Polymeric and Hybrid Membranes for Achieving Ultra-Low Sulfur Fuel Requirements

The gasoline quality is one of the key environmental issues for all oil refineries worldwide, and sulfur content is a very important parameter in tight fuel specifications [1]. High sulfur content of fossil fuels, in particular gasoline and diesel fuel, pollutes the environment and causes serious human health problems when burned [2, 3]. Sulfur dioxide (SOx) is a primary emitter, boosting nitrogen oxide (NOx) levels in vehicle exhausts, forming acid rain, and fueling global warming. It may also have adverse effects on ecosystems and the built environment, contribute to respiratory illnesses, and induce heart diseases and asthma [3, 4]. Moreover, sulfur compounds present in feedstocks corrode the equipment, deactivate catalysts, and diminish the quality of products in petrochemical process plants [5]. Hence, the regulatory limitations on sulfur content are tightening; most developed countries and China have intended to lower it to less than 10 ppm [6, 7]. For these environmental problems and the needs of industrial applications, various desulfurization methods have been developed, such as hydrogenation desulfurization (HDS), extractive desulfurization, oxidative desulfurization, biological desulfurization, adsorption desulfurization, and membrane technology [8]. Although HDS is the most common and effective industrial method for reducing sulfur content to less than 1 ppm, it requires significant equipment investments, high hydrogen consumption, and harsh operating conditions of high temperatures and pressures. It can lower the octane number of gasoline by saturating olefins and aromatics [8, 9]. In contrast, membrane pervaporation desulfurization (PV) has emerged as a promising alternative because the process comes with several advantages, such as low operation and investment costs, high separation efficiency, easy operation, and easy scalability to adapt to changes in the flow of the procedure [10]. PV purifies specific components from a liquid stream via partial evaporation with a non-porous selective membrane. Commercial processes like the S-Brane technology of W.R. Grace & Co. demonstrated its industrial applications at substantially lower temperatures and pressures, giving a much lower requirement for hydrogen in combination with HDS processes. Polymeric Membranes: The majority of membrane applications employ polymeric materials because of their low material cost, permeability, mechanical stability, and ease of processing and fabrication over a range of applications [11].

Polymers such as polydimethylsiloxane (PDMS) [12, 13], PEG [14], and polyurethane (PU) [15] have been widely used in desulfurization. However, many challenges associated with polymeric membranes prevent their performance and long-term stability. One of the most pronounced issues is that high loading of permeable components can cause membrane swelling, making it less selective, and it might lead to chemical degradation, especially when temperature and pressure become higher [16-18]. In addition, polymeric membranes typically experience a “trade-off” phenomenon between permeability and selectivity, where increased selectivity reduces flux and the other way around, limiting overall performance [19-21]. Furthermore, various polymeric membranes show unsatisfactory internal separation capacity, resulting in relatively low enrichment factors for sulfur compounds [22]. In addition, their relatively unreactive nature makes them quite rigid to undergo chemical modifications, and sharpen enrichment factors at higher temperatures. In addition, their relatively unreactive nature makes them quite rigid to undergo chemical modifications and sharpen enrichment factors at higher temperatures. Although hydrogenation (HDS) is the industrial backbone of desulfurization, its demanding operating requirements (high temperatures and pressures), high costs, and negative impact on octane number make it unsuitable for achieving ultra-low sulfur requirements (< 10 ppm) economically. In contrast, membrane pervaporation (PV) technology has emerged as a promising alternative that reduces energy and process requirements. However, conventional polymer membranes continue to struggle with the challenge of polymer swelling and the inherent trade-off between permeability and selectivity. The knowledge gap and the urgent need for this review lie in providing a comprehensive critical evaluation of the latest next-generation membranes, particularly MMMs enhanced with advanced materials (such as MOFs) [23]. These have demonstrated revolutionary results in overcoming the permeability/selectivity trade-off and enhancing chemical stability [21]. Evaluating the current performance of these materials and the separation mechanisms they use (such as facilitated transport via π-complexes) places them in the context of future work aimed at achieving sustainable industrial viability.

