Sample
|
Adsorbent Weight
(g)
|
Bed Height
(cm)
|
Bed Diameter
(cm)
|
Pressure Drop
(bar)
|
1.6 mm 13X-beads |
50 |
7.5 |
3.2 |
0.264 |
Monolith - 600 cpsi |
27 |
7.5 |
3.2 |
0.178 |
Monolith - 800 cpsi |
42 |
7.5 |
3.2 |
0.192 |
Results and Discussion
Materials Characterization
The crystallinity of the zeolite adsorbents was confirmed by XRD
(Figure 1a ). As evident, the major characteristics peaks
observed in these spectra were consistent with those of zeolite 13X
powder,17 indicating that the crystallinity was not
affected during either the extrusion or pellitization processes. It is
also worth noting here that, in the monoliths several of the minor
diffractive indices exhibited greater intensities compared to the
zeolite beads. This could have possibly been attributed to variation
between the sources used to manufacture the zeolite or slight
differences in the hardening procedures. From the TGA experiments
(Figure 1b ), it was also shown that the monoliths exhibited a
greater weight loss (20%) compared to the beads (10%) below 300 °C.
This further suggested that the monoliths contained higher amount of
organic components than the beads, because the weight loss in both
samples could likely be attributed to removal of additional moisture,
and the elevated loss in the monoliths indicated that a greater quantity
had been adsorbed. Instead, the monoliths’ weight loss exhibited a
smooth profile which was nearly parallel to that of the beads and is
indicative of the removal of a single species.18 The
difference in weight loss could also be explained in terms of zeolite
content which is lower in the monoliths (90 wt%) relative to binderless
beads.
Figure 1. (a) XRD profiles and (b) thermogravimetric analysis
curves for 1.6 mm beads, 600 and 800 cpsi monoliths.
The N2 physisorption isotherms and pore size
distributions are shown in Figure 2 while the textural
properties of the samples are summarized in Table 2 . In the
N2 physisorption profiles (Figure 2a ), all
three 13X adsorbents displayed type I isotherm with H4 hysteresis,
suggesting microporous nature of the materials and also the presence of
slit-type mesopores formed during the formulation
process.19 These differences were further evident in
the pore distributions (Figure 2b ), where significant
reductions in pore volume were observed from the monoliths to the beads.
It is also worth noting here that the honeycomb monoliths also exhibited
slight mesoporosity at ~4 nm pore diameter. As we
reported recently,12 this could have been caused by
the binder removal process, which burns out the organic components and
produces a hierarchal pore structure.
Figure 2. (a) N2 physisorption and (b) NLDFT
pore distributions for beads, 600 and 800 cpsi monoliths.
As shown in Table 2 , the BET surface areas were found to be
662, 548, and 571 m2/g for the binderless beads, 600
cpsi monolith and 800 cpsi monolith samples, respectively. the surface
areas of the monoliths ~ 83% of that of binderless
beads which is due to lower zeolite content of the monoliths (i.e., 90
wt%). These differences in surface area were to be expected from TGA,
XRD, and N2 physisorption, which all suggested the
monoliths’ formulation process decreased the number of accessible pores.
This was further supported by the monoliths’ slight (8-9%) reduction in
micropore volume from the monoliths. Nevertheless, it is worth noting
that the differences in pore volume between the three samples were,
overall, small. For this reason, they could all be considered comparable
in further testing.
Table 2. Textural properties of 13X zeolite beads and 600 and
800 cpsi monoliths.