Advances in mixed-matrix membranes for biorefining of biogas from anaerobic digestion

This article provides a comprehensive review of the state-of-the-art technology of polymeric mixed-matrix membranes for CO2/CH4 separation that can be applied in medium, small, and domestic biogas systems operating at low pressures (0.2–6 kPa). Critical data from the latest publications of CO2/CH4 separation membranes were analyzed, considering the ratio of CO2/CH4 permeabilities, the CO2 selectivity, the operating pressures at which the membranes were tested, the chemistry of the polymers studied and their gas separation mechanisms. And the different nanomaterials as fillers. The intrinsic microporous polymers (PIMs) were identified as potential candidates for biomethane purification due to their high permeability and selectivity, which are compatible with operation pressures below 1 bar, and as low as 0.2 bar. This scenario contrasts with other polymers that require pressures above 1 bar for operation, with some reaching 20 bar. Furthermore, the combination of PIM with GO in MMMs was found to not influence the permeability significantly, but to contribute to the membrane stability over time, by preventing the structural collapse of the membrane caused by aging. The systematic analysis here presented is a valuable resource for defining the future technological development of CO2/CH4 separation membranes for biogas biorefining.


Introduction
Biogas is a strategic fuel against the dependence on fossil fuels, especially to replace natural gas (Atelge et al., 2021).Biogas is a mixture of gases with a high content of methane, and it is produced by the anaerobic digestion of biomass, with a composition as detailed in Table 1 (Rick Wanek, 2011;Occupational Safety and Health Administration OSHA, 2019;Piergrossi et al., 2019;Yang et al., 2020).The upgrading of biogas refers to physical, chemical, and biological processes to enhance its properties by removing unwanted compounds and producing purified biomethane.Carbon dioxide (CO 2 ) is the second most abundant component in biogas, and it is chemically inert to the combustion reaction.However, CO 2 reduces the calorific value of the biogas to 20-25 MJ/m³, compared to biomethane with 35-40 MJ/m³.Other undesirable components in biogas include water, hydrogen sulfide (H 2 S), and siloxanes, which even at trace levels can cause corrosion, deteriorate the biogas systems, and potentially harm human health (Baena-Moreno et al., 2019).
Millions of biogas plants are currently in operation in the world ranging from domestic to very industrial scales.Anaerobic digestion technology integrates a wide variety of biosystems, in contrast to natural gas refining that currently takes place at a few hundred industrial facilities around the world.Various cleaning methods have been developed to improve the quality of the biogas at small and medium scale, mainly to remove H 2 S, siloxanes, and water.However, there are no widespread or affordable methods for CO 2 removal for smaller scales (Sanders et al., 2013;Sutanto et al., 2017).Specific polymer membranes and their composites with fillers in mixed-matrix membranes (MMMs) have been shown to selectively separate gas mixtures taking advantage of both the physical and chemical interactions between membrane material and gas molecules (Vu et al., 2003;Cheng et al., 2019).MMM refining has the potential to be incorporated into biogas systems as a scalable and flexible technology for different system dimensions.MMM technology has lower costs, easier operation, reduced equipment size, higher efficacy, and a smaller carbon footprint than other capture and filtration technologies, such as adsorption filtration systems and cryogenic distillation; or chemical methods like caustic soda washing (Chuah et al., 2021).Therefore, the MMM technology for CH 4 /CO 2 gas separation and biogas upgrading is feasible to implement in systems at smaller scales.
This review presents a comprehensive and updated visualization of the advances in the adaptation of polymeric membrane technology and MMMs for biogas biorefining in medium-and small-scale biogas systems.This review is elaborated by collecting, processing, and analyzing the main characteristics and variables of polymeric membranes used for CH 4 /CO 2 gas separation and their modification with fillers.The objective is to identify the optimal characteristics for MMMs in the context of medium and small-scale anaerobic digestion, considering the impact these systems would have in practice, such as operating conditions, component separation efficiencies and reported yields, which are the main indicator for development of anaerobic digestion projects.This work contributes to the development of sustainable and efficient solutions for biogas biorefinery as a renewable resource of growing importance in the transition to a low-carbon economy, as an alternative for countries with developing economies.