The object of this study is to obtain an integrated and comparative investigation of polymer membranes applied in membrane evaporation desulfurization. The project will develop a comparative critical review of polymer materials, the principle of operation of polymer separation, and current performance characteristics, including positive/negative aspects in each case. Furthermore, in this study, an attempt has been made to provide both neglected and emergent areas on membrane evolution as well as recommendations for the design of next-generation membranes with enhanced performance. This includes solving current challenges and placing them into the context of future work toward improving the desulfurization process for fuels.

3.1 Solution-diffusion mechanism

The separation of sulfur-containing compounds from fuels using membrane technologies is governed by intrinsic transport characteristics that control the migration of species within the membrane structure. Polymeric membranes are widely applied in this context because they offer cost-effectiveness, operational simplicity, mechanical robustness, and suitability for large-scale industrial applications [9, 25-27]. The degree of efficiency of polymer membranes as a function of the nature and chemical structure of the polymer material, temperature, and pressure during work under the membrane. The concentration of sulfur-containing components in the feed significantly affects their transport through dense membranes, a process that is predominantly governed by the solution–diffusion mechanism. This model is widely known as the transport model of non-porous membranes, which popularized pervaporation separation membranes [28]. This mechanism proceeds through three sequential stages: initial adsorption of sulfur molecules at the membrane surface, followed by diffusion of the dissolved species within the polymer matrix, and finally desorption on the permeate side of the membrane. In this circumstance, increasing the adsorption capacity could be attributed to the higher adsorption force of fuel components on the polymer material. Alternatively, membranes based on PEG polymers have a relatively high adsorption capacity for some sulfur compounds, such as thiophene, due to their polarity [29-31]. The rate of intrinsic diffusion is affected by several factors, most notably the flexibility of the polymer chains and the size of the available free spaces, in addition to the molecular size of the transported compound [32, 33]. Various studies have shown that PDMS membranes exhibit a transition behavior dominated by the diffusion phase, rather than dissolution. It is worth noting that the reverse adsorption phase is often rate-non-limiting when appropriate vacuum conditions are applied on the permeation side [34, 35]. Despite its simplicity, the solution–diffusion model is constrained by the well-known permeability–selectivity trade-off, where increased permeation rates are often accompanied by reduced separation efficiency for sulfur compounds [36]. This behavior is clearly observed in PDMS membranes, where increasing flux leads to decreasing selectivity, and vice versa. Moreover, membrane swelling induced by high concentrations of permeating species can reduce selectivity and accelerate chemical degradation, thereby limiting the long-term industrial applicability of polymeric membranes [37]. On the other hand, the facilitated transport mechanism is a promising option to overcome this challenge, as this mechanism relies on a reversible chemical reaction between sulfur compounds and transporters embedded within the membrane [38]. Transition metal ions have been used as effective carriers in this context, such as silver (Ag⁺), copper (Cu²⁺), nickel (Ni⁺), manganese (Mn⁺), and lead (Pb⁺), as these ions form π-complex bonds with the aromatic rings of thiophene, enhancing its adsorption and transport across the membrane [39]. There are notable discrepancies that exist in the performance of polymeric membranes such as PEG, PDMS, and PI. These variations originated mainly from chemical structure differences among polymers. PEG membrane systems show higher flux due to their flexible and hydrophilic ether linkages, resulting in enhanced solubility and diffusion of polar sulfur compounds. In contrast, PI-based membranes, characterized by rigid aromatic backbones and low chain mobility, achieve higher selectivity but reduced flux. PDMS-membrane-based membranes are highly permeable to swelling, further demonstrating the inherent permeability–selectivity trade-off. Such distinctions underscore how polymer chain rigidity, polarity, and free-volume distribution govern transport behavior under the solution–diffusion mechanism.

