Separation of ethane-ethylene and propane-propylene by Ag(I) doped and sulfurized microporous carbon

Highlights

• Ag(I) doped and sulfurized microporous carbons were synthesized.

• Carbons characterized with porosity, SEM-EDX and XPS.

• The carbon was selective to C₂H₄ and C₃H₆ in mixtures with C₂H₆ and C₃H₈.

• IAST-based selectivity was calculated adsorption breakthrough was simulated.

• The selectivity to alkenes were attributed to pi-pi complexations.

Abstract

Separation of light olefins from their respective paraffins by adsorption is an attractive strategy due to the sustainable and inexpensive nature of adsorption process. It is well understood that the presence of Ag(I) in the adsorbent favors the adsorption of ethylene and propylene (olefins) over ethane and propane (paraffins) owing to the $\pi$ -$\pi$ complexation between Ag(I) and $\pi$ bond present in olefins. In this research, Ag(I) doped microporous carbons were synthesized from furfuryl alcohol as carbon source. It is also shown that prior sulfurization of the carbon greatly favored Ag(I) content owing to the affinity of sulfur to Ag(I). All the carbons were successfully characterized with pore textural properties, SEM-EDX imaging and x-ray photoelectron spectroscopy (XPS). Only the carbon with highest Ag(I) content (2.5 at.%) and BET specific area (1193 m²/g) favored the adsorption of ethylene and propylene over ethane and propane, respectively. IAST-based selectivity values were also employed to calculate binary adsorption isotherms for ethane-ethylene and propane-propylene mixtures. The IAST-based selectivity values of ethylene/ethane and propylene/propane were in the range of 4.5 to 2.5 and 5 to 2.4, respectively. Finally, column breakthrough and pulse chromatographic peaks were simulated to confirm the separation of paraffin-olefin pairs on thus carbon. To the best of our knowledge, it is the first report of the separation of light paraffin and olefins in a carbon-based adsorbent and by harnessing the $\pi$ -$\pi$ complexation.

Introduction

Separation of light olefins from their paraffin counterparts is one of the key separation needs of today [1]. Ethylene and propylene are two such light olefins and for their applications, they need to be separated from ethane and propane, respectively. The key usage of the light olefins is in the polymerization industries where polyethylene (polythene) and polypropylene are the two main polymerization products of ethylene and propylene, respectively. Besides polymers, different types of specialty chemicals are also produced from light olefins whereas light paraffins are mostly used as fuel [2]. In the last decade, the production of these olefins increased to over 50% (e.g., 25 trillion tons of ethylene per annum) [3] and with increase in demand from developing countries, the production is expected to increase further [4]. It is estimated that global annual production of ethylene and propylene has exceeded 200 million tons [1]. Usually, in petroleum industry, the olefins are synthesized by thermal decomposition, fluid catalytic cracking or thermal cracking of gas oils [[5], [6], [7]]. In order to meet the increasing demand of olefins, a recent technology involved the dehydrogenation of paraffin to produce olefin [[8], [9], [10]], however, thermodynamics constraints limited the equilibrium conversion to 20–40% [3] only. Therefore, a suitable separation is required to recover the olefins from the product mixtures. The polymer grade olefin should have a purity of11 > 99.5% in order to avoid undesirable products during polymerization.

Owing to the very similar chemical properties of an olefin with respect to its corresponding paraffin, it is very challenging to separate ethylene from ethane and propylene from propane. The kinetic diameters of ethane, ethylene, propane and propylene are 4.44, 4.16, 4.3 and 4.5 Å, respectively [12]. Owing to the very close kinetic diameter of ethane/ethylene or propane/propylene pairs, it is very difficult to separate them by size exclusion basis. However, due to the small difference in boiling points of ethane/ethylene and propane/propylene pairs (Ethane: −89 °C; Ethylene: −103.7 °C; propane: −42 °C; propylene: -47.6 °C, all ambient pressure), the most state-of-the-art technology to separate light paraffin-olefin is cryogenic distillation, which has been used for almost 70 years without any significant improvement [13]. As the distillation needs to harness only a small difference in boiling point of paraffin and olefins (especially for propane/propylene mixtures) in the cryogenic region, the unit operation is performed under extreme conditions of −25 °C/23 bars for ethane/ethylene [14,15] and −30 °C/3 bar for propane/propylene separation [13] to obtain high quality and polymer grade olefins [16]. The distillation unit is also quite big, consisting of 100–150 trays [17]. Because of such operation, the Department of Energy (DOE) estimated that about 0.12 Quads (1 Quad = 10¹⁵ BTU) of energy was used annually for the separation of paraffin-olefin as early as 1991 [18] and it increased significantly today. A single unit of ethane/ethylene separation costs about $500 million [13]. Purification of ethylene and propylene alone accounts for 0.3% of global energy use [1], which is equivalent to the annual energy consumption of a small nation, like Singapore [19]. Because of such a large capital investment and operational cost, research is directed towards developing an inexpensive and smaller separation system that can efficiently separate light olefins from their respective paraffins. Different alternative technique that has been introduced in the separation of light paraffin and olefins are membrane separation [[20], [21], [22]] organic solvent-based sorbents [23] and nanoporous adsorbents [24,28].

A very large array of nanoporous adsorbents have been examined for separation of paraffin and olefins, like MOFs, zeolites or silica. As mentioned earlier, owing to similar size of the paraffin/olefin pair, affinity-based adsorption has always been a popular choice to selectively adsorb and separate the paraffin or olefins. Although few adsorbents demonstrated preference to paraffin (like ethane [12,19,24] or propane [24]), the majority of the research has been dedicated towards preferential adsorption of olefin and it mainly performed by $\pi -\pi$ complexation with the help of Ag(I) or Cu(I) species doped or grafted on the sorbent surface. π- π complexation has been implemented in different types of adsorbent systems, like, cation exchanged [25,26] or CuCl dispersed zeolites [27]. Ag(I) is often used to graft with MOFs [28] or PAFs [29] through sulfonate functionalization and the resultant adsorbents demonstrated superior selectivity towards olefins. Alumina [30], or silica [11,31] was also used to support the systems for π complexations. A detailed review of these systems can be found elsewhere [27,32].

Although π- π complexation can influence an olefin adsorption in the mixture of olefin and paraffins, it has not been investigated for carbon-based nanoporous adsorbents. Carbonaceous adsorbents have several advantages over other adsorbents, such as easy and inexpensive synthesis method, very high and tunable surface area, high micropore volume, and remarkable chemical and thermal stability. In this research, we have synthesized a polymer-derived and highly microporous carbon followed by its sulfurization and chemical grafting of Ag(I) on its surface. This Ag(I) grafted microporous carbon was studied with adsorption of ethane/ethylene and propane/propylene pairs to investigate their separations. To the best of our knowledge, this is the first report on separation of alkane-alkene by Ag(I) doped carbon.