Mechanisms for gas separation through polymeric membranes
The permeability is an intrinsic property of a gas in a specific material, reflecting its inherent capacity to allow molecules to go through under specific conditions.Permeability (P) is defined in Eqs 1, 2 by the penetrant diffusion coefficient of transport through the membrane (D), the solubility coefficient (S), the penetrant concentration (C), and partial pressure (p) (Galizia et al., 2017): The permeability and solubility are influenced by the chemical and physical interactions between the permeating molecules and the membrane material.The transport of molecules through membranes relies on diffusion, solution, adsorption, and molecular sieving mechanisms within the polymer matrix as illustrated in Figure 1 (Li et al., 2015;Cheng et al., 2019).The transport process begins with the absorption of gas molecules onto the membrane surface.Then, the diffusion through the membrane matrix is driven by the concentration gradient, the affinity of the gas molecules for the polymer matrix and the ratio of size and shape of the molecules relative to the pore size within the membrane structure.For biogas upgrading into biomethane, the selectivity of either CH 4 or CO 2 is achieved by taking advantage of the differences in the physical and chemical interactions of the molecules with the membrane material.The sieving effect allows, by kinetic diameter, selectively pass through the molecules through the membrane pores.Additionally, the chemical affinity between specific gas molecules and the membrane material can enhance selectivity by preferentially absorbing certain species (Li et al., 2015;Cheng et al., 2019).

Historical development of mixedmatrix membrane technology for gas separation
In 1979, within the context of natural gas refining, the first gas separation station using polymeric membranes made of polysulfone (PSF) was built (Galizia et al., 2017).Commercial PSF membrane technology was developed by Permea, specifically for H 2 , N 2 , Ar and CH 4 purification plants (Bollinger et al., 1982).The polymer  [(Occupational Safety and Health Administration OSHA, 2019;Rick Wanek, 2011;Yang et al., 2020;Piergrossi et al., 2019)].
vapor recovery applications were estimated at $1.5 billion annually by 2017, with natural gas treatment alone accounting for $300 million per year (Chuah et al., 2021).
In recent years, advances in membrane technology have shifted towards the incorporation of fillers into polymer matrices, resulting in mixed-matrix membranes (MMMs).Some of the incorporated fillers are carbonous materials, graphene oxide (GO) and few layer graphene (FLG), metal-organic frameworks (MOFs), and organosilicon materials as polyhedral oligomeric silsesquioxane (POSS), which significantly enhance the longevity of their properties reducing problems as aging and plasticization (Shen et al., 2015;Xin et al., 2015;Castarlenas et al., 2017;Feijani et al., 2018;Al-Maythalony, 2019;Li et al., 2020).While the inclusion of fillers offers promising avenues for enhancing the membrane performance, challenges such as dispersion of the filler within the polymer matrix, the potential for agglomeration, and ensuring the long-term stability and resistance to fouling must be addressed (Li et al., 2015;Cheng et al., 2019).
The polymeric MMMs developed for the separation of CH 4 /CO 2 mixtures present obstacles for their implementation in low pressure biogas systems since their physical characteristics are not able to obtain viable separations with these operating conditions (Ebrahimi et al., 2016;Kheirtalab et al., 2020;Raouf et al., 2020;Sainath et al., 2021).Their performance over time, in which the physical aging and plasticization of the polymer reduces the permeability and selectivity is also a concern that limits their application to industrial scales.Although several polymers have shown outstanding properties so far, the natural gas refining industry is still largely dominated by CA, with 80% of the 2012 market dominated, followed by PI (Sanders et al., 2013;Jansen et al., 2016).
More recently, with the development of nanomaterials for implementation in MMM, ultrathin membranes from combinations of nanofillers have also been developed, with exceptional results in their performance.Researchers have developed graphene oxide membranes with ZIF-8, organosilica with reduced graphene oxide, and individual sheets of graphene oxide, among others.Despite their great performance, this type of membrane lacks two-of-three fundamental aspects in the list required for its implementation: performance-feasibility of implementation-manufacturing costs (Li et al., 2018;Sainath et al., 2021;Zhao et al., 2021).

