Two-Dimensional Zeolite Materials: Structural and Acidity Properties.

Zeolites are generally defined as three-dimensional (3D) crystalline microporous aluminosilicates in which silicon (Si4+) and aluminum (Al3+) are coordinated tetrahedrally with oxygen to form large negative lattices and consequent Brønsted acidity. Two-dimensional (2D) zeolite nanosheets with single-unit-cell or near single-unit-cell thickness (~2–3 nm) represent an emerging type of zeolite material. The extremely thin slices of crystals in 2D zeolites produce high external surface areas (up to 50% of total surface area compared to ~2% in micron-sized 3D zeolite) and expose most of their active sites on external surfaces, enabling beneficial effects for the adsorption and reaction performance for processing bulky molecules. This review summarizes the structural properties of 2D layered precursors and 2D zeolite derivatives, as well as the acidity properties of 2D zeolite derivative structures, especially in connection to their 3D conventional zeolite analogues’ structural and compositional properties. The timeline of the synthesis and recognition of 2D zeolites, as well as the structure and composition properties of each 2D zeolite, are discussed initially. The qualitative and quantitative measurements on the acid site type, strength, and accessibility of 2D zeolites are then presented. Future research and development directions to advance understanding of 2D zeolite materials are also discussed.


Introduction
Zeolites are generally defined as three-dimensional (3D), crystalline, microporous aluminosilicates that have demonstrated enormous framework variety and pore connectivity [1]. The presence of aluminum (Al 3+ ) in zeolites imposes net negative charges on the framework that are often counterbalanced by organic/inorganic cations or protons. Therefore, zeolites are endowed with ionic exchange capabilities as well as Brønsted acidity. Over time, these materials have been selected and manipulated to suit specific applications as both catalysts and molecular sieves [2,3]. In addition to the preparation and applications of 3D zeolite materials, the development of quantitative structure/activity relationships for chemistry in zeolites is being pursued. Various experimental methods have been explored to investigate zeolite acidity, including the quantification of acid site concentration, strength, and affinity. The structure/activity model built from acidity understanding was developed to describe the performances of zeolites in their corresponding applications. The reviews published by R. J. Gorte [4] and Y. Román-Leshkov [5] well-documented the results and progress on acidity investigation in 3D zeolite materials.
In comparison to 3D zeolites, two-dimensional (2D) zeolites are an emerging type of nanoporous materials [6][7][8][9][10][11][12]. They are often made from the 2D layered precursors that contain stacked sheets of one-to-two unit cell or smaller thicknesses that are linked by weak van der Waals forces or hydrogen  Nu-6(1) (New (ICI, Imperial Chemical Industries) with sequence number six (one)), a precursor of zeolite type NSI (Nu-6(2) (six)), was the first officially reported 2D zeolite material, prepared in 1983 for synthesizing its 3D calcined counterpart, Nu-6(2) [56]. In the early stages of zeolite studies, a complete understanding of the crystallization mechanism was lacking, and, thus, proper control of the crystallization process and resultant zeolite morphology was infeasible. The emergence of 2D zeolite materials did not arouse intensive research interests until MCM (Mobil Composition of Matter)-22 (an MWW (MCM-22 (twenty-two)) topology) was reported in the 1990s [57]. Following this discovery, Edinburgh University (EU)-19 (NSI-CAS (cesium aluminosilicate) intergrowth) [58], RUB-15 [59], RUB-39 [60] and PREFER (precursor of ferrierite) [61] precursors for producing EU-20b, siliceous SOD (sodalite) zeolite, RUB-24, RUB-41, and ferrierite (FER), respectively, were discovered in similar synthesis processes. The implication of the layered MCM-22 zeolite material was to recognize that 2D zeolite precursors such as MCM-22(P) and others mentioned above could be modified into various hierarchical zeolite structures to include properties that differentiated them from conventional 3D zeolite materials with rigid structures. The first successful investigation into the design of a unique
