercial oxidation reactors and equipment that come in contact with the MC catalyst solution must be cladded with titanium in order to minimize corrosion. This desire to discover an alternative halogen-free homogeneous oxidation catalyst to the MC catalyst has been a significant motivation for many. 

The mechanism of oxidation of / -toluic acid with MC catalyst has not been studied in great detail since the early paper by Ravens (which was one of the first papers on MC cobalt-bromide oxidation) [66]. We assume that the mechanism of oxidation of the second methyl group is the same as the first one, but the reaction rate constants may be very different for /j-toluic acid-derived radicals and hydroperoxides [33]. This is an area that deserves further investigation. 

Since the discovery of the MC catalyst for the oxidation of alkylaromatic hydrocarbons, significant research activities have been dedicated toward the development of alternatives to this catalyst [9, 68]. One motivation for the research is to eliminate Br from the process. This would allow one to reduce unit corrosion and downgrade metallurgical requirements for the process equipment. A bromide-free catalyst could also eliminate the formation of CHjBr (discussed in Section 4.2.8). Another motivation for the alternative oxidation catalyst is to reduce or eliminate usage of HOAc and, therefore, reduce variable cost. Despite the extensive research efforts, however, no known alternative catalysts could generate the oxidation rates and product selectivity observed from the MC system. 

Another approach to develop a bromineless catalyst is in the replacement of bromine with a nonhalogen substitute. In 1996, Y. Ishii first reported that an organic compound, iV-hydroxyphthalimide (NHPl), could replace bromide in the MC catalyst [82, 83]. Numerous investigators have since tested the scope and limitation of these imides [84-89]. Other groups have probed the kinetics and mechanisms of these catalysts [88, 90, 91]. The reader is referred to Chapter 16 for a detailed discussion of this chemistry. 

Oxidation processes that could potentially eliminate reliance on a valuable solvent have been explored by a number of researchers. In these cases, the homogeneous catalyst must be modified so that it is soluble in a hydrocarbon, which serves simultaneously as a feedstock [101]. As discussed earlier, expanded solvents such as CO2 and HOAc have also been explored with conventional MC-type catalysts [79,102,103]. Supercritical H2O as a solvent was explored for Mn-Br catalyst for pX oxidation [104-109]. Combinations of ionic liquids and HOAc were recently disclosed by UOP investigators for pX oxidation to TA [110]. Lastly, the use of HOAc in a spray reactor in combination with the MC catalyst for pX oxidation was explored by Li. etal. [111]. 

Since its discovery over 50 years ago, much has been learned about the MC catalyst, and this overview presented the current view of the authors on key aspects of MC oxidation chemistry. The unmasldng of the inner worldngs of this catalyst has revealed that each of the catalytic species plays unique roles in complex catalytic cycles that lead to fast and highly selective conversion of pX... 

THERMODYNAMIC AND KINETIC STUDIES TO ELUCIDATE THE AMOCO Co/Mn/Br AUTOXIDATION ( MC ) Catalyst"... 

The various catalytic systems on the base of transition metal compounds have been used for the alkylarens oxidation with molecular oxygen. And all of them catalyzed alkylarens oxidations mainly to the products of deep oxidation [6, 34]. One of the most striking examples is the oxidation of alkylarens into carbonyl compounds and carbonic acids by dioxygen in the presence of so-called MC-catalysts (Co(ll) and Mn (II) acetates, HBr, HOAc) [6]. 

LLDPE grades based on metallocene (mC) or single-site catalysts (SSC) can show a certain effect of LCB. This is because mC catalysts are able to incorporate vinyl-terminated chains as comonomers. This is considered as a possible way to improve the processing behaviour of these narrow MWD materials. The catalyst system used as well as the polymerisation conditions play an important role in the amount of LCB reached in the final product. Figure 5.12 compares two different mC - LLDPE with and without LCB. 

The electropolymerization of MN4-MC catalysts on carbon supports is another possibility concerning the fabrication of modified electrodes. Qiu et al. [54] reported the development of a glucose biosensor based on poly-Ni-Pc/MWCNTs. The enzyme glucose oxidase (GOx) was also immobilized on the surface of the functionalized electrode. The sensor gready improved the emission of luminol electrochemiluminescence (ECL) in the presence of H2O2, one of the products of glucose oxidation by GOx. 

Table 12.1 lists the data on the copolymerization of propylene with 1-butene and 1-pentene in liquid propylene medium. It is seen that different initial concentrations of the MC catalyst significantly affect the activity of the MC-MAO catalytic system (experiments 1, 8). A decrease in the concentration of the MC-based complex during its formation, as was shown in Ref. [25], brings about an increase in the activity of the catalytic system by a factor of nearly 2 from 240 kg PP/(mol Zr h) (experiment 1) to 440 kg PP/(mol Zr h) (experiment 8). Addition of the comonomers influences the activity and molecular mass of the polymers. Thus, even at small contents of 1-butene (below 3.4 mol%) and 1-pentene (below 1.4 mol%) in the monomer mixture, the activity of the catalytic system increases appreciably. In the case of copolymerization with 1-butene, the 5deld of the copolymer increases by a factor of 2-3 relative to that for the homopolymerization of propylene for copolymerization with 1-pentene, the 5deld of the copolymer increases by a factor of 1.5. A further increase in the concentration of these comonomers decreases the rate of polymerization. The activation effect of small additives of less reactive comonomers is referred to as the comonomer effect. ... 

It is known that a medium has a marked effect on the reactivity of comonomers. For copol5miers s mthesized in liquid propylene, relative to those synthesized in toluene, a smaller content of the comonomer in the monomer mixture is needed to ineorporate the same amount of higher linear olefin into the pol5mier ehain. The above values of reactivity ratios, for the copolymerization of propylene with higher a-olefins, are distinctive features of the proeess in liquid propylene and are associated with the nature of active centers on sterieally hindered isospecific MC catalysts. ... 

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