*Polymer-Based Membranes for C3+ Hydrocarbon Removal from Natural Gas DOI: http://dx.doi.org/10.5772/intechopen.103903*

Another promising reverse-selective glassy polymer class for C3+ hydrocarbon separation and removal are substituted polynorbornenes (PNBs). Many valuable correlations between gas-permeability and polynorbornene structure have been summarized in prior reviews [12, 19, 71]. More recently, the development in the field of next generation polynorbornene-based polymers, their limitations and challenges for targeted gas separations was discussed also in a recent review [99]. These studies show that solubility-controlled permeation of gaseous hydrocarbons (C1 C4) is characteristic for both addition polymerization-type polynorbornenes (APNBs) with Si-containing side-groups and ring opening metathesis polymerization type polynorbornenes (ROMP PNBs) with flexible Si-O bonds in side-groups (**Figure 12**). Summary of C3+ hydrocarbon permeation properties for APNBs and ROMP PNBs membranes under pure- and mixed-gas can be found in **Table 7**. In general, the membrane C4H10/CH4 selectivity increased significantly as increasing the length of side groups (e.g. Me, Et, *n*-Pr, *n*-Bu). Among these polynorbornenebased polymers, both distributed APNBs-Si(OPr)3 and ROMP-Si(OBu)3 membranes exhibited higher C4H10/CH4 pure gas selectivities of 48.8 and 32.4, respectively [76].

Although many studies investigated the permeation properties using pure gases, fewer reported separation performance under multicomponent gas feed streams [21]. Sundell et al*.* [75] reported a route toward the production of *exo* ROMP and addition-type polynorbornenes and polytricyclononenes (APTCN) through the stereochemical control afforded by the reductive MizorokiHeck reaction. All addition-type and tricyclononene-based polymer membranes (e.g. APTCN-SiMe3 and APTCN-Si(OEt)3) show improved mixed gas C4H10/CH4 selectivities compared to APBNs-Si(OEt)3 and ROMP-Si(OEt)3 membranes (**Figure 13**) under the same testing conditions, due to the increased polymeric chain-spacing. These polynorbornene-based polymers demonstrate solubilityselective permeation with mixed gas selectivities that exceed commercially used PDMS.

#### **Figure 12.**

*Structures of metathesis and addition polynorbornenes bearing different substituents for gas separation applications.*

#### **Figure 13.**

*Mixed gas C4H10 permeability vs.C4H10/CH4 selectivity for APBNs-Si(OEt)3 (black squares), ROMP-Si(OEt)3 (red circles) and substituted-APTCN (blue triangles) under C3+ rich multicomponent gas mixtures containing 1.5–4 vol% C4H10 at 25°C and 800 psi.*

## **5.2 Rubbery polymers**
