**Molecular Size and Structure in Pyridine Extracts of Upper Freeport
Coal**

**as Separated by M41S Sieving**

*Jerry E. Hunt, Randall E. Winans, and Langqiu Xu*

Chemistry Division

Argonne National Laboratory

9700 South Cass Ave.

Argonne, IL 60439

**Introduction**

Coal extracts have long been used to obtain coal material in solution
form that can readily be characterized. However, what part of the total
coal structure these extracts represent is not completely known.
Pyridine has been a particularly good solvent for coal; for example, the
extractability of Upper Freeport has been shown to be as high 30%.
Although pyridine extracts of coal have been referred to as solutions,
there is good evidence that they are not truly solvated, but are
dispersions which are polydisperse in particle size.^1^ The particle
sizes may span the size range from clusters of small molecules (a few Å)
to extended clusters of large particles (a few hundred Å), not unlike
micelles, where the functional groups of molecules which interact
favorably with the pyridine solvent lie at the surface of particles.

Mesoporous silicates are attractive candidates for separations due to
their high surface areas and porous nature. MCM-41 is one member of a
new family of highly uniform mesoporous silicate materials introduced by
Mobil, whose pore size can be accurately controlled in the range 1.5
Å-10 nm.^2,3^ This recently discovered M41S class of zeolites should be
useful to effect size separation, due to their large pore sizes and thus
their potential for the separation of larger compounds or clusters. True
molecular sieving on the size range of molecular and cluster types found
in coal solutions should be possible with M41S materials by tuning the
pore size.

We have synthesized a mesoporous silicate material with a surface area
of approximately 1100 m^2^/g and pore sizes of approximately 25 Å and 33
Å. The results of a study on the ability of this mesoporous materials
(M41S) to be used as stationary phases for separations of coal complexes
in pyridine is the subject of this paper.

**Experimental**

**Coal Samples**. The coal used in this study is the mv bituminous Upper
Freeport coal (APCS 1) of the Argonne Premium Coal Samples series.^4^ We
have used a new room temperature extractor, the Gregar extractor and
traditional refluxing in pyridine to extract the coal. Approx­imately 10
g of coal was extracted in the Gregar extractor for one week. The
extract was filtered and stirred with the M41S material for one week.
This was then filtered and the zeolite-containing coal was extracted
with methylene chloride. The extraction removed the coal material from
the zeolite quantitatively.

A similar quantity of coal (10 g) was extracted by pyridine reflux for
one week. The extract was divided and stirred for one week in 33 Å and
25 Å M41S material. After filtering, the zeolite material containing the
coal was extracted with methylene chloride. Approximately 25% of the
coal material was recovered from each zeolite by methylene chloride
extraction. Further extraction with chlorobenzene resulted in only a few
additional percent of coal.

**Mesoporous silicate synthesis**. Synthesis of MCM-41 was an approach
combining the advantages of several literature works^2,3,5^. Different
chain length of surfactants, ranging from dodecyl­trimethylammonium to
octadecyl-trimethylammonium bromide, were used as template to construct
a periodic mesophase. The silica source used was tetramethyl
orthosilicate. The synthesis was carried out at room temperature in the
solution of methanol and sodium hydroxide mixture. Pure silica versions
of these zeolites were prepared to alleviate irreversible absorption by
aluminum sites.

**Characterization Methods.** Mass spectra were recorded on a Kratos
Maldi III instrument for laser desorption (LD) and a Kratos MS-50TA for
low voltage high resolution mass spectrometry (LVHRMS). For the LD
spectra, the laser intensity for the N~2~ laser (337 nm) was kept at ion
appearance threshold for all samples. TGA-DTA (thermal gravimetric
analysis and differential thermal analysis) measurements were obtained
on a SDT 2960 from TA Instruments. For these samples, measured against
an alumina standard in a 100 mL/min O~2~ flow with a temperature ramp of
10°C/min to 800°C. Total organic loss were calculated by measuring the
weight loss over the approximate temperature range of 200-600°C. XRD
analyses were carried out on a Rigaku Miniflex+ instrument using CuKα
radiation, a NaI detector, a 0.05° step size, and a 0.50°, 2/min scan
rate. XRD and SAXS showed pore sizes of 25 Å and 33 Å for the two
M41S-class zeolites.