The framework of the β-HLS layer has a cup-shaped cage topology with 4-and 6-MR pores. Such a cage is comparable to a sodalite cage split into two fractions with trimethylammonium (TMA) + cations incorporated into them as templates. Na + ions and H 2 O molecules are located between two β-HLS layers, with two interlayer distances (4.0 Å and 4.6 Å) varying alternately. The layer thickness is 7.2 Å, and the inter-layer distance of terminal silanol or siloxy groups (d(OH· · · OH)) is~3.2 Å. HUS-1 and RUB-55 have similar structural properties to those of β-HLS, except that the inter-layer distance of HUS-1 (~1.5 Å-2.6 Å) is shorter and RUB-55 (~7.7 Å) is longer, respectively, than that of the 2D β-HLS precursor. In addition, the symmetry of these two types of 2D AST layers is slightly distorted compared to β-HLS silicate [82]. HUS-5 is the precursor of HUS-1 that undergoes fewer washings (one-to-three times) than HUS-1 (thorough washing) during preparation. The crystal structure of HUS-5 is the same as that of HUS-1, but the interlayer distance is~4.0 Å, and TMA + , Na + , and hydrated H 2 O are present in the interlayer. For 2D layered silicate with an SOD topology, RUB-15 is formed by cutting out a section of the SOD framework perpendicular to the [011] direction ( Figure 3). RUB-51 has the same 2D silicate layer as RUB-15 but with a different stacking sequence and template intercalation. The stacking sequence of the 2D layers in RUB-51 is AA rather than ABAB as in RUB-15 [83]. The obvious differences between the RUB-15 and DLM-2 layers lie in the space group and the resulting arrangement of the water molecules between the 2D SOD layers [84]. In comparison with those of AST layers, the SOD layers have a low Q 3 :Q 4 (Q 3 : three-connected [SiO 4 ] tetrahedral structure; Q 4 : four-connected [SiO 4 ] tetrahedral structure) ratio and a larger inter-layer distance.  Both AST and SOD zeolites have very small micropore sizes (as listed in Table 1) and are not often considered for catalysis applications. In comparison to 3D AST and SOD structures, the 2D layered silicates with AST and SOD topologies have a sheet of cup-shaped voids made of 4-and 6-MR connections, which indicates possible site accessibility. All 2D AST and SOD layers are made of siliceous species. The HUS-1 layered silicate is silylated to form the dimethylsilane (DMS)-HUS material, which has 8-MR micropores in the interlayer space [85]. Except for this attempt, none of the other 2D layered precursors have been successfully delaminated or pillared to form 2D AST or SOD derivatives. One of the main challenges in developing 2D derivatives is the structural instability of these 2D precursor silicate layers. The removal of an organic template by acid washing or thermal treatment often leads to the formation of an amorphous structure. The other main challenge is the high Q 3 :Q 4 ratio, which indicates significant H-bonding, which easily leads to self-condensation in post-treatment processes to form products with a reduced structural order. Given these considerations, 2D AST and SOD layered silicates have rarely been studied for the formation of any derivative structures, acidity properties, or catalysis applications.

2D Layered Precursor
Similar to most 2D layered materials that have siliceous compositions, PLS-1 and its 3D zeolite CDS-1 cannot behave as solid acid catalysts because the framework is constructed solely of tetrahedral SiO 4 units. Recently, B- [123], Ge- [124], and Al- [95,116] containing PLS-1 and their layered iso-structures were prepared. CDS-1 zeolites with these heteroatoms were also made by direct calcination of these 2D layers. Derivatives of 2D layers with the CDO zeolite topology are currently being explored. As shown in Table 3, interlayer-expansion by the silyation [115,116,118,119,125] and pillaring [33] of PLS-1 has led to COD zeolites with expanded and pillared structures.
