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.