**Results and Discussion**

The Gregar extractor extracted only 7.45% of the Upper Freeport coal
into pyridine. The LD mass spectrum of the coal-zeolite extract showed
only minimal ion intensity. The LVHRMS of this sample was used to
estimate the number of aromatic rings present in the extracts. The
results show that the largest mole % of hydrocarbon containing material
was zero rings (5.5%), one ring (2.5%), two ring (0.8%) species. There
is a small amount of material that can be attributed to multiple ring
systems above three rings (2.87%). The reason for the low extractability
using this technique may be a result of room temperature pyridine
extraction as opposed to refluxing pyridine for the traditional
technique. Although the amount of material is less, it is interesting
that the majority of this is essentially aliphatic with very little
aromatic character. The refluxing pyridine extracted 29.6% of APCS 1,
which is consistent with published results of extractabilities of this
coal.

We used TGA to determine the weight loss as a function of temperature.
From this data we can determine the total amount of organic material in
the mesoporous zeolite. The TGA data shown in **Figure 1** indicates
that the 33 Å pore size material contains a larger amount of coal
extract from the pyridine solution than the 25Å M41S mesoporous
material. This is shown as the weight loss per cent of the total sample
weight of 22.6% for the 33Å material versus 15.6% for the 25Å material
(Fig. 1 & 2).

![](media/image1.wmf){width="2.8in" height="2.0868055555555554in"}

**Figure 1**. The TGA of 33Å M41 S material with pyridine extract of
Upper Freeport coals.

![](media/image2.wmf){width="3.0555555555555554in"
height="2.1840277777777777in"}

**Figure 2**. The TGA of 25Å M41 S material with pyridine extract of
Upper Freeport coals.

Certainly, very polar material will bind to the silica. In the case of
both M41S mesoporous materials only 25 % can be removed from the
zeolite. The more nonpolar will be extracted in the washes of
coal-infused M41S material. The TGA indicates that very low molecular
weight material is evolved at about 60°C, which we assign to occluded
pyridine. The major part of the organic material begins to come off at
roughly 300°C and peaks at 440°C for the 33Å material and at 500 for the
25Å material. In addition, there is a distinct shoulder at 460 in the
33Å and at 520 in the 25Å which may indicate different organic compounds
or clusters of compounds. The peak at 675 in the 33Å material is only
1.5% of the total weight, but may be related to very tightly bound (very
polar) organic material on the silica.

![](media/image3.png)

**Figure 3**. The laser desorption mass spectrum of methylene chloride
extract of APCS 1 imbibed M41S zeolite. 25 Å pore diameter (upper panel)
33 Å pore diameter (lower panel).

The laser desorption mass spectra of the extracts from the
coal-containing mesoporous zeolites is shown in **Figure 3**. Here there
is a clear distinction in mass between the 25 Å and the 33 Å sieves. The
average mass of the 33 Å extract is higher than that of the 25 Å extract
by roughly 44 mass units. This would correspond to an additional ring
system. Since LD chiefly accesses the aromatic content of a sample, we
believe that this difference can be attributed to one additional ring in
the 33 Å over the 25 Å extract. That is, the separation of the extract
using mesoporous sieving by M41S-class zeolites is effectively
differentiating small ring sizes.

![](media/image4.wmf){width="3.154166666666667in"
height="2.557638888888889in"}

**Figure 4**. Comparison of aromatic ring size from HRLVMS of methylene
chloride extracts of 25Å and 33Å M41S sieves loaded with pyridine
extracts of Upper Freeport coal (APCS 1).

Clearly, the average molecular size of an aromatic molecule of molecular
weight 300 is much smaller than either pore size. This suggests that the
separation takes place on larger clusters of molecules of similar ring
sizes. We suggest that molecular clusters which are manifested in the LD
spectra as relatively small aromatic molecular ions are separated as
large clusters of molecules. Evidence from small angle neutron
scattering of pyridine coal systems shows a high polydispersity in
cluster size. We may be accessing one portion of that range in the 20-30
Å size range. Further experiments with larger pore size mesoporous M41S
materials are underway.

The LVHRMS results comparing the 25 Å and 33 Å is shown in **Figure 4**.
The average aromatic ring size is an estimate based on the number of
double bond equivalents calculated from the high resolution data. The 33
Å shows an increased aromatic ring size over the 25 Å.

**Acknowledgment**

This work was supported by the Division of Chemical Sciences, Office of
Basic Energy Sciences, U.S. Department of Energy, under contract
W-31-109-ENG-38.

**References**

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