Influence of binary swelling solvents: Mechanism of action Page: 2 of 4
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APCS coal samples (Beulah-Zap, Illinois #6 and Lewiston-Stockton) were opened
under argon and 30 mg portions were immediately placed into vials and covered
with 2 mL of a toluene spin probe solution spiked with pyridine. Each sample was
then swelled for 18 hours and worked up as previously described? The
concentration of the incorporated spin probes was determined by EPR spectroscopy.
RESULTS AND DISCUSSION
Illinois #6 Subbituminous Coal
The retention of spin probe VII (amine substitutent) in Illinois #6 is shown in
Figure 2. It can be seen that oscillations occur in the concentration of spin probe
retained as the amount of pyridine that is added to the swelling solvent is increased.
These oscillations decrease in intensity as the concentration of pyridine in the
solvent solution is increased up to 2% pyridine in toluene. From a 2% up to 5%
pyridine concentration (not shown) there is no significant change in the retention of
spin probe VII. The largest changes in spin probe retention are observed for
concentrations of pyridine less than 0.5%. A three fold increase in spin probe
retention is observed upon the addition of 500 ppm pyridine (0.05%) to the toluene
Figure 3 illustrates the effect of pyridine concentration in toluene on the retention
of spin probe ViiI in Illinois #6, where the size of the probe is the controlling factor.
The effects are similar to those observed for the retention of spin probe VII,
although the extent of retention is decreased by 90%. Significant oscillations in spin
probe retention are observed for concentrations of pyridine less than 2%. As above,
increases in pyridine concentration beyond 2% have very little effect on the
retention of spin probe VIII.
Since retention of spin probe VII was much greater than that observed for spin
probe VIII, it seems that structural changes, brought about by small amounts of
pyridine, provide for significantly increased accessibility of the spin probe to active
sites in Illinois #6 APCS coal. The enhanced retention of VII Indicates that the
active sites are capable of strong interactions with the amino group.
The fact that the addition of 0.1% pyridine causes a large decrease in retention for
both spin probes shows that a structural change is primarily responsible since spin
probe VIII has no functional interactions. The decrease in retention of spin probe
VIII indicates either that the structure of the coal collapsed, blocking access to the
coal micropores, or that the structure opened up to such a large extent that the spin
probes could not be trapped. It seems likely at this point that the structure is opened
to an extent such that the spin probes are removed during the cyclohexane wash
since the concentration of spin probe VII is still greater than the retention observed
in the absence of pyridine. Evidently very small amounts of pyridine open the
structure of the coal enough so that "pockets" of active sites are made accessible to
the spin probes.
It is possible that opening the structure only slightly allows for diffusion of the spin
probes into the structure, while somewhat greater opening of the structure allows
for pockets to be formed which trap the spin probes more effectively. In this way,
the coal pockets could trap polar spin probes of the requisite size with hydrogen
bond interactions exactly like an inclusion compound (as opposed to a simple
intercalation process). As the structure is opened further, the larger pockets can no
longer trap the guest molecules, and so the retention of the spin probes in the
At 0.2% pyridine, the concentration of the retained spin probe VIII drops even
further; however, the retention of spin probe VII increases again. This shows that
the structure has opened even further (decrease in spin probe VIII), allowing greater
accessibility to hydrogen-bonding sites (increase in spin probe VII), but not creating
any a4ditiolal pockets which might trap the non-hydrogen bonding probes (spin
As the amount of pyridine is increased to 0.4%, a decrease in retention is observed
for spin probe VII while an increase in retention is observed for spin probe VIII.
This indicates that pyridine causes a poisoning of the active sites available for
interaction with the polar spin probe, while at the same time opening the structure
to create more areas that are able to trap small spin probes. It should be noted that
the amount of amino substituted spin probes retained at this pyridine concentration
is still much greater than that of the non-substituted spin probes, indicating that
there is still a significant number of active sites available for trapping polar spin
At 0.6% pyridine, an increase in retention is observed for spin probe VII while a
decrease is observed for spin probe VIII. At this point and beyond, further
oscillations in the retention of each spin probe appear to be due to competing
processes of opening micropores, opening larger structural areas, and poisoning of
active sites with pyridine.
With higher concentrations of pyridine both the period and the amplitude of the
oscillations decreased. After the concentration of pyridine reached 10%, the spin
retention observed is similar to that observed for pure pyridine. The data suggest
that complete structural opening occurs so that if this is desired, it may be possible to
achieve this goal without wasting large amounts of an expensive, toxic, strong
Spin probe VII retention in Beulah-Zap lignite swelled in toluene spiked with up to
1.2% of pyridine is expressed as a function of pyridine concentration in Figure 4.
Again an oscillatory behavior is observed for spin probe retention as the
concentration of pyridine in the swelling solvent solution is increased. Similar to
the behavior of Illinois #6, the oscillations observed for the retention of spin probe
VII in Beulah-Zap have decreasing periods as the concentration of pyridine is
increased. Increasing the pyridine concentration above 4% has little effect on the
amount of spin probe retention. It is likely that the structure of the lignite has then
been completely opened by disrupting all hydrogen bonded networks, as was
observed when pure pyridine wag used as the swelling solvent.
The retention of spin probe VIII (size dependence) in Beulah-Zap lignite as a
function of pyridine concentration in the toluene swelling solvent is shown in
Figure 5. Oscillatory behavior similar to that of Illinois #6 is observed. However,
after 0.6%, additional pyridine has a negligible effect on the incorporation of the spin
probe. Addition of 100 ppm pyridine (0.01%) to the swelling solvent for Beulah-Zap
has almost no effect on the retention of spin probe VIII. When the concentration of
pyridine is increased to 200 ppm (0.02%), a large increase in the retention of spin
probe VIII is observed, while a small, yet significant decrease in the retention of spin
probe VI is observed. The large increase in spin probe VIII retention indicates that
hydrogen bonds which block access to the interconnected micropore network have
been disrupted without significantly affecting the macromolecular structure.
Although a greater number of micropores was made available, a decrease in the
retention of spin probe VII was observed. This would seem to indicate that in
Beulah-Zap the pyridine necessary to provide initial access to the micropore
structure competes significantly for the active hydrogen bonding sites available to
the amino spin probes.
As the concentration of the pyridine in the swelling solvent is increased from 200
ppm to 500 ppm (0.05%), a large decrease in the retention of spin probe VIII occurs,
but a corresponding increase in the retention of spin probe VII is observed. The
large decrease in retention of spin probe VIII indicates that the macromolecular
structure was opened to a significant extent. This disruption of hydrogen bonds in
the macromolecular structure caused a dramatic increase in the available active sites
for hydrogen-bonded interactions, as evidenced by the large increase in retention of
spin probe VII.
A further increase in the concentration of pyridine to 700 ppm (0.07%) results in an
increase in retention of spin probe VIII and a huge decrease in retention of spin
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Ding, R.; Tucker, D. & Kispert, L.D. Influence of binary swelling solvents: Mechanism of action, article, December 31, 1995; United States. (digital.library.unt.edu/ark:/67531/metadc670229/m1/2/: accessed September 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.