Analysis of mixed-matrix membranes performance for biomethane purification
The implementation of membranes for biomethane purification at small and medium scale anaerobic digestion systems requires their viable operation at low pressure.The data of separation performance of membranes for CH 4 /CO 2 separation from four previous major reviews (Koros et al., 1988;Vu et al., 2003;Galizia et al., 2017;Comesaña-Gándara et al., 2019), and multiple investigations (Park et al., 2008;Koros et al., 1988;Puleo et al., 1989;Sada et al., 1989;Mizumoto et al., 1993;Hachisuka et al., 1995;Pinnau, 1996;Al-Masri et al., 1999;Zimmerman and Koros, 1999;Lin and Chung, 2001;Yang et al., 2001;Nagel et al., 2002;BUDD et al., 2005;Shida et al., 2006;Wang et al., 2007;Ghanem et al., 2009;Fritsch et al., 2011;Ma et al., 2012;Li et al., 2015;Shen et al., 2015;Xin et al., 2015;Althumayri et al., 2016;Ebrahimi et al., 2016;Bernardo et al., 2017;Castarlenas et al., 2017;Alberto et al., 2018;Feijani et al., 2018;Li et al., 2018;Al-Maythalony, 2019;Kheirtalab et al., 2020;Li et al., 2020;Raouf et al., 2020;Yang et al., 2020;Luque-Alled et al., 2021a;Sainath et al., 2021;Zhao et al., 2021;Mohsenpour et al., 2022) have been analyzed and visually organized in this section (See database in Supplementary Material).The data is presented in graphs of CO 2 /CH 4 selectivity versus CO 2 permeability and incorporate the visualization of variables such as polymer, fillers, and the year of publication of the research.The pressure at which the gas separation was carried out is also included in the analysis, which is a variable that may not be as important for large industrial natural gas operations but is very significant for low-pressure biogas systems.Special attention has been devoted to recent research on MMMs using different fillers.The Pineau (2015) and Robeson (2008) describe limits based of permeability vs. selectivity historic data points, in order to describe the behavior of Limits that were added to the graphs as benchmarks, derived from other studies and described in equations (3-5) (Comesaña-Gándara et al., 2019), where α CO2/CH4 is the separation factor of the gases.
Figure 3 shows the selectivity of CO 2 /CH 4 as a function of CO 2 permeability, and the pressure of operation is indicated as a color scale.The data points are shown in grey for the cases where the pressure information was unavailable.The pressures at which membranes have been studied for CO 2 /CH 4 separation range from 0.2 to 20 Bar.It is observed that larger pressures (red dots) were used for the materials located further below the Pineau and Robeson limits, and for membranes with lower permeability.The lower pressures (blue dots) were used for materials located close to the Pineau and Robeson limits.
Figure 4 displays the same set of data, but in this case the polymer material is indicated with the color scale (Park et al., 2008;Koros et al., 1988;Puleo et al., 1989;Sada et al., 1989;Mizumoto et al., 1993;Hachisuka et al., 1995;Pinnau, 1996;Al-Masri et al., 1999;Zimmerman and Koros, 1999;Lin and Chung, 2001;Yang et al., 2001;Nagel et al., 2002;Vu et al., 2003;BUDD et al., 2005;Shida et al., 2006;Wang et al., 2007;Ghanem et al., 2009;Fritsch et al., 2011;Ma et al., 2012;Sanders et al., 2013;Li et al., 2015;Shen et al., 2015;Xin et al., 2015;Althumayri et al., 2016;Ebrahimi et al., 2016;Bernardo et al., 2017;Castarlenas et al., 2017;Galizia et al., 2017;Alberto et al., 2018;Feijani et al., 2018;Li et al., 2018;Al-Maythalony, 2019;Comesaña-Gándara et al., 2019;Kheirtalab et al., 2020;Li et al., 2020;Raouf et al., 2020;Yang et al., 2020;Luque-Alled et al., 2021a;Sainath et al., 2021;Zhao et al., 2021;Mohsenpour et al., 2022).The trends discussed above can be correlated to polymer chemistry by analyzing the clusters of points on the graph.The use of conventional polymers, such as CA, PC, and PMDA, require higher operating pressures, concentrating below the Robeson limit.The recently developed polymers are between the strip formed by the limits of Robeson and Pineau on the highest permeability values.Between these limits, two groups of polymers can be discerned (Atelge et al., 2021): TRs with higher selectivity, lower permeabilities, and noticeable presence of PI, located at the left end of the band,; and (Occupational Safety and Health Administration OSHA, 2019) the PIMs, which operate at pressures below 1 bar and are located at the far right of the range, with lower selectivity and higher permeability.These results indicate that progress in MMMs that can operate at lower pressure for medium and small biogas systems may be possible with TRs and PIMs, due to their higher free volume and higher selectivity and permeability values.The use of TR and PIMs discards the need for high pressure in which plasticization and aging increase membrane failure rates.The polymers at the right side of the graph, showing higher values of CO 2 permeability are both the PIM-1 and the PTMSP, however, the CH 4 /CO 2 selectivity of PIM-1 is around 7 times higher than PTMSP (16.2 for PIM-1 vs 2.2 for PTMSP).
