Recent progress of porous geopolymers: nanoporosity regulation toward fundamental applications

ABSTRACT Porous geopolymers have been the promising subject over the last decade owing to the functional domains from high surface area, structural stability upon heat and chemists, as well as eco-friendly and economy. Especially, the nano-sized geopolymers are playing a significant role in extensive applications. Here, we summarize recent progress in the development of nanoporous geopolymers in the applications of adsorption as well as catalyst supporter. Generally, microporous geopolymers were synthesized through gas bubbling or combination of zeolite to form partial porous structure, while mesoporous geopolymers were produced by the using of various structure directing agent. However, it remains a challenge to obtain functional materials with narrow porous distribution. Our recent work show that the uses of well-defined template method give rise to finely designed nanoporous geopolymers, which exhibited extremely excellent adsorptivity, solid acidic capacity as well as effective carbon-replication. We discussed well the relationship between nanoporosity regulation and corresponding performances to conclude that just using the bubbling, the compositing of zeolite, or traditional rough template method is difficult to form homogeneous pore structures. By analyzing the connection between the uniform porous structure and its related properties, the well-defined template-method would be recommended to generate emerging nanoporous structures with superior functions.


Introduction
A large amount of research have been devoted on geopolymer since Davidovits initially reported on its semi-crystalline 3D aluminosilicate structure in the 1970s [1,2] With the increasing demand for environmental sustainability as well as green energy, geopolymer has been viewed as a potential material to replace cement.Among the various sources, fly ash, and metakaolin have become the most practical ones used to prepare geopolymers.Fly ash is a well-known waste derived from coal-fired electricity generation, providing outstanding commercial utilization [3,4].While the metakaolin as a recently rising source is considered as a highly efficient one owing to the properties in mechanism and transport field, as well as high compressive strength by excellent filling effect [5][6][7][8][9][10].However, these dense counterparts possess high surface energy, resulting in low permeability, dispersivity, and processability [11][12][13][14].
Given these trends, porous geopolymer (PG) nowadays has become one of the promising inorganic materials because of the unique combination of structural stability, mechanical properties, as well as eco-friendly and economic process benefiting from the locally available abundant sources [15][16][17].The research trend of PGs among the geopolymer research can be seen from the increase in the number of publications on this topic in the last decade.Figure 1 shows the number of peerreviewed journal papers (journals based on science citation index, conference proceedings, dissertation thesis are excluded) obtained searching Web of Science for publications.Along the increasing of geopolymer-related research, that of porous geopo-lymer also increases accordingly (see Figure 1a) and shows a remarkable increase in the last few years (see Figure 1b).
However, there was almost no breakthrough in extending application fields of PGs since the fabrication of PGs are based on air forming, giving rise to macroporous structure, mainly limited to adsorption, filtration, etc [18][19][20][21][22][23][24].generate fundamental macroporous geopolymer with enhanced durability index and high compression strength, recommended a potential material for industrial application [26].
As well known, nanoporous materials possess a high specific surface area compared to macroones, which is favorable in providing efficient surface access and diffusion for effective mass transfer [27][28][29].In contrast to macropores, mesopores are more conducive to the transport of reactants or solvents, while micropore is beneficial to small molecules.Recently, significant efforts have been devoted to the nanoporosity regulation of geopolymers for a much better replacement and applications [30][31][32][33][34][35].Though, most of them failed to obtain nanoporous geopolymer with intense pore size distribution [36][37][38][39][40].
In this review, recent progress of nanoporous geopolymers are reviewed from the review of micro-and mesopore regulation, followed by corresponding applications.Micropores were introduced through gas bubbling or by combining zeolite to increase adsorptivities [41][42][43][44][45][46][47][48].Mesopores were derived using various structure directing agents to produce high porosity for catalytic performance [33,[49][50][51][52].We fabricate ordered nanoporous geopolymers through welldefined template method [53][54][55].The finely ordered pore structures exhibited effective carbonreplication.Furthermore, we summarized how the geopolymer engender nanoporosity associated with diverse applications, hoping to provide a novel sustainable strategy for multifunctionalized highly porous materials.Against this backdrop, an aspiring idea was emerged that PGs could be fabricated into functionalized materials, which could be a charming candidate for the current porous materials including silicas, zeolites, MOFs, et al.

