1. Results and Discussion
  2. 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.