Materials 2020, 13, 1822 11 of 52 [115,116,118,119,125] and pillaring [33] of PLS-1 has led to COD zeolites with expanded and pillared structures. HPM (nanostructured hybrid biohybrid and porous materials)-2 is a new layered organosilicate containing 2D layers with an MTF topology, yielding zeolite MTF by calcination [100]. The MTF structure contains 8-MR micropores along the c-axis direction, which separates the 2D layers normal to b-axis direction, as shown in Figure 6. Strong hydrogen bonds and close arrangement of silanols in adjacent layers exist in HPM-2. Therefore, attempts to delaminate or pillarize HPM-2 using previously reported recipes applied to other 2D layered materials failed. The interlayer expansion with dimethyldichlorosilane was successful, but the obtained material was non-microporous [100]. All 2D building layers discussed up until now are dense layers characterized by the fact that they do not contain 8-MR or larger pores perpendicular to the 2D layers. CIT (California Institute of Technology)-10, a layered precursor to the siliceous RTH zeolite framework, is an exception, as it contains 8-MR channels going through the layer (Table 2 and Figure 6). In addition, CIT-10 can be directly calcined to form pure-silica RTH (SSZ (Standard Oil Synthetic Zeolite)-50 [126]) or can be expanded by a silyating agent (e.g., dichlorodimethylsilane or diethoxydimethylsilane) to form CIT-11, which is stable following calcination (calcined material denoted as CIT-12) [101].  HPM (nanostructured hybrid biohybrid and porous materials)-2 is a new layered organosilicate containing 2D layers with an MTF topology, yielding zeolite MTF by calcination [100]. The MTF structure contains 8-MR micropores along the c-axis direction, which separates the 2D layers normal to b-axis direction, as shown in Figure 6. Strong hydrogen bonds and close arrangement of silanols in adjacent layers exist in HPM-2. Therefore, attempts to delaminate or pillarize HPM-2 using previously reported recipes applied to other 2D layered materials failed. The interlayer expansion with dimethyldichlorosilane was successful, but the obtained material was non-microporous [100]. All 2D building layers discussed up until now are dense layers characterized by the fact that they do not contain 8-MR or larger pores perpendicular to the 2D layers. CIT (California Institute of Technology)-10, a layered precursor to the siliceous RTH zeolite framework, is an exception, as it contains 8-MR channels going through the layer (Table 2 and Figure 6). In addition, CIT-10 can be directly calcined to form pure-silica RTH (SSZ (Standard Oil Synthetic Zeolite)-50 [126]) or can be expanded by a silyating agent (e.g., dichlorodimethylsilane or diethoxydimethylsilane) to form CIT-11, which is stable following calcination (calcined material denoted as CIT-12) [101].
RUB-18, also known as octosilicate or ilerite, is the layered precursor of RWR zeolite [47,50,102,103]. The layered backbone is composed of four 5-MR pores as building units ( Figure 6 and Table 2). Upon calcination, RUB-18 transforms into RUB-24, a zeolite with an RWR framework topology [51,127]. RUB-24 is a small-pore zeolite with a one-dimensional (1D) pore system consisting of straight and non-intersecting 8-MR channels. Though the structure analysis showed that the pores of the (idealized) silica framework are empty, nitrogen sorption experiments showed that there is no "free" access to the pore volume. However, compared to RWR zeolite, the 2D precursor, RUB-18, has many established capabilities, such as the interlamellar sorption of water and organic molecules, ion-exchange due to the interlayered hydrated counter-cations, and post-treatment to form multiple types of derivative materials (Table 3). For example, the original Na-form RUB-18 can be exchanged into the H + -form, which can behave as a proton conductor [103,109,128,129]. It can also be exfoliated into nanosheet layers [16,24], swollen to form organic-inorganic composites [30,130], and pillarized to form inorganic [110][111][112]131,132] or organic [113,114] pillared structures. Though RUB-18 does not contain heteroatoms, the pillarization treatment is able to introduce transition metals or organic functional groups that enable catalytic reactions over the derived 2D zeolite materials. not contain 8-MR or larger pores perpendicular to the 2D layers. CIT (California Institute of Technology)-10, a layered precursor to the siliceous RTH zeolite framework, is an exception, as it contains 8-MR channels going through the layer (Table 2 and Figure 6). In addition, CIT-10 can be directly calcined to form pure-silica RTH (SSZ (Standard Oil Synthetic Zeolite)-50 [126]) or can be expanded by a silyating agent (e.g., dichlorodimethylsilane or diethoxydimethylsilane) to form CIT-11, which is stable following calcination (calcined material denoted as CIT-12) [101].   In the category of 2D layered materials that have the 10-MR zeolite framework topology, six materials have been reported. Among them, one of them is aluminophosphate, while the rest of them possess the aluminosilicate composition. The presence of heteroatoms (e.g., Al) in the framework enables the materials to function as acid catalysts for catalysis applications, which is distinct from the 2D layers with 8-MR and 6-MR zeolite frameworks. In addition, the 10-MR micropores exist in some of these 2D layers (e.g., MFI and MWW), offering 2D layers that are directly capable of adsorption and catalytic applications. In addition, nearly all of them can be post-treated to form 2D derivative structures, as summarized in Tables 4-7 below.