Geopolymers and their porous structures
Geopolymers are a class of aluminosilicate-based amorphous inorganic materials consisting of a polymeric Si-O-Al framework.The strong dipole moment between Si, Al, and O induces strong interactions with water, which accelerate the gel formation.The gels could be condensed with air, zeolites, or small organic molecules to form porous precursor, resulting micro-or mesoporous structures.In the other case, the polar chemical bond from the Si-O-Al framework can be partially dissolved in strong alkaline solution.The leached small fragments provide an opportunity to connect with pre-organized micelles, which gave rise to ordered pores upon removal of the micelle templates.

Microporosity regulation
Microporous materials with their superb textural properties and structural diversity have the potential to achieve new applications in many fields, including low-energy separation processes, adsorptions, catalysts, and so on, yet these materials are facing practical challenges in scale-up synthesis [56].Especially, microporous geopolymer is an important consideration in thermal conductivity.According to the Knudsen effect, thermal conductivity is strongly related to microporous architecture [57].Normally, geopolymers possess relatively low microporosity compared to meso-and macroporosity for the stacking of the micro-sized cages during the gel formation.Recently, micropores were developed based on the frustration of cage stacking through generating gas bubbles in situ   reactions trapped in the slurry [41].The bubbles were also introduced by rapid stirring of the mixture of surfactants and geopolymer gels [42].However, the pore formation from above methods usually accompanied macroporous structure, which is difficult to control pore size distribution, resulting in irregular porosity.
Another effective method for microporous geopolymer is developed by the combination of zeolite into geopolymer gel.Recently, Han and coworkers have realized the preparation of zeolite X/foam geopolymer composites using fly ash (FGX is short of zeolite X accreted with coal Fly ash-based Geopolymer), and zeolite was insitu conversed by foam geopolymer gel through maintaining alkali concentration inside the samples to enhance the zeolite content [45].FGX exhibited high porosity with the BET surface area (SA) of 579.69 m 2 /g, which is comparable with the commercial zeolite (Commercial Zeolite X is abbreviated as CZX) (SA is about 339.59 m 2 /g).As shown in (Figure 2(a,)), CZX and FGX showed type I (P/P 0 <0.1) and type IV isotherms with a type H4 hysteresis loop (P/P 0 >0.1), indicating the existence of narrow fissure holes including some cracks that inevitably formed in CZX, along with many voids on the pore walls of FGX, resulting hierarchical pore connectivity which was beneficial in gas adsorption applications [46].The pore size distribution diagram (Figure 2(c,)) showed that FGX had a higher micropore volume than CZX, owing to the porous geopolymer supporter.The advances was exhibited from the CO 2 adsorption.It exhibited a maximum CO 2 equilibrium adsorption capacity of 7.91 mmol/g, much better than 5.61 mmol/g of CZX.The higher adsorption capacity under different dynamic adsorption was possibly due to the larger contact area between CO 2 and FGX due to more adsorbent in the packed-bed column, which significantly prolonged the breakthrough time.In addition, it showed good regeneration and cyclic properties.
Shao and coworkers developed composited geopolymer to obtain a novel nano-filtration material (NFM) ZM@SCS [47].The zeolite/geopolymer composite was synthesized from curing and hydrothermal treatment of amorphous geopolymer.ZM contains 25 vol.% micropores (see Figure 3(a)).The strong crystallization of zeolite within composited system induced larger cavities or channels, which is beneficial for NFM as organic pollutants filter.It was used for water treatment toward various organic pollution.It exhibited ultra-fast permeability with a high water flux of 340-440 L/(m 2 •h•MPa), rejection rate above 95% and good reusability, owing to the structural features (see Figure 3(b)).It was found that alkaline activator during the process is the key to control the channel formation for the system.The low-cost and high-performance zeolite/geopolymer composite (see Figure 3(c)) is hoped to be developed in other filtration area.
Feng and coworkers have developed cocrystallization to obtain geopolymer-type zeolitelike microporous catalytic material GC-WIFA (GC-WIFA is an abbreviation of Geopolymer Catalyst from Waste Incineration Fly Ash) as shown in Figure 4 [48].The co-crystal could be easily loaded with nickel to form Ni/NiO-GC-WIFA catalyst, which exhibited the isotherm adsorptiondesorption curves closer to the type I, denoting that the typical microporous structure was remained.It performed efficient hydrogenation of levulinic acid, with 100% conversion and 94% yield (see Figure 4(a-)).In addition, the catalyst revealed high stability (see Figure 4(e)).Upon five recycling, it remained above 98% activity, which is expected to be used in various high-value fields.