3.2 Facilitated transport via PI-complexation

An effective strategy for enhancing membrane performance involves facilitated transport, which relies on specific and reversible interactions between the target sulfur compounds and functional carriers incorporated within the membrane matrix. In fuel desulfurization, the preferred components (such as thiophene compounds) often contain pi-bonds. In this approach, metal ions such as silver (Ag⁺), copper (Cu⁺), lead (II) (Pb²⁺), nickel (II) (Ni⁺), and cerium (IV) (Ce⁴⁺) act as carriers that selectively enhance the transport of sulfur-containing species across the membrane [40]. The essential process involves the creation of π-complexes by these ions in conjunction with aromatic thiophene molecules [39, 41] For instance, various research results demonstrate how metal ions can play such a role. In the case of in situ synthesized metal cation incorporated membranes, the formation of pi complexes between thiophene species and silver ions has been suggested to explain the enhancement in membrane flux and selectivity. As the silver ions have a vacant d orbital as an acceptor and sulfur in thiophene can provide a free pair of electrons, this assumption is justified [42]. Similarly, the Cu²⁺ copper ion may bind reversibly and specifically with thiophene by π-complex formation, which is more selective. Furthermore, copper and nickel ions can also bind to the delocalized electrons in the aromatic ring of thiophene and create a pi-complexation reaction [43]. Experimental results demonstrate that ions such as Cu²⁺, Mn²⁺, and Pb²⁺ can simultaneously improve thiophene selectivity while increasing membrane flux at elevated operating temperatures. This confirms the facilitated transport mechanism [39]. Although the strength of the interaction between the metal ions and thiophene may vary (with Ni (II), Ce (IV), and Cu (II) interacting with variable strength, where Ni (II) > Ce (IV) > Cu (II)). The most affinity ions, such as Cu²⁺, may exhibit the best overall performance due to their high loading capacity and superior free volume properties of the PDMS-dopamine/copper membranes [44]. Even silver nanoparticles (AgNPs) can synergistically facilitate thiophene transport, with Ag⁰ acting as a Lewis acid and thiophene as a Lewis base, leading to an affinity between them, as well as a π-π stacking interaction that accelerates at higher temperatures [40]. These findings demonstrate that careful selection of suitable metal ions can significantly improve membrane performance, enabling separation efficiencies that surpass the conventional permeability–selectivity limitations observed in pervaporation processes.

3.3 Pore flow and size exclusion in hybrid membranes

Understanding the mechanisms of mass transfer in such membranes contributes to their development, which is not only based on the traditional adsorption and diffusion model, but extends to include the contribution of fillers in creating new paths for the penetration of molecules, through either pore-flow mechanisms or size exclusion, or both, in addition to interaction with preferential adsorption.

Pore-flow transport has been widely reported as one of the key contributors to enhanced membrane permeability when inorganic fillers are incorporated into polymer matrices [45]. Experimental studies indicate that these fillers introduce additional interconnected pathways within the membrane structure, which facilitate molecular transport and reduce mass transfer resistance. For instance, the incorporation of MIL-101(Cr) into PDMS membranes has been shown to generate supplementary free-volume regions that act as efficient transport channels, leading to improved permeation performance [46]. Similar trends have been observed in zeolite-based hybrid membranes, where the intrinsic porosity of fillers such as CuY promotes the diffusion of small sulfur-containing molecules through the polymer phase [47]. These observations suggest that membrane performance is strongly influenced not only by filler type but also by dispersion quality and polymer–filler interfacial compatibility [48]. Even at the level of modified polymers, the introduction of PEG groups into polyimide-block-polyethylene glycol membranes can lead to a “significant increase in permeation pathways” [14]. The variation in performance among hybrid membranes incorporating different fillers could be explained by differences in filler dispersion and interfacial compatibility with the polymer matrix. Zeolite-filled membranes generally show enhanced selectivity through rigid molecular sieving, whereas MOF- and COF-based fillers often increase permeability by creating additional nanoporous transport channels. Inconsistent dispersion or formation of interfacial voids may cause conflicting flux–selectivity trends under seemingly similar operating conditions. Therefore, the overall separation efficiency depends not only on filler type but also on the degree of polymer–filler interaction and microstructural homogeneity. Table 1 provides an overview of different membrane systems and challenges mentioned in the literature.