The second layered (fluoro)aluminophosphate, denoted as EMM-9 (ExxonMobil Material #9), was reported recently by using a fluorine-medium and 4-(dimethylamino)pyridine (DMAP) as the organic structure-directing agent [135]. The 2D layers of EMM-9 are composed of STI (stilbite) composite building units, and DMAP cations are located between the layers. The layered EMM-9 structure is closely related to the 3D framework structure of EMM-8 and can be transformed to EMM-8 via calcination (Figure 7). EMM-8, which exhibits the SFO (SSZ-51 (fifty-one)) framework, contains 12and 8-MR channels. Adjacent layers need to translate by 1/3a (4.8 Å) and 1/2b (6.8 Å) relative to one another before they are condensed along the c-axis. Though EMM-9 belongs to the category of 2D layers with a 12-MR zeolite topology to be discussed in Section 3.4 below, we include it here due to its compositional similarity to [F,Tet-A]-AlPO-1. The current studies on 2D layers with AFO and SFO AlPO zeolite topologies have been focused on synthesis and structure elucidation. 2D derivatives of these two 2D layered framework types have not been reported in literature.

2D Layers with FER Framework Topology
FER zeolite is a medium-pore aluminosilicate, including 2D intersecting channels with 8-MR channels (4.8 Å × 3.5 Å) along the [010] direction and 10-MR channels (5.4 Å × 4.2 Å) along the [001] direction. The 2D layers with the FER topology cut through the 10-MR channels, and, therefore, they only contain 8-MR pores (see Figure 5 in Section 3.2). Due to its close structural relationship with CDO zeolite, the FER framework topology has been identified in a number of 2D layered precursors (Table 4 and 2D layer precursors of CDO in Table 2). The differences between the crystal structure among these related 2D FER precursors result from the stacking sequence of FER layers, i.e., the displacement of layers parallel to the a-b plane and/or interlayer distance [136], which is the consequence of incorporating different templates in syntheses. Among these FER precursors, the most prominent ones are PREFER, PLS-3, ICP (Instituto de Catálisis y Petroleoqu ḿica)-2 and ERS (EniRicerche molecular Sieve)-12 (Table 4).

2D Layers with FER Framework Topology
FER zeolite is a medium-pore aluminosilicate, including 2D intersecting channels with 8-MR channels (4.8 Å × 3.5 Å) along the [010] direction and 10-MR channels (5.4 Å × 4.2 Å) along the [001] direction. The 2D layers with the FER topology cut through the 10-MR channels, and, therefore, they only contain 8-MR pores (see Figure 5 in Section 3.2). Due to its close structural relationship with CDO zeolite, the FER framework topology has been identified in a number of 2D layered precursors (Table 4 and 2D layer precursors of CDO in Table 2). The differences between the crystal structure among these related 2D FER precursors result from the stacking sequence of FER layers, i.e., the displacement of layers parallel to the a-b plane and/or interlayer distance [136], which is the consequence of incorporating different templates in syntheses. Among these FER precursors, the most prominent ones are PREFER, PLS-3, ICP (Instituto de Catálisis y Petroleoquímica)-2 and ERS (EniRicerche molecular Sieve)-12 (Table 4).   PREFER was synthesized in a fluoride-media in the presence of 4-amino-2,2,6,6tetramethylpiperidine template [61,160]. The layer orientation is very straightforward: The stacking of FER layers occurs along the a-direction without translation in the b-or c-directions. Upon template elimination by calcination, the ordered 3D FER structure is formed through the condensation of the surface silanol groups. The layered PLS-3 silicate was prepared by a solid-state reaction using an H + -form of layered silicate (kanemite) as a silica source and tetramethylammonium as the SDA. The structure of PLS-3 is similar to that of the PREFER layer but with a smaller interlayer distance [91,161]. ICP-2 can be obtained in fluoride medium from aluminosilicate gels, using the chiral cation (1R,2S)-dimethylephedrinium (DMEP) as the SDA. It is a core-shell structure where the shell is composed of the organic cations arranged as supramolecular dimers surrounding the inorganic FER cores [137]. Similar to PREFER, ICP-2 can also be obtained in an Al-free form. The layered ERS-12 silicate is synthesized using the TEOS (tetraethyl