Introduction
Effective separation of olefin/paraffin is critical for modern chemical
industry,1-4 in which olefins are primarily obtained
from the hydrocarbons steam cracking mixtures to serve as
feedstocks.5,6 Removal of impurities from olefins must
be maximized to ensure the high quality of subsequently
derived-products.7,8 Typical methods for
olefin/paraffin separation include cryogenic
distillation,3,9 liquid – liquid
extraction,10 adsorptive
separation,11-13 and membrane
separation.14-16 Among all these approaches,
adsorptive separation is considered to be a promising alternative
compared to the current main technologies owing to its high selectivity
and flexibility, mild operating condition and low capital
cost.3,13,17 Indeed, successful adsorptive separation
largely relies on the performance of adsorbents employed. During the
past several decades, a variety of porous materials, including
aluminosilicate zeolites,12,18,19 carbon molecular
sieves,20,21 and metal – organic frameworks
(MOFs),22,23 have been extensively explored, tested
and applied for adsorptive separation of olefin/paraffin mixtures.
Synthetic zeolites remain the most popularly utilized adsorbents in
various industrial processes due to their high energetic and structrual
stability, and low cost.24,25
Ion-exchange has been primarily focused on tuning the pore structure and
adsorption affinity to enhance adsorption capacity and
selectivity.26,27 According to earlier reports, a
variety of ion-exchanged zeolite sorbents have been employed for
selective adsorption of olefins over paraffins. Sakai et
al. 28 reported that compared with the parent zeolite
NaX membrane, the selectivity of Ag-exchanged zeolite X membrane for
olefins increased from 3.63 (NaX) to 55.4 (AgX) for a propylene/propane
(50:50) mixture. In addition, Anson et al. 29studied the effect of cation type on the performance of ion-exchanged
ETS-10 in ethane/ethylene separation. They concluded that the adsorption
selectivity of ethylene over ethane decreased following the order of Na
> K > Li > Cu ≈ Ba >
Ba/H > La/H. This trend appears to be completely opposite
to the pressure swing adsorption results. Moreover, it was reported that
Cu-exchanged natural Chilean zeolite, which mainly consists of
clinoptilotite and mordenite, showed increased number of adsorption
sites and enhanced interaction energy in ethylene adsorption, which led
to significantly increased adsorption capacity.30
For practical applications, conventionally synthesized zeolite powder
has to be fabricated as macroscale particles/pellets with additional
binder to ensure relative low pressure drop across the sorbent
bed.31,32 Nevertheless, the binder introduced is an
adsorption inert component, which results in reduced adsorption
capacity.31,33 Moreover, the binder component may lead
to undesired side reactions and/or diffusion
inhibition.33,34 To minimize the negative impacts of
binder on the performance of industrial adsorbents, recently, we
developed a one-pot route to synthesize binderless zeolite A pellets by
using in situ hydrothermal transformation of silica gel
precursors.35
In current work, we report synthesis of spherical binderless zeolite X
pellets via in situ hydrothermal conversion of silica gel
precursors. The synthesized binderless zeolite X exhibits significantly
enhanced adsorption performance compared with commercially available
binder-containing zeolite X products. Employing multiple analysis
methodologies, we further characterized the resulting solid samples as
the synthesis length varies and deduced the in situ conversion
mechanism from precursors to zeolite X. Further, the impacts of ion type
and exchange degree on olefin adsorption were evaluated experimentally
by 1-hexene adsorption and computationally with GCMC simulation, in
which we successfully correlate adsorption capacity and guest – host
interaction energy with type of cation and degree of exchange for
1-henexe adsorption on ion-exchanged zeolite X.