Size exclusion: The size exclusion mechanism is a fundamental principle in particle size-based separation, whereby the membrane or filler pores can allow smaller molecules to pass through while preventing larger ones [49]. In some studies, this mechanism provides an explanation for the membrane’s selectivity; for example, PDMS membranes are observed to exhibit higher selectivity toward thiophene than 2-methyl-thiophene, and this difference is attributed mainly to the variation in molecular sizes of these solutes [50]. This suggests that the variation in molecular sizes directly affects the ability of molecules to penetrate membrane pores or available channels.

However, the role of size exclusion is not always dominant and depends on the relationship between pore size and the sizes of the molecules to separate. In certain MOF membranes, e.g., UiO-66-NH₂ membranes [51], where the pore diameters are greater than the kinetic diameters of thiophene (0.53 nm) and hydrocarbons like n-octane (0.43 nm), the experiments indicate that molecule exclusion should not significantly influence the enrichment factor. Under such circumstances, size variations might not be drastic enough to cause adamant exclusion; therefore, the relevance of other processes, such as preferential adsorption, is realize [51]. Fillers also help to alter the internal structure of the polymer membrane, for instance, by enhancing fractional free volume (FFV) and intersegmental d spacing [44, 52]. These changes in the internal structure mean more space for molecular motions, enhancing permeability. Synthesis techniques like cross-linking also restrict the movement of polymer chains, thus controlling the effective pore size or fractional free volume, which can affect size-based separation [53, 54]. To maximize the effectiveness of the membrane separation process, pore flow, size exclusion, and preferential adsorption must be combined. Preferential adsorption of sulfur compounds (such as thiophene, which is more polar than hydrocarbons) is an essential part of most desulfurization systems, particularly those MOF or transition metal ion type [10, 23]. Molecules of sulfides, adsorbed, passed on effectively through the membrane by channels and routes formed by fillers. For example, the incorporation of copper (Cu²⁺) or silver (AgNPs) ions into membranes through the formation of S-M bonds or π-complexations could enhance thiophene permeation through the membrane. Through chemical interaction in desulfurization processes, these enhance the performance of the porous structure and allow enhanced overall and integrated separation performance [39, 55]. Thus, a comprehensive comparison of different membrane systems, their applications, and associated challenges reported in the literature is provided in Table 1.

Table 1. Membrane-based desulphurization techniques: Materials, applications, and challenges compared

While the membrane material is the prime element that determines performance and separation efficiency, the overall performance of the system is vitally affected by external operating conditions. To improve the efficiency of membrane efficiency in desulfurization, it is essential to understand how these conditions—such as temperature, pressure, and sulfur compound concentration—control the flow and permeability characteristics and their interaction with the membrane structure. These process conditions are critical parameters to determine the efficiency of membrane separation, controlling the characteristics of flow and permeable media as well as interaction at the surface of a membrane. Table 3 shows the performance of polymer based desulfurization membranes according to the previously published scientific literature.

