A nonwoven supported mixed matrix membrane for CH4/N2 separation

Abstract

Efficiently enriching low-concentration CH4 is pivotal for enhancing the utilization of unconventional energy sources and mitigating greenhouse gas emissions. This study focuses on modifying the overall performance of CH4/N2 separation membranes. A novel mixed matrix membrane (MMM) with a reinforced substrate structure was developed through a straightforward dip-coating technique. This MMM incorporates a polytetrafluoroethylene (PTFE) porous membrane as the supporting framework, while a composite of block polymer (styrene-butadiene-styrene) and metal-organic framework (Ni-MOF-74) forms the selective separation layer. Comprehensive characterization of Ni-MOF-74 and the fabricated membranes was conducted using X-ray diffraction, scanning electron microscope, Brunauer-Emmett-Teller analysis, and gas permeance tests. The findings indicate a robust integration of the PTFE porous support with the membrane layer, enhancing the mechanical stability of the MMM. Under optimal conditions, the mechanical strength of the PM20 membrane (containing 20% Ni-MOF-74) was observed to be 37.7 MPa, representing a remarkable increase compared to the non-reinforced MMM. Additionally, the PM20 membrane exhibited an impressive CH4 permeation rate of 92 barrer (1 barrer = 3.35 × 10^-16 mol·m·m^-2·s^-1·Pa^-1) alongside a CH4/N2 selectivity of 4.18. These results underscore the MMM's substantial performance and its promising potential in methane enrichment applications.

1. Introduction

Compared to traditional fossil fuels like coal and oil, coalbed methane presents distinct advantages, notably in cleanliness and reserve abundance, positioning it as a viable and eco-friendly energy alternative. The development and strategic utilization of coalbed methane carry profound implications for energy sustainability, as outlined in prior research [1-3].

A critical aspect of harnessing coalbed methane effectively lies in the enrichment of methane, a widely acknowledged bottleneck in the field [4]. Industry currently employs several CH4/N2 separation techniques, notably pressure swing adsorption, cryogenic separation, and the adsorption method. These methodologies, while mature, confront challenges including cumbersome separation apparatus and elevated operational costs, particularly for low CH4 concentration coalbed gas, which demands excessive energy for enrichment [5-7]. Membrane separation processes have emerged as a promising solution, offering precision, efficiency, minimal energy consumption, and an environmentally benign footprint [8-10].

The current landscape of membrane materials for CH4/N2 separation predominantly includes carbon nanotube (CNT) membranes, metal-organic framework (MOF) membranes, and mixed matrix membranes (MMMs) based on styrene-butadiene-styrene (SBS) triblock copolymers [11]. For instance, Gilani et al. [12] developed composite membranes by embedding CNTs in anodized aluminum oxide (AAO), achieving CH4/N2 selectivity in the range of 1.80-3.85 at room temperature, but with limited CH4 permeance of (8.2-9.5) × 10^-8 mol·s^-1·Pa^-1. Dou et al. [13] fabricated a continuous MIL-100 (In) membrane via an in-situ hydrothermal method on an AAO substrate, achieving a selectivity of 3.38 and a CH4 permeance of 8.1 × 10^-7 mol·m^-2·s^-1·Pa^-1. Wang et al. [14] incorporated [Ni3(HCOO)6] filler into SBS polymer to create a novel composite membrane, which under optimized conditions displayed a CH4/N2 selectivity of 2.7 and a CH4 permeance of 181 barrer (1 barrer = 3.35 × 10^-16 mol·m·m^-2·s^-1·Pa^-1). Xue and Liu [15] prepared a mixed matrix membrane with modified SBA-15 and SBS polymer, exhibiting a high CH4/N2 selectivity of 4.8 but a lower CH4 permeance of 20 barrer. These studies have positively impacted the separation performance of CH4 in coalbed gas, yet they encounter challenges such as complex fabrication processes, thick membrane layers (70-100 μm), and comparatively low CH4 permeance [16-18]. Research indicates a crucial interplay between the microstructure, mechanical strength, and gas permeance of membranes for effective CH4 enrichment, where achieving a balance remains a key challenge.