Frontiers in Chemistry frontiersin.org06 Guerrero Piña et al. 10.3389/fchem.2024.1393696structure is not in thermodynamic equilibrium conditions, so it tends to reorganize over time to reduce the free energy and approach equilibrium states, i.e., aging, which reduces free volumes and, consequently, permeability.Figure 5A illustrates the aging of PIM-1, in which the pores of the structure collapse with time and the free volume is reduced (Bernardo et al., 2017).The reduction of free volume affects the CO2 permeability through the membrane.As with plasticization, increasing the thickness reduces the advance of the aging phenomenon on the material (Alberto et al., 2018).The incorporation of nanofillers in PIM-1 MMMs enhances the stability of the membrane over time, promising a transformative impact in the field of filtration technology.Amongst nanostructured carbon fillers and metal-organic nanostructures displays improved performance and lower cost compared to graphene nanoporous membranes (Baena-Moreno et al., 2019) synthesis of such polymer/ nanocarbons composites can take place via different synthesis approaches in situ synthesis and mixing.Figure 5 illustrates the structural organization for PIM-1 and the interaction between GO.The GO acts as a mechanical enforcement that prevents the collapse or flow of the PIM-1 structure that causes aging and plasticization as illustrated in Figure 5B, The data of some MMMs were included as empty circles in Figure 4.The addition of fillers does not improve the gas separation performance in most cases, as the GO-MMMs do not concentrate at edges of the polymer clusters, do not exhibit any specific pattern, and do not display extreme values of selectivity or permeability.Incorporating fillers in MMM reduces the permeability slightly, by blocking the flow of molecules in the structure.The performance improvement over time of PIM-1 MMMs for different nanofillers can be observed in Figure 6, showing the behaviour of CO 2 permeability drop over time and CO 2 /CH 4 selectivity for eight different funcionalizations of PIM-1: GO (graphene oxide), GO-ODA (graphene oxide-octadecyl amine), rGO-ODA (reduced graphene oxide-octadecyl amine), rGO-OA (reduced graphene oxide-octyl amine), hGO-ODA (holey graphene oxide-octyl amine), GO-APTS and PIM-1 POSS (polyhedral oligomeric silsesquioxanes) Althumayri et al., 2016;Alberto et al., 2018;Luque-Alled et al., 2021a;Luque-Alled et al., 2021b;Mohsenpour et al., 2022).Molecular representations of every filler can be observed in Figure 7.The development of PIM/GO MMMs balances efficiency improvements with cost effectiveness, offering a viable path to widespread adoption of this filtration technologies (Chuah et al., 2021).