Table 3. Comparative summary of polymer-based membranes used in desulfurization applications

Temperature has a direct impact on membrane performance [37]; Figure 3A and 3B illustrate the effect of temperature on flux and enrichment factor. Several studies have reported that increasing temperature results in higher permeability flux [37, 72]. This is attributed to the enhanced movement of molecules within the membrane and the high vapor pressure of sulfur compounds, which increases the driving force for transport across the membrane [73]. For example, the permeate flux of the 6FDA-BDAF membrane increased from 7.96 to 37.61 kg/(m²·h) when the operating temperature was raised from 50 to 90℃ [5]. However, the effect of temperature on the enrichment factor is not constant, as in some cases an initial increase is observed, followed by a subsequent decrease [53], or a continuous decrease with the rise in temperature [58, 68, 74]. This is due to the trade-off between adsorption selectivity and diffusion selectivity, where increasing temperature reduces the difference in solubility between sulfur compounds and hydrocarbons and thus leads to a decrease in selectivity [55, 75]. The effect of temperature on membrane permeability is commonly described using the Arrhenius relationship [76]. This relationship describes how the permeability of a particular component changes with absolute temperature:

$J=A \exp \left(-\frac{E p}{R T}\right)$     (1)

where, J is the permeability flux, A (or J_o) is the pre-exponential factor, Ep is the activation energy of pervaporation and R is the gas constant.

To improve readability and reduce reliance on large tables, the experimental and reported performance data of polymeric and MMMs are summarized in graphical form. Instead of presenting extensive tabulated values of flux and enrichment factor for each membrane–feed system (see e.g., PDMS/PAN, PDMS/PEI, PEG/PES, PI-based composites, and PU blends). The results were converted into comparative graphical figures to enhance readability and reduce reliance on large data tables. Figure 3 illustrates the combined influence of operating temperature and membrane material on sulfur separation performance. Figure 3A presents a heat map of the average conditional enrichment factor (EF) per 10℃ interval, organized by membrane type, allowing a comparison between pure polymer membranes and composite mixed-matrix membranes (MMMs). This visualization highlights the synergistic effect of material composition and operating temperature on separation efficiency. Figure 3B shows a scatter plot of permeability flux (J, in kg/(m²·h))versus enrichment factor (EF), where polymeric and MMM systems are distinguished. The point size reflects the operating temperature, while the trend lines illustrate the relationship between temperature and performance for key material groups. Figure 4 further depicts the relationship between permeability flux and enrichment factor for a range of polymeric and MMM membranes used in desulfurization, illustrating the well-known permeability–selectivity trade-off. Different membrane types are identified using distinct symbols, providing a comparative overview of their separation performance. These visualizations provide an immediate and clearer alternative to classical tables, allowing trends to be more easily identified and cross-comparisons among various polymers, fillers, and operating conditions to be performed. The data was processed and analyzed using Python (version 3.13.7).

Figure 4. Relationship between permeability flux (J) and enrichment factor (EF) for polymeric and mixed-matrix membranes used in desulfurization, illustrating the permeability–selectivity trade-off among different membrane types

Permeability pressure is one of the factors determining the driving force between the two sides of the membrane [68]. Results indicate that increasing it leads to a decrease in permeation flux as a result of a decrease in the partial pressure difference [53]. Regarding the enrichment factor, some studies have shown that it may initially increase and then decline [77]. The driving force for mass and heat transfer in a membrane evaporation (PV) process is determined by the chemical potential difference between the permeate and the feed. This difference is created by applying a vacuum or sweep gas flow at the permeate side. It is explained by the solution-diffusion equation, which includes the permeate pressure [78]:

$J_i=\left(\frac{P_i}{L}\right)\left(c_{i 0} e^{-\frac{V_i p_0}{R T}}-c_{i p} e^{-\frac{V_i p_p}{R T}}\right)$     (2)

where, J_i is the molar flux of component i (mol·m⁻²·s⁻¹), Pi is the permeability of component i. L is the membrane thickness (m), $c_{i 0}$ and $c_{i p}$ are the concentrations of component i at the feed and permeate interfaces within the membrane (mol·m⁻³). $V_i$ Is the partial molar volume of component i (m³·mol⁻¹). $p_0$, $p_p$ are the feed and permeate pressures (Pa). R is the universal gas constant (J·mol⁻¹·K⁻¹). In addition, T is the absolute temperature (K).