orthosilicate) and TMAOH templates, which can also crystallize as germanosilicate, but not as alumino-and titanosilicate [138]. Calcined ERS-12 is, however, very different with respect to the calcined PREFER. This is caused by the fact that half of the silanol pairs on neighboring layers remain uncondensed during calcination, preventing the formation of a fully connected FER zeolite framework. In order to obtain the ordered FER framework, the layer must be shifted by 1/3c and 1/2b. Similar to 2D layers with a CDO topology, layers with a FER topology have many 2D derivatives since delamination, pillarization, and silylation have been practiced in this type of 2D material. The delamination of PREFER was first done by Corma and co-workers by swelling the precursor in an aqueous CTAB (cetrimonium bromide)/TPAOH (tetra-n-propylammonium hydroxide) solution (pH 12.5) followed by ultra-sonication to exfoliate the 2D layers [14,63,64]. The as-produced FER monolayers are named ITQ-6, which has partial amorphization due to high pH condition. In 2011, Katz's group [162] used a mild non-aqueous condition that contained a mixture of CTAB, tetrabutylammonium fluoride (TBAF), and tetrabutylammonium chloride (TBACl) in a dimethylformamide (DMF) solvent to swell PREFER. Afterwards, concentrated HCl was added to result in delamination of the swollen PREFER (denoted as UCB (University of California at Berkeley)-2). The characterization shows that the UCB-2 material does not have an amorphous structure and maintains the 2D layer structural integrity. The pillarization of swollen PREFER formed pillared FER, designated as ITQ-36 [34,120]. ZSM-55 has 2D FER layers, and it condenses to the CDO topology upon calcination. The pillaring of ZSM-55 was done recently, and it produced an ordered pillared FER at room temperature, as well as a structure with disorganization and partial layer degradation at high temperature (373 K) [33]. The interlayer expansion of FER layers by silylation formed new zeolite structures with larger 12-MR micropores [115,163] and 14 × 12 MR [164] materials, as noted by Wu et al. Due to the prominent 8-MR pore structure within 2D FER layers and the diversification of 2D derivatives and compositions, 2D FER zeolites have been proven to be efficient catalysts for different reactions. Heteroatoms such as Al, Ge, Ti, and B can be incorporated into the structures as well ( Table 5). The pillaring process introduced additional elements such as Fe [34], Cr [34], and Sn [165], which further diversifies the acidity of FER-based 2D zeolite materials. 12MR Al -a information reported as pore classification (i.e., mesopore), dimension (in Å) and/or pore ring size (MR).

2D Layers with HEU Framework Topology
As shown in Figure 8, the HEU (heulandite) zeolite framework contains a pore channel system with openings consisting of a 10-MR (3.1 Å × 7.5 Å) channel in the [001] direction as well as 8-MR micropores (3.6 Å × 4.6 Å) in the [001] direction (Table 4). Additionally, there is another set of 8-MR pores (2.8 Å × 4.7 Å) along the [100] direction. Materials with the HEU framework are divided into two distinct classes based on their Si/Al ratio. Those with an Si/Al ratio of less than four are known as heulandite, and those with an Si/Al ratio of greater than four are known as clinoptilolite or silica-rich heulandite. The 2D layers with an HEU topology are all high-silica layered aluminosilicate. CIT-8P is obtained from a low-water synthesis in fluoride media, with diquaternary amine as the SDA [139]. The condensation of CIT-8P by calcination produced CIT-8, which has the HEU topology. It should be noted that Ti and Al heteroatoms can be incorporated into the produced HEU zeolite. The layered silicates, HUS-2 [96] and HUS-7 [140], are comprised of 4-, 5-, and 6-MR with a framework topology similar to that of HEU-type zeolite. HUS-2 is synthesized using amorphous silica, sodium hydroxide, and choline hydroxide as the SDA. The change in SDA to biphenyl and benzyltrimethylammonium hydroxide led to the HUS-7 precursor. Silylation with the trichloromethylsilane of HUS-2 and subsequent calcination led to a microporous HUS-10 zeolite [166]. The Ti-species is intercalated into HUS-2, which leads to photooxidation applications [167], expanding on the siliceous derivative, which has only been considered for adsorption purposes.