Significant improvements in delaying physical aging in PIM-1 membranes by incorporating modified graphene oxide (GO) (Alberto et al., 2018).Octylamine (OA) and octadecylamide (ODA) were used to branch GO, followed by a chemical reduction of graphene oxide.The optimal concentration for ODA-rGO, OA-GO, and OA-rGO was found to be 0.05%wt.According to Figure 6, over 160 days, CO 2 permeability decreased less in modified membranes compared to pure PIM-1: 69% in pure PIM-1, 45% in PIM-1/0.05%GO-ODA, 49% in PIM-1/ 0.05% rGO-ODA, and 39% in PIM-1/0.05%wt rGO-OA.Comparisons indicate better performance of GO over rGO, and OA over ODA in terms of initial permeability and aging delay.Results for combinations of PIM-1 with OA and GO were not reported, leaving a gap in the analysis of these combined variables (Alberto et al., 2018).
voids within its structure that slightly improved the performance previously reported by (47), enhancing both the initial permeability and the result after 155 days.As it can be seen in Figure 6, PIM-1 hGO demonstrated an initial permeability of 6,146 barrer and a final permeability of 3,763 barrer, with an initial and final CO 2 /CH 4 selectivity of 11.6 and 15.6 respectively.These results signify an appreciable advancement over non-holey graphene oxide.However, is notable that even the unmodified PIM-1 membrane in this study showed similar final CO 2 permeabilities than some of the MMMs in the previous study, suggesting that the baseline PIM-1 membrane already exhibited excellent performance before functionalization (Luque-Alled et al., 2021a).
Further research by Luque-Alled et al. (2021b) explored different modifications of GO in PIM-1 membranes.In this study, they compared PIM-1, PIM-1 GO, and PIM-1 GO-APTS functionalization, resulting in improvements in selectivity rather than permeability after 150 days.The results, showed in the permeability and selectivity graph in Figure 6, showed a decline from initial permeability of 6,190 barrer for PIM-1 vs. 5,660 for PIM-1 GO-APTS and 5,072 for PIM-1 GO, and after 3 days the behavior was similar among all three materials, ranging from 3,283, 3,305 and 3,458 for PIM-1, PIM GO-APTS and PIM-1 GO, respectively.However, CO 2 /CH 4 selectivity improvements were significant, with PIM-1 GO-APTS showing a rise from 12.6 to 21.7.These results suggest that APTS's addition to GO not only enhances the distribution within the polymer matrix and contributes to a higher free volume, but also promoting more efficient CO 2 and CH 4 separation, playing a significant role in the transport of these two molecules (Luque-Alled et al., 2021a).
PIM-1 membranes were first functionalized with POSS (polyhedral oligomeric silsesquioxane) by (Rowe et al., 2009), marking an enhancement in gas separation performance more than a reduction in permeability reduction over time.This modification led to an enhancement of CO2 permeability of initial 8,026 barrers vs. 7,195 barrer of PIM-1.Both the initial and final permeabilities showed improvements, with final permeability values recorded at 3,524 barrer for PIM-1 POSS and 3,048 for PIM-1.Additionally, this modification resulted also in increases in selectivity, from initial values of 12.3 vs. 13.3 to finals 14.6 vs. 16.1, reflecting the benefits of POSS's organosilicon structure, being remarkable as a functionalization that enhances both selectivity and permeability at the same time, a non-usual behavior in the rest of the previous functionalizations showed in Figures 6, 7, which follows the usual trade-off relationship between selectivity and permeability (Luque-Alled et al., 2021a).
When Few Layer Graphene (FLG) were added to the PIM-1, PIM-1 FLG in Figure 6, a notable enhancement in CO2 permeability was reported (Althumayri et al., 2016).Initially, permeability was reported to reach up to 12,700 Barrer, 248% higher than the 5,120 Barrer for the unmodified PIM-1.Even after aging, FLGmodified membranes maintained a permeability of 9,240 Barrer, compared to only 3,670 Barrer in the unmodified samples this also represents an enhancement in aging resistance, reporting a reduction of 27.24% for PIM-1 FLG vs. a 28.32% in PIM-1.This substantial increase highlights the effectiveness of FLG in improving the gas separation properties of PIM-1 membranes (Althumayri et al., 2016).Illustration of chemical structures for different fillers in PIM-1 MMM.Based on (Althumayri et al., 2016;Alberto et al., 2018;Luque-Alled et al., 2021a;Luque-Alled et al., 2021b;Mohsenpour et al., 2022).
