The rate of coking increases generally with the
reactant pressure. Thus in m-xylene disproportionation
on mordenites [22] the coking rate and the coking/
disproportionation rate ratio for Pm_xyl~ne=0.2 bar
were about 2 times greater than for Pm_xylene=0.06
bar. The use of hydrogen in the feed was found to
decrease the formation of coke and to improve the
zeolite stability for typical reactions of acid catalysis
such as toluene [23] and cumene disproportionation
[24]. The effect is generally more pronounced when
hydrogen is used at high pressure or activated by redox
compounds. Obviously, the formation of coke is still
more significantly reduced when there is a change
from an acid to a bifunctional mechanism.