Inspired by the innovative syntheses of 2D MFI nanosheets using quaternary ammonium surfactant templates, a range of 2D MFI derivative structures have been prepared from direct hydrothermal crystallization. As summarized in Table 6, unilamellar [170][171][172], self-pillared [143,173], and nanosheet aggregates [174,175] with interconnected macro-/meso-/micropores have been created in the past decade. The unilamellar MFI nanosheets are synthesized when the SDA is in the hydroxide form (e.g., [170,172]. The usage of nanocrystal-seeded growth triggered by a single rotational intergrowth in the presence of bis-1,5(tripropyl ammonium) pentamethylene diiodide (denoted as dC5) SDA also synthesized high-aspect-ratio MFI nanosheets with a thickness of 50 Å (2.5 unit cells) [171]. Self-pillared MFI (self-pillared pentasil, SPP) nanosheets were achieved by intergrowth with their 90 • counterparts and with a small amount of MEL acting as a fourfold symmetric connector using the tetrabutylphosphonium cation as an SDA [173]. Following the usage of biphenyl and naphthyl groups in the SDA to synthesize SCZN-1, the usage of bolaform amphiphilic molecules with bi-quaternary ammonium head groups and biphenyl groups ([C 6 H 13 4 ) as the SDA synthesized MFI nanosheets joined with a 90 • rotational boundary; this was named SCZN-2 [143]. All of these structures have an interconnected meso-and micro-porosity that facilitates mass transport for separation and catalysis applications. Inspired by the innovative syntheses of 2D MFI nanosheets using quaternary ammonium surfactant templates, a range of 2D MFI derivative structures have been prepared from direct hydrothermal crystallization. As summarized in Table 6, unilamellar [170][171][172], self-pillared [143,173], and nanosheet aggregates [174,175] with interconnected macro-/meso-/micropores have been created in the past decade. The unilamellar MFI nanosheets are synthesized when the SDA is in the hydroxide form (e.g., [170,172]. The usage of nanocrystal-seeded growth triggered by a single rotational intergrowth in the presence of bis-1,5(tripropyl ammonium) pentamethylene diiodide (denoted as dC5) SDA also synthesized highaspect-ratio MFI nanosheets with a thickness of 50 Å (2.5 unit cells) [171]. Self-pillared MFI (selfpillared pentasil, SPP) nanosheets were achieved by intergrowth with their 90° counterparts and with a small amount of MEL acting as a fourfold symmetric connector using the tetrabutylphosphonium cation as an SDA [173]. Following the usage of biphenyl and naphthyl groups in the SDA to synthesize SCZN-1, the usage of bolaform amphiphilic molecules with bi-quaternary ammonium head groups and biphenyl groups ( as the SDA synthesized MFI nanosheets joined with a 90° rotational boundary; this was named SCZN-2 [143]. All of these structures have an interconnected meso-and micro-porosity that facilitates mass transport for separation and catalysis applications.  Besides the direct synthesis method, traditionally practiced methods have also been used to produce 2D MFI nanosheet derivatives. The exfoliation of layered MFI nanosheets was first performed by Tsapatsis's group using a polymer-melt-blending technique. Through the polystyrene melt blending, polymer removal, density gradient centrifugation, and redispersion steps, the as-obtained monolayer MFI nanosheets were further fabricated as molecular sieve membranes [18]. Following previous work, a more facile route was developed [19] in which the exfoliated multilamellar MFI zeolite from polystyrene-melt-blending was treated by a piranha solution to remove the organic residue. In 2017, Fan's group reported the exfoliation of multilamellar MFI by suspending the layered zeolite precursors in telechelic liquid polybutadiene followed by brief shearing or sonication at room temperature [20]. The intercalation of TEOS into the multilamellar MFI zeolites followed by hydrolysis has been used to produce SiO 2 -pillared MFI [35,179]. Other metal oxide pillared cases have also been reported, such as titanosilicate [42], magnesium oxide, zinc oxide [180], and tin-silica [176]. An organic pillaring case was reported in Liu's group, where acid extraction and UV light irradiation were sequentially employed to remove the SDA in multilamellar MFI zeolite, followed by intercalation of acrylic silsesquioxane (1,4-bis(triethoxysilyl)benzene, BTEB) molecules between multilamellar MFI layers [36]. It should be noted that the pillared MFI zeolites contain mesopores created by the inorganic pillar species sitting between MFI layers that is parallel to the zigzag channels and perpendicular to the straight channels within the layers.

