**5.3 Mixed matrix membranes (MMMs)**

MMMs have been explored to develop materials for enhanced separation performance of conventional membrane. Different types of inorganic fillers, such as silica, TiO2, fumed silica, graphene oxide (GO), activated carbon (AC), MOFs, zeolites, ZIFs, POSS etc., have been incorporated into PDMS, POMS, PTMST, and PU membrane matrix for air separation [119], CO2 removal and capture [120, 121], pervaporation [122], hydrogen separation [83, 123], and hydrocarbon separation [10, 65, 94, 124–129]. The concept of using MMMs for light and heavy hydrocarbons separation and recovery in petrochemical and chemicals industries has attracted much interest and research in recent years. Several studies, such as olefin/ paraffin separations (e.g. C2H4/C2H6, C3H6/C3H8, *n*-C4H8/*i*-C4H10), aromatic/aliphatic separations (e.g. toluene/n-hexane, *p*-xylene/*o*-xylene), vapor/gas separations (e.g. C3+/N2, C3+/H2 and C3+/CH4) were discussed in a recent review by Najari and coworkers [125]. Here, summary of C3+ hydrocarbon permeation properties of mixed matrix membranes (e.g. PTMSP, PTMGP, PDMS, POMS, PU and modified rubbery siloxane) in pure and mixed gases can be found in **Table 9**. Khanbabaei et al*.* [55] prepared PDMS-fumed silica MMMs for removing C4H10 from CH4. Results show the decreased C4H10/CH4 mixed gas selectivity for both neat membrane and MMMs. The MMMs exhibited 38% increment in C4H10 permeability and simultaneous 30% increase in C4H10/CH4 selectivity under C4H10/CH4 (3/97 mol%) binary gas mixture with adding >10 wt% of fumed silica, compared to neat PDMS membrane. Mushardt et al*.* [89] evaluated the separation of C3+ hydrocarbons of MMMs composed of POMS and 10–40 wt% AC from permanent gas streams under varying operating conditions (e.g. feed and permeate pressure, temperature, feed gas compositions). Best performance was achieved at highest average fugacity of C4H10 caused by increasing feed pressure, high permeate pressure and high concentration of C4H10 in feed mixture. At 40 bar, POMS/20 wt% AC MMMs exhibited higher C4H10/CH4 mixed gas selectivity of 33 and C4H10 permeance of 18 mN3 /(m<sup>2</sup> . h.bar) under multicomponent gas mixtures including 2 vol% of C4H10. Gomes et al*.* [65] reported the preparation of PTMSP/silica MMMs with incorporation of 20– 40 nm silica by sol–gel copolymerization of tetraethoxysilane (TEOS) with different organoalkoxysilanes. When methyltriethoxysilane (MTEOS) or *n*octyltriethoxysilane (OTEOS) were used, the obtained MMMs exhibited higher separation performance (C4H10/CH4 mixed gas selectivities of 12.7 and 13.9 at 1 bar and 30°C, respectively).

In a recent investigation, Yang et al*.* [95] described a method to design and produce novel, crosslinked siloxane/POSS MMMs for enhanced C3+ hydrocarbon recovery from natural gas. Dual-functional POSS nanofiller (OS-POSS-VTMO) containing silicon hydride moiety (☰–Si–H) and trimethoxylsilicon groups (–Si (OMe)3) was synthesized and used as both a crosslinking agent and nanofiller (**Figure 18**) in modified rubbery siloxane (Ter-PDMS) membrane matrix. Under


*Natural Gas - New Perspectives and Future Developments*

