Molecular physically entrapped adsorbed

Figure 5.7 Steps in forming physically entrapped (adsorbed) molecular catalysts by sol-gel processing. (Adapted from [81].)...

One characteristic of in situ NMR experiments is that there is typically a wide range of correlation times characterizing molecular motion. Some species will be essentially immobile as a result of strong chemisorption to the catalyst surface or physical entrapment, as in the case of a coke molecule. Other species may reside exclusively in the gas phase or else be partitioned into adsorbed and gas phase populations in slow exchange on the NMR time scale owing to diffusional constraints. Figure 7 shows an example. At high temperatures, methanol and dimethyl ether are partitioned between the gas phase and adsorbed phase on zeolite HZSM-5 [31 ]. For many adsorbates on zeolites, especially at reaction... 

The relatively small quantities of soft coke physically entrapped within the pore structure that were amenable to extraction after demineralisation (soft coke II) have considerably higher molecular masses, lower atomic H/C ratios and higher heteroatom contents than their easily extractable counterparts (Table 2). If the more strongly adsorbed soft coke is the major precursor of hard coke, then it is a case of concentrating the polars as opposed to the large polycyclic aromatic hydrocarbon moieties from the initial gas oil. Further, the aliphatic nature of the soft coke indicates that considerable condensation is required to form large aromatic structures. 

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