Adsorbate bonding-selectivity in partial

OYAMA ET AL. Adsorbate Bonding Selectivity in Partial Oxidation... 

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

As mentioned above, it is reasonable to assume that this tetrahedral V species forms at defect sites (hydroxyl nests) in the zeolite framework, but is stabilized by this interaction in a well defined environment through V-O-Si bonds. As indicated by the characterization data, the local coordination of vanadium must be different from that found for well dispersed vanadium sites on silica. This stabilization probably limits the unselective metal-bonded propane or propylene adsorption, in agreement with the role of adsorbate bonding on the selection of partial and total oxidation pathways of ethane on vanadium supported on silica (76) and in agreement with IR evidence (Fig. 

Cellulose contains adsorbed water which is held in the glucopyranose structure by hydrogen bonding, hence the separation proceeds via a partition mechanism. Cellulose materials are used almost exclusively for separating hydrophilic substances, for instance, amino acids and sugars in contrast to silica gel and alumina which are used for the separation of lipophilic compounds. Similar eluants, as for the PC application, can be selected. The partial structure of the cellulose molecule is shown in Figure 3.4. 

In the investigation of hydrocarbon partial oxidation reactions the study of the factors that determine selectivity has been of paramount importance. In the past thirty years considerable work relevant to this topic has been carried out. However, there is yet no unified hypothesis to address this problem. In this paper we suggest that the primary reaction pathway in redox type reactions on oxides is determined by the structure of the adsorbed intermediate. When the hydrocarbon intermediate (R) is bonded through a metal oxygen bond (M-O-R) partial oxidation products are likely, but when the intermediate is bonded through a direct metal-carbon bond (M-R) total oxidation products are favored. Results on two redox systems are presented ethane oxidation on vanadium oxide and propylene oxidation on molybdenum oxide. 

Fig. 32. (a) Selected internal coordinates and partial charges for p-fluoronitrobenzene adsorbed on the zeolite model. p-Fluoronitrobenzene is weakly hydrogen bonded to the zeolite, but not protonated. Values in parentheses are those of an isolated, neutral p-fluoronitrobenzene molecule, (b) Optimized geometry for p-fluoroaniline adsorbed onto the zeolite. In this case, we started with the proton on the zeolite the optimization resulted in protonation of the adsorbate. (Reprinted with permission from Nicholas et al. (82). Copyright 1995 American Chemical Society.)... 

Gas-adsorption processes Involve the selective concentration (adsorption) of one or more components (adsorbates) of a gas (or vapor) at the surface of a microporous solid (adsorbent) The attractive forces causing the adsorption are generally weaker than those of chemical bonds and are such that, by Increasing the temperature of the adsorbent or reducing an adsorbate s partial pressure, the adsorbate can be desorbed The desorption step Is quite Important in the overall process First, desorption allows recovery of adsorbates In those separations where they are valuable, and second, It permits reuse of the adsorbent for further cycles ... 

Type 1. Consecutive reactions. The common feature of these examples (Scheme 2.68) is that the product formed in the first step is capable of reacting further under essentially the same reaction conditions. If the requirement for selectivity is to stop the process after the first step, a variety of approaches can be attempted. For example, in case (a) both consecutive steps belong to the same type of chemical process. Therefore to ensure the selective hydrogenation of the alkyne to the alkene, it is necessary to utilize a catalyst that permits the reduction of the triple bond but not the double bond. This requirement is met in Lindlar s catalyst, a palladium metal catalyst adsorbed on a carbonate that is partially deactivated with lead (Pd-CaC03-Pb0). 

Hydrogenation studies of conjugated dienes show that the diene is more strongly adsorbed than the monoene produced on partial hydrogenation. This means that the monoolefin is displaced from the catalyst surface by the diene. Also, the double bond of the monoene does not isomerize in the presence of the diene.9 " Therefore, maximum selectivity is obtained when the reaction conditions are such that sufficient diene is available to the catalyst to displace the monoene product. Low hydrogen availability can enhance selectivity at higher conversions. 

---