Chemistry of Bond Scission Processes

Heteroatom removal from petroleum feeds is important for improving their processability and for eventual production of clean environmentally friendly fuels. 

Zeolites are not typically employed in converting these classes of compounds, but their impact on zeolitic catalyst performance in FCC and hydrocracking, coupled with the occasional use of zeolite based catalysts to assist in their conver- 

Reactivities of different compounds vary greatly within each class and require different mechanistic pathways among classes. For example, nitrogen compound reactivities line up in the following order [63]  

This order is roughly proportional to their basicity and molecular weight Likewise, sulfur compound reactivities roughly increase in the order  

Optimum catalysts in use today for hydrodesulfurization typically consist of Mo or W metals promoted with Co and supported on weakly acidic alumina supports. Increasingly stringent regulatory requirements have pushed sulfur specifications for transportation fuels to les s than 10 ppm for diesel fuels and 30 ppm for gasoline 

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Note, however, that chain ends 4 and 5 may give different chemistry to those formed in termination by disproportionation (2, see Scheme 8.5) or the processes under (a) above. Chain scission (3 to the double bond will not lead to a MMA propagating species. It is not established whether the presence of these ends will give impaired thermal stability. 

The central cleavage of P-carotene 1 is most likely the major pathway by which mammals produce the required retinoids il), in particular, retinal 2, which is essential for vision and is subsequently oxidized to retinoic acid 3 and reduced to retinol 4. An alternative excentric cleavage of 1 has been reported involving scission of the double bond at C7-C8 producing P-8 -apocarotenal 5 (2a,2b) which is subsequently oxidized to 2 (Fig. 1) (2c). The significance of carotene metabolites such as 2, 3 and 4 to embryonic development and other vital processes such as skin and membrane protection is a major concern of medicinal chemistry. 

In this work we have avoided consideration of specific mechanism for decrease in molecular weight with liquefaction, including in particular the perceived need for thermal scission of C-0 and C-C bonds during conversion. Thus we view conversion simply as a process requiring some kind of nonspecific reduction chemistry. 

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