Mesoporosity regulation
Owing to the high specific surface area, mesopores are more conducive to the transport of reactants or solvents.Despite the intrinsic mesoporosity of geopolymers, the densely packed particles limited the permeation as well as applications [50].Recently, Seo and coworkers have purposed the pore directing method with paraffin oil into geopolymer gel.After extracting the oil, it formed mesoporous geopolymer with large pore size, exhibiting good adsorption and removal efficiency for large adsorbates including proteins, toxins, and cells [58].Furthermore, the pore directing method has been optimized by the modification of agent structures.In 2018, Barbosa et al. fabricated uniform mesoporous geopolymer using soybean oil as structure-directing agent [49].The curve of adsorption equilibrium for methyl violet 10B fitted the Freundlich and Sips models, exhibiting fast kinetics, with the adsorption capacity of 276.9 mg/g at 328 K.
In 2020, Bouna et al. substituted the oils with organic surfactant of cetyltrimethylammonium bromide (CTAB) to synthesize mesopores [33].Such surfactant could be easily aggregated into micelles at geopolymer surface, which is beneficial to form mesopore structure (see Figure 5(a)).The reaction condition was optimized at 70°C retaining the molar formulation ratios of H 2 O/Na 2 O = 15, Si/Al = 2 and Na/Al = 1.5, giving rise to specific surface area up to 128 m 2 /g and pore volume about 0.14 cm 3 /g.These materials have performed in the retention of cationic dyes.For example, the retention for methylene blue was up to 183 mg/g (see Figure 5(b)).
Dong and coworkers purposed activation of geopolymer using alkalic defecting method.The calcination together with Na 2 CO 3 resulted mesoporous surface arounded by Na + and Ca 2+ , which give rise to the opportunity to be ion-exchanged as shown in Figure 6.It was applied for water purification exhibiting efficient adsorption rate of Cd (II) up to 97.84% [51].It was noted that the most suitable secondary kinetic model (R 2 = 0.9936) illustrates that the Cd (II) adsorption process was led by chemisorption, accompanied by physical adsorption, safeguarding public health.
In addition, efforts have been devoted to developing multi-functioned fragments.Upadhyay and coworkers have prepared geopolymer gels containing copper (II) ions, which further condensed into CuO/geopolymer framework [52].It showed comparable or better activity than the copper oxide heterogeneous catalysts since geopolymer supporter reduced the activation barriers based on the charge transfer effect.Although the BET surface area decreased from 413 m 2 /g to 180 m 2 /g as shown in Figure 7(a,), it exhibited efficient catalytic activity of 67% conversion for the oxidation of aldehydes as shown in Figure 7(c).
The above extensive research on the mesopore regulation were mainly focused on modifying directing agent structure.However, the modification of small directing agent could difficultly control the huge geopolymer system.

Ordered Mesoporous Geopolymer
To solve the limitation of directing agent, it needs to set the penetration point at the fundamental framework of geopolymer.We proposed an innovative idea that if kaolinite is activated by strong alkali to generate homogeneous small fragments, which could provide transient state enabling the efficient functionalization with directing agents.It might result in regularly arranged pore structures after condensation closely around micelles.In addition, the fabricated porous geopolymer should be more competitive than commercial porous materials owing to the low cost of kaolinite clay.
Recently, we carried out a novel activating method based on reversible cycles between covalent bonding of polarized Si-O-Al upon alkali and condensation [53][54][55].Upon dehydroxylation of kaolinite, meta-kaolin was prepared.It was further degraded into regular small fragments after activating with KOH solution as described in Figure 8 (a).While, we also modified aggregation of CTAB to form positively charged micelles with the diameter of ~3 nm, which could easily absorb negatively charged fragments to be condensed around micelles [54].Removal of directing agent (CTAB micelles) gave rise to the hexagonally arranged uniform mesopores (EPG-2, EPG is short of Ewha Porous Geopolymer), in which the regularity of pores could be controlled from the CTAB concentration see Figure 8(b).
The ordered meso-channels could replicate porous carbon rods as shown in Figure 8(c,), which exhibited high CO 2 adsorption capacity owing to the high specific surface area.EPG-2 was also used as stable drug carrier after wrapping with Tween 60 by the strong charge interaction (see Figure 9(a)) [55].The meso-channels could effectively capture ammonia since the pore frameworks exhibited controllable acidic sites (see Figure 9(b)).Therefore, niclosamide (NIC), an anthelmintic drug could be stably loaded inside the pores through interattraction between acid and base, increasing solubility of NIC.The composite was used for the in-vivo oral administration and exhibited both improved T max and t 1/2 , suggesting better formulation than MCM-41 and SBA-15.Based on the current topic that nano-clays could be a potential pseudo-antibodies for COVID-19, ordered mesoporous geopolymers are expected to be important candidates in targeting therapy [59].
In order to enhance pore stability by thermal, as well as expand the application of geopolymer, the directing agent of CTAB was substituted by polymeric agent.Polymeric agent composed of hydrophobic block and hydrophilic section could self-assemble into micelles through phase separation in aqueous solution [60].Compared to the cationic micelles from CTAB, P123 as amphiphilic polymeric agent is easily aggregated into uniform micelles surrounded by numerous hydrophilic groups, which could sufficiently attract the geopolymer fragments for further condensation (see Figure 10(a)).
The condensation came out not only on the surface of the micelles but also thick polyethylene glycol layer, giving rise to stable framework of EPG-1.Hexagonal type uniform mesopores of ~7.20 nm were extricated upon removal of P123 [53].The well-ordered large meso-channels could effectively replicate porous carbon wires of EPC-1 as shown in Figure 10(b).The well-ordered large meso-channels (red lines) could effectively replicate porous carbon wires (purple arrows) of EPC-1 as shown in Figure 11(a,).In addition, there were small intrawall pores (yellow arrows) resulted from phase coincidence between the micelles and fragments, contributing to high porosity.
It showed inverse structure and size compared to the template channels, suggesting thermal stability of the pore geopolymer structures (see Figure 11(c)).The specific surface area was up to 711 m 2 /g and mesopore volume was 0.56 cm 3 /g from BJH analysis of EPG-1, and replicated EPC-1 with high porosity.EPC-1 performed high CO 2 adsorption capacity up to 26.30 mmol/g as shown in Figure 11d, which turns out to be better than any commercial carbon materials as well as those from porous silica.The excellent performance of EPC-1 could recommend the advantages of geopolymer, indicating it as a prospective material in future.
Based on these advantages, the above method to generate homogeneous small fragments can be recommended to produce highly ordered mesoporous geopolymers in combination with directing agents.They exhibited outstanding performance as solid acid, adsorbents, and templates, indicating contest ability owing to the low cost.