5.3 Type and concentration of sulfur compounds

The efficiency of sulfur compound separation has been shown to depend strongly on the molecular characteristics of the permeating species and their affinity toward the membrane material [47, 60]. Several studies report that compounds with lower molecular weight generally exhibit higher permeation flux and enrichment factors compared to bulkier sulfur species, following an inverse relationship with molecular size [79]. Increasing sulfur concentration in the feed phase typically enhances permeation flux due to the higher driving force; however, this improvement is often accompanied by a decline in the enrichment factor [53]. This behavior has been attributed to membrane swelling, which reduces selectivity by enlarging transport pathways [42]. Notably, once a critical concentration is reached, further increases in sulfur content tend to have a limited impact, as the membrane approaches a swelling equilibrium state [80, 81]. The sulfur enrichment factor is used as an indicator of the separation selectivity of sulfur compounds. It is defined as the ratio of the sulfur content in permeate to the sulfur content in the feed [82]:

$\alpha=\frac{C^{\prime} A}{C A}$     (3)

where, $\alpha$ is the sulfur enrichment factor, C′A is the sulfur content in the permeate, CA is the sulfur content in the feed.

The solubility parameter defined as the square root of the cohesive energy per molar volume [83]:

$\delta=\left(\frac{E_{c o h}}{V}\right)^{\frac{1}{2}}$     (4)

where, δ is the solubility coefficient (J/cm³)^1/2, E_coh is the cohesion energy (J/mol) and V is the molar volume (cm³/mol).

5.4 Membrane thickness

Membrane thickness is a crucial structural parameter influencing permeation behavior, as numerous studies have demonstrated an inverse relationship between active-layer thickness and permeability flux [84, 85]. Thinner membranes generally allow faster mass transport due to reduced diffusion resistance, which explains why normalized flux values are often used when comparing membranes of different thicknesses [85, 86]. In addition, hydrodynamic conditions play a secondary role; increasing the feed flow rate or Reynolds number has been reported to lower boundary-layer resistance, resulting in modest improvements in both flux and enrichment factor [80, 82]. These effects are commonly interpreted using the solution–diffusion framework, which incorporates membrane thickness as a key variable governing mass transfer [71]:

$J_i=P_i \frac{c_i F}{\delta}$     (5)

$P i=\frac{J_i * l}{(P i o-P i l)}$     (6)

where, J_i is the fractional flux of the component, Pi is the permeability of the component i, c_iF is the concentration of the component i in the bulk fluid, δ is the film thickness, Pi is the permeability of component i. J_i is the total flux of component i and l is the membrane thickness (m).

Polymer and hybrid membranes are among the most promising alternatives for sustainable desulfurization. A critical evaluation of 40 pilot studies shows that the variability in performance of these membranes stems primarily from differences in polymer polarity, chain stiffness, and the quality of the interfacial bond between the filler and the matrix. Polyethylene glycol (PEG) membranes show higher flow rates due to hydrophilic ether bonds, while polyimide (PI) systems achieve superior selectivity thanks to their solid aromatic structure and low free volume. Although conventional membranes such as PDMS and PEG remain cost-effective, they suffer from significant swelling and a trade-off between permeability and selectivity. The incorporation of inorganic fillers such as metal-organic frameworks (MOFs) has proven capable of overcoming this trade-off and simultaneously improving both selectivity and flux. However, challenges such as clumping, poor reproducibility, and inter-compatibility issues remain major obstacles to the industrial scaling up of MMMs. To move from laboratory performance to industrial scale, future work must urgently focus on three areas that stem directly from these constraints:

(a) Conducting long-term stability tests using real fuel streams (e.g., liquid catalytic cracking fuel) rather than model fuels;

(b) Using advanced molecular simulations to predict sulfur transport pathways and optimize filler distribution; and

(c) Developing asymmetric thin-layer composite membranes and “smart” fillers to enhance structural robustness. Considering these challenges and opportunities, polyimide/MOF-based MMMs emerge as a strategic candidate for industrial applications.