Diadsorbed species

The present economic and environmental incentives for the development of a viable one-step process for MIBK production provide an excellent opportunity for the application of catalytic distillation (CD) technology. Here, the use of CD technology for the synthesis of MIBK from acetone is described and recent progress on this process development is reported. Specifically, the results of a study on the liquid phase kinetics of the liquid phase hydrogenation of mesityl oxide (MO) in acetone are presented. Our preliminary spectroscopic results suggest that MO exists as a diadsorbed species with both the carbonyl and olefin groups coordinated to the catalyst. An empirical kinetic model was developed which will be incorporated into our three-phase non-equilibrium rate-based model for the simulation of yield and selectivity for the one step synthesis of MIBK via CD. 

In contrast to kinetic models reported previously in the literature (18,19) where MO was assumed to adsorb at a single site, our preliminary data based on DRIFT results suggest that MO exists as a diadsorbed species with both the carbonyl and olefin groups being coordinated to the catalyst. This diadsorption mode for a-p unsaturated ketones and aldehydes on palladium have been previously suggested based on quantum chemical predictions (20). A two parameter empirical model (equation 4) where - rA refers to the rate of hydrogenation of MO, CA and PH refer to the concentration of MO and the hydrogen partial pressure respectively was developed. This rate expression will be incorporated in our rate-based three-phase non-equilibrium model to predict the yield and selectivity for the production of MIBK from acetone via CD. 

Diadsorbed diolefins, 30 33 Diadsorbed species, 30 61, 71 Diagonalized matrix, 32 284—286, 288 ammonia synthesis, 32 294—297 n-butane dehydrogenation, 32 309-313 butenes isomerization, 32 305-308 1-butene to 1,3-butadiene dehydrogenation, 32 297-298... 

In order to interpret the remarkably high activity of platinum to promote isomerization of neopentane to isopentane, the direct formation and involvement of metalla-cyclobutane intermediate 18 was suggested.152,158 This a,y diadsorbed species bonded to a single platinum atom is in accordance with the existence of platinum complexes and the ability of platinum to catalyze a-y exchange.156,159... 

It was also shown that bond breaking does not necessarily require direct bonding of the carbon atoms of the bond to be broken to surface metal atoms. For example, neither 1,2- nor 1,3-diadsorbed species can explain the favored cleavage of 2,2,3,3-tetramethylbutane to yield isobutane. A 1,4-diadsorbed intermediate, however, could account for the hydrogenolysis of the quaternary-quaternary bond.255... 

This mechanism is similar to the olefin metathesis reaction. When the molecule structure permits formation of 1,3-diadsorbed species, this reaction can occur via Ti-allyI adsorbed complexes.271... 

For the VPO catalyst, for some unknown reason, which may be related to the much lower reaction temperature, the Cp—H bond breaks much more readily than the C —H bond, and dehydrogenation becomes the dominant reaction. For the higher alkanes, the formation of 1,3-diadsorbed species is very favorable that Cp—H bond breaking and dehydrogenation is not an important reaction any more. 

The amount of the cis isomer formed in the hydrogenation of 10 at various hydrogen pressures, however, is always considerably lower150 than that formed in the hydrogenation of 9. All these observations can be interpreted by invoking tr-allyl-adsorbed species (11 and 12) instead of 1,2-cr-diadsorbed species as surface intermediates in the hydrogenation over Pd (equations 11 and 12)150. Of the two species, 12 formed from 10 (equation 12) is expected to result in a higher trans ratio. 

The aa-diadsorbed species (adcarbenes) either do not exist or are in much lesser proportion than other species on a Pd surface during exchange reactions, even at relatively high temperatures (>200°C) (47). It should be noted that these reactions on nickel or rhodium occur via aa-diadsorbed species (39, 45). 

Figure 2 shows the 1,2- and 1,4-diadsorbed octalone, each in the cis and trans configuration. It can be seen that the cis 1,4-diadsorbed species is less hindered than is the trans species, and, thus, m-0-decaione formation should be favored by a 1,4-addition sequence. In the 1,2-diadsorbed species, little difference between cis and trans adsorption can be detected, and, therefore, nearly equal mixtures of the two isomeric decalones should be formed from a 1,2-addition process. Thus, the product distribution data given in Table II can be understood if it is considered that the reaction is a combination of 1,2- and 1,4-addition processes. The more polar the solvent, the more the 1,4-addition occurs and, as... 

Initial distributions of products from reactions of ethane and higher alkanes in excess D2 show that step (c) in Scheme 1 is reversible. Thus, interconversion of monoadsorbed and aj3-diadsorbed species can be very rapid especially on Pd and Rh before desorption of alkane. This interconversion is now referred to as the a(3 process. 

Burwell (1) disagreed with Rooney s solutions to both problems. First, he maintained that the ad-diadsorbed species is eclipsed vicinal diadsorbed alkane. Part of his evidence for this view was that only molecules containing 2 or more vicinal hydrogen atoms that are already eclipsed or may easily move into eclipsed positions undergo the a0 process. For example, bicyclo[2,2,1] heptane (Fig. 1) initially exchanges only one set of 2H atoms, those that are eclipsed on C2 and... 

FIG. 5. Roil-over mechanisms of 1,2-diadsorbed species (1,11-13). Reprinted with permission from J. Amer. Chem. Soc. 88, 4SSS (1966). Copyright by the American Chemical... 

The roll-over mechanism is now clearly established and this brings us back to Problem B, the nature of the a/3-diadsorbed species. Apart from the argument concerning the necessity for eclipsed pairs of vicinal hydrogens for the aj3 process to operate, Burwell (7) stressed that in his view conversion of certain cyclic... 

Now that the a/3-diadsorbed species is known to be tr-complexed olefin, the simplest interpretation of rollover is that the metal-olefin bond breaks the free olefin has then a transient existence in the gas phase and can migrate from one type of site to another. That this occurs to an appreciable extent even at ambient temperatures starting with alkane in excess D2 may seem surprising but is powerful support for the olefin migration step postulated in hydrocracking and hydroreforming on dual-functional catalysts. 

Because transition metals other than Pt (as far as they have been examined) have not such a propensity to form a metallocyclobutane directly from a gem-dimethyl group the way to ready isomerization of neopentane is barred. However, an alkane such as isobutane may have an indirect route to the ay-diadsorbed species provided that 1,2-migration of hydrogen is relatively easy (60d), as follows. 

Tanaka has recently reviewed the hydrogenation of ketones with an emphasis on the mechanistic aspects of the reaction.233 Numerous references related to this subject can be found in his article. Deuteration of cyclohexanones and an application of NMR spectroscopy to the analysis of deuterated products have revealed that on ruthenium, osmium, iridium, and platinum, deuterium is simply added to adsorbed ketones to give the corresponding alcohols deuterated on the Cl carbon, without any deuterium atom at the C2 and C6 positions, while over palladium and rhodium the C2 and C6 positions are also deuterated.234 A distinct difference between rhodium and palladium is that on rhodium deuterium is incorporated beyond the C2 and C6 positions whereas on palladium the distribution of deuterium is limited to the C2 and C6 carbons.234,235 From these results, together with those on the deuteration of adamantanone,236 it has been concluded that a Tt-oxaallyl species is formed on palladium while deuterium may be propagated by an a, 3 process237 on rhodium via a staggered a, 3-diadsorbed species. 

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