Perspective and future
In this review, we focused on the most recent development of nanoporosity regulation of geopolymer including micro-and mesopores, and summarized corresponding performance in diverse applications.The progress are impressive, but also remains challenges.The most emphasizing one is that fine-tuning of detailed interaction between geopolymer fragments and directing agents critically contributed to the pore formation.However, there was an urgent need of application extending since the utilization of ordered PGs are limited in fields of adsorption, separation, supporter, and template.This mini review was looking forward to further promoting nanoporosity regulation.Meanwhile, these innovative routes accelerate the development of porous geopolymers in future applicable in various fields such as enantioselectivity and battery membranes [61,62].

Figure 1 .
Figure 1.Trends in the number of publications on geopolymer and porous geopolymer over the past 10 years (a), and number of publications of porous geopolymer in the last few years (b).

Figure 2 .
Figure 2. The Ar isotherms of the CZX (a) and FGX (b) samples and the pore size distributions of the CZX (c) and FGX (d) samples.Reproduced from ref [45].With permission.Copyright 2022 elsevier.

Figure 3 .
Figure 3. (a) pore diameter distribution of the ZM sample.(b) Cyclic filtration performance of MB solution through ZM@SCS.(c) Proposed mechanism for the filtration removal of organic pollutants by the ANA-ZM@SCS.Reproduced from ref [47].With permission.Copyright 2020 Elsevier.

Figure 5 .
Figure 5. (a) Schematic preparation of geopolymers samples.(b) Mechanism of adsorption of MB on porous and surface of geopolymer matrix.Reproduced from ref [33].With permission.Copyright 2020 Elsevier.

Figure 7 .
Figure 7. (a) N 2 adsorption-desorption isotherms, (b) BJH pore size distribution of the geopolymer and CuO/Geo.(c) A schematic of the proposed reaction mechanism for the oxidation of benzaldehyde to benzoic acid over CuO/Geo.Reproduced from ref [52].With permission.Copyright 2023 Elsevier.

Figure 8 .
Figure 8.(a) Schematic routes to obtain porous geopolymer from kaolinite and porous carbon replication from porous geopolymer.TEM images of porous geopolymers (b, c) and porous carbon (d).Reproduced from ref [54].With permission.Copyright 2020 the Royal Society of Chemistry.

Figure 9 .
Figure 9. (a) Schematic diagrams showing the synthesis of NIC nanohybrid coated with tween 60 via a simple physical coating method.(b) The types of intra-pore interaction models depending on structural properties of NIC-PG, NIC-MCM-41, and NIC-SBA -15 nanohybrid.Reproduced from ref [55].With permission.Copyright 2021 Elsevier.