Sustainable Separations of C₄‐Hydrocarbons by Using Microporous Materials

Abstract

Petrochemical refineries must separate hydrocarbon mixtures on a large scale for the production of fuels and chemicals. Typically, these hydrocarbons are separated by distillation, which is extremely energy intensive. This high energy cost can be mitigated by developing materials that can enable efficient adsorptive separation. In this critical review, the principles of adsorptive separation are outlined, and then the case for C4 separations by using zeolites and metal–organic frameworks (MOFs) is examined. By analyzing both experimental and theoretical studies, the challenges and opportunities in C4 separation are outlined, with a focus on the separation mechanisms and structure–selectivity correlations. Zeolites are commonly used as adsorbents and, in some cases, can separate C4 mixtures well. The pore sizes of eight‐membered‐ring zeolites, for example, are in the order of the kinetic diameters of C4 isomers. Although zeolites have the advantage of a rigid and highly stable structure, this is often difficult to functionalize. MOFs are attractive candidates for hydrocarbon separation because their pores can be tailored to optimize the adsorbate–adsorbent interactions. MOF‐5 and ZIF‐7 show promising results in separating all C4 isomers, but breakthrough experiments under industrial conditions are needed to confirm these results. Moreover, the flexibility of the MOF structures could hamper their application under industrial conditions. Adsorptive separation is a promising viable alternative and it is likely to play an increasingly important role in tomorrow's refineries.

Efficient Separation of n‐Butene and iso‐Butene by Flexible Ultramicroporous Metal‐Organic Frameworks with Pocket‐like Cavities

“…Separation of C4 hydrocarbons is an important task but challenging for chemical industry due to their similar physical and chemical properties . Among the separation of C4 hydrocarbons, 1,3‐butadiene was firstly separated, the remaining streams mainly contain n ‐butene ( n ‐C 4 H 8 ) and iso ‐butene ( iso ‐C 4 H 8 ) . Separation of n ‐C 4 H 8 and iso ‐C 4 H 8 is a critical process for the production of high purity of n ‐C 4 H 8 and iso ‐C 4 H 8 , which are both fundamental feedstocks in high demand in petrochemical industry .…”

Thermodynamic Separation of 1-Butene from 2-Butene in Metal–Organic Frameworks with Open Metal Sites

“…[19][20][21] Separation of n-butene mixtures has also been studied using several different zeolitic frameworks. 16 Notably, all-silica RUB-41 was demonstrated to adsorb both cis-and trans-2-butene selectively over 1-butene in liquid-phase adsorption and breakthrough experiments. 22 Unfortunately, the saturation capacities of this zeolite are quite low (≤ 1.0 mmol/g for the 2-butenes) and the origin of its selectivity is not well understood.…”

“…Relevant physical properties of C 4 olefins and paraffins. 16 Given their similar boiling points and comparable sizes and physical properties (Table 1), the C 4 hydrocarbons are particularly challenging to separate. In contrast to lighter C 2-3 hydrocarbon mixtures, where the components differ in their degree of unsaturation, the presence of olefin and paraffin isomers leads to diverse mixtures of molecules bearing few chemical handles that can be exploited in viable separation schemes.…”

“…The Sorbutene process of UOP uses molecular sieves to produce 1-butene in 99.2% purity from Raffinate II, although the yield suffers somewhat from in situ isomerization. 11 Various studies have explored the potential of zeolites to effect useful C 4 hydrocarbon separations, 16,18 with particularly notable success in the selective removal of 1,3-butadiene from C 4 mixtures. [19][20][21] Separation of n-butene mixtures has also been studied using several different zeolitic frameworks.…”

Alternatives to Cryogenic Distillation: Advanced Porous Materials in Adsorptive Light Olefin/Paraffin Separations

“…Last, a perspective on possible directions to push the limit of the research in this field is presented. C4 olefin/paraffin separation, a subset of light olefin/paraffin separations, will not be discussed in this review to avoid redundancy with the existing review on C4 hydrocarbon separations using microporous materials …”