This review comprehensively examined the historical progression in polymeric membranes and their approach to be incorporated in medium-scale biogas refining using mixedmatrix membranes.Throughout the 1980s and 1990s, the development and application of new polymers like PSF and the CA evolved, and these materials have now captured between 80% and 90% of the current CH 4 /CO 2 separation market.The turn of the millennium saw the emerging of PIMs and TRs, two families of polymers with enhanced free volume and improved performance.Advancements in MMMs in the last decade, particularly with integrating carbonous "graphene like" materials, organosilicon materials, and MOFs, stand out in this journey.These recent innovations in materials present significant advantages over traditional refining methods, including enhanced selectivity and permeability, low-pressure operation, and improved resistance to plasticization and aging.This approach not only boosts operational efficiency but also enables the deployment of biogas technologies on smaller and medium scales, paving new paths for energy sustainability.
The polymers that stand out for their high permeability to CO 2 and that enable lower operating pressures due to their structure and spatial organization, are PIM-1 and PTMSP.However, PIM-1 exhibits a more efficient CO 2 /CH 4 selectivity that exceeds the PTMSP average by more than seven times (ratios of 16.2 for PIM-1 versus 2.2 PTMSP).Selectivity tests for the PIM-1 were carried out at pressures below 1 bar, with successful operational measurements even achieved at 0.2 bar.This pressure range favors the possibility of implementation in smaller-scale biogas systems compared to other membrane materials, indicating the most significant potential across different materials for anaerobic digestion at medium and minor scales.
The most recent studies show that incorporating nanofillers into polymeric membranes aims to improve their long-term performance, despite not doing so immediately, and may even slightly reduce it due to the use of bulky materials, creating physical obstruction of mass transport within the polymeric matrix.However, MMMs show increased efficiency throughout their useful life, strengthening the membrane resistance to plasticization and aging due to improved mechanical stability.The diversity of the results obtained outcomes highlight the complexity of membrane enhancements, reporting aging mitigation at the expense of reduced initial and final permeabilities, indicating a trade-off between longevity and performance.Conversely, some investigations showcase improved CO2/CH4 selectivity or initial and final permeabilities without a corresponding aging resistance, suggesting variability in nanofiller effects.Notably, few-layer graphene has shown outstanding increases in permeability, up to 300%.However, these advancements are not uniformly observed across all studies, with some polymers exhibiting superior performance without nanofillers.
Future research should focus on optimizing nanofiller dispersion and membrane fabrication processes to harness potential enhancements in aging resistance, permeability, and selectivity.The integration of graphene-like materials with other structurally stable nanofillers could further improve material stability and performance over time.Developing and refining membrane fabrication protocols is critical, as studies show substantial discrepancies in performance, even when comparing between PIM-1 reference material across the studies, with selectivity variations over 10 points and initial permeability differences exceeding 2,500 Barrer, causing differences over 30% between reference materials.Improved protocols for nanofiller dispersion could enhance initial material properties, leading to better overall membrane performance, combining aging resistance with improved initial and final permeability and selectivity.
To further enhance the development of mixed-matrix membranes, exploring materials or structural functionalizations that combine graphene-like materials with other nanofillers with high free volume, structural stability and high number of reaction points can be beneficial.Such combinations could create membranes with increased free volume and structural stability, potentially leading to significant improvements in both selectivity and permeability over time.While recent studies have shown enhancements in these properties individually, standardized protocols and optimized synthesis processes could improve results in baseline material and then integrate these advances more effectively, leading to reach superior membrane performance.
of Costa Rica Institute of Technology-TEC-VIE, for enabling the funding of this research.Our gratitude also goes to Lic.Daniela Zúñiga and the students of Materials Engineering, Mariana Rojas and Gipsy Durán, for their collaboration in the preliminary work.Special thanks to the master's program in Science and Technology for Sustainability and its faculty, particularly to Dagoberto Arias, PhD.Lorena Varela and PhD.Tomás Guzmán, for their invaluable advice.

TABLE 1
Common components of biogas obtained by anaerobic digestion of biomass.Based on