Page 368 - Adsorbents fundamentals and applications

DESULFURIZATION OF TRANSPORTATION FUELS 353

Table 10.16. Summary of energies of adsorption (E)for
thiophene and benzene in kcal/mol, calculated from mo-
lecular orbital theory
Sorbent E ads (Thiophene) E ads (Benzene)

+ −
Cu Z 21.4 20.5
+ −
Ag Z 20.0 19.1
(Z − denotes zeolite anion using the cluster model shown in
Figure 8.3; From Yang et al., 2002.)

Ab initio molecular orbital calculations were performed for the bonding be-
+
+
tween benzene or thiophene with Cu and Ag exchanged zeolites (Yang et al.,
2002; Takahashi et al., 2002). The results on energy calculations are summarized
in Table 10.16. As described in Chapter 8, the energy predictions from ab initio
methods (e.g., at high levels of basis sets of Gaussian) are fairly accurate. The
calculations showed that the π-complexation bonds for thiophene are stronger
than that with benzene, and that Cu(I)Y adsorbs more strongly than AgY does;
that is, the relative strengths of the π-complexation bonds follow:

For the same sorbate: Cu > Ag +
+
For the same sorbent: thiophene > benzene

Furthermore, the predicted bond strengths in the neighborhood of 20 kcal/mol
are well-suited for purification.
Vapor-phase adsorption isotherms of benzene and thiophene on various sor-
bents including CuY and AgY were measured first in order to assess their
suitabilities for sulfur removal, as well as for the removal of aromatics. Direct
correlations have yet to be established between the vapor-phase isotherms and
the liquid-phase isotherms. However, some efforts have already been made in
this direction by using the potential theory approach where log (C s /C) replaces
log (P s /P ) in the potential energy expression (C s is the saturated concentration
in solution and P s is the saturated vapor pressure, see Manes, 1998). A sorbent
capable of adsorbing thiophene at very low partial pressures in the gas phase
should also be able to do so in the liquid phase.

NaY, AgY, and Cu(I)Y. Figure 10.53 shows the equilibrium isotherms of ben-
◦
zene and thiophene on NaY at 120 and 180 C. More benzene was adsorbed
on NaY than thiophene at pressures <10 −2 atm. From Table 10.15, benzene
has a higher polarizability. This result indicates that the van der Waals interac-
tions and the interaction between the field (of Na ) and induced dipole domi-
+
nated the adsorption; both interactions depend on the polarizability (see Chap-
ter 2). At higher pressures over 1 × 10 −2 atm, pore filling occurred and the
benzene adsorption amount became smaller because benzene has a larger molar
volume than that of thiophene. Na-Y does not have selectivity for thiophene;