2D Layers with MWW Framework Topology
The MWW zeolite contains two independent pore systems. One system is defined by sinusoidal 10-MR channels with dimensions of 4.1 Å × 5.1 Å, and the other system consists of supercages delimited by 12-MR channels with dimensions of 7.1 Å × 7.1 Å × 18.1 Å. The consecutive supercages are connected through slightly distorted elliptical 10-MR windows (4.0 Å × 5.5 Å). One of the most prominent representative materials is the MCM-22 zeolite [57]. 2D layers with an MWW topology have many variations, which are differentiated by the layer ordering and interlayer repeat due to different synthesis conditions. Among these, the most prominent layers are MCM-22(P) [57,145], EMM-10P [146,147], ERB (EniRicerche-Boralite)-1 [148,149], MCM-56 [62,150,181], UZM-8 [152], SSZ-70 [153,154] and UJM-1P (Uniwersytet Jagiellonski Material #1) [157]. MCM-22(P) has layers stacked in vertical alignment with separation 2 Å longer (27 Å c-unit cell repeat) than in the complete 3-D framework (25 Å repeat) to which it converts upon calcination; hydrogen bonding between surface silanols was proposed as the interlayer connection maintaining the aligned in-register stacking. ERB-1 is a borosilicate zeolite material whose random stacking along the c-axis at well-defined distances is attributed to the piperidine molecules present in the interlayer region [148]. EMM-10P is closely related to MCM-22(P), but its layers are stacked without vertical alignment and are believed to be twisted off-register or otherwise disordered in-plane but still hydrogen bonded through silanols [147]. Upon calcination, EMM-10P partially converts to an ordered MWW structure, but, to a large extent, the stacking disorder persists when the layers fuse together. MCM-56 is a non-ordered material that can be regarded as a single layer collection, i.e., 'partially delaminated,' immature MCM-22 [182]. UZM-8 has a similar framework topology to that of MCM-56, but its inter-layer distance is larger than that of MCM-56 and smaller than that of MCM-22(P) [152]. UJM-1P is a new multi-layered and slightly expanded 2D MWW precursor, which was obtained by prolonging synthesis of the mono-layered MIT-1 material (see discussion in next paragraph). It is easier to swell with surfactants than MCM-22(P), which indicates a weak interlayer connection that may be due to the special SDA molecules lining the surface of its layers [157]. The as-synthesized SSZ-70(P) using imidazolium SDAs has a layered structure possessing some feature similarities to MCM-22(P), but the calcined form (e.g., SSZ-70) has different crystallographic features and catalytic performance to those of MCM-22 [153,154].
2D layers with an MWW zeolite topology have been explored extensively to produce numerous derivative structures. Delamination, pillarization, and interlayer expansion have all been investigated using these types of precursors (Table 7). First, MCM-22(P) has undergone numerous post-synthetic treatments that result in a plethora of layered and unilamellar zeolite structures [76]. A unilamellar MWW structure has also been developed by the delamination of the layered precursor and swollen material, resulting in MCM-56 [150] and ITQ-2 [13], respectively. More recently, the one-pot synthesis of MWW nanosheets was successfully implemented using an organic SDA containing both hydrophobic and hydrophilic portions to produce the desired 2D MWW structure that contains disordered single-layer MWW averaging 25 Å thickness and 150 nm length [67]. Another method of producing delaminated MWW nanosheets involves exfoliation in a mildly basic aqueous solution of pH 9, which results in the successful delamination of MCM-22(P) to form UCB-1 [22], and the Al-SSZ-70 zeolite precursor to form UCB-3 [183]. Pillared MWW (PMWW) was created by pillaring the MWW layers with SiO 2 , maintaining the 10-MR sinusoidal channels and hourglass shaped pores (half of the supercages in MWW) in the intact layers and mesopores between the layers. A silica source can be introduced as a post-synthetic technique to stabilize the gap between individual MWW layers and produce an ordered product (IEZ-MWW) from MCM-22(P) and disordered structure (EMM-10, EMM-12 [184]) from the disordered precursor, EMM-10P [147]. The slight interlayer spacing of MCM-22(P) can be manipulated further through swelling (MCM-22(P)-sw) with organic templates and pillaring for stability following calcination (MCM-36) [25]. SSZ-70 is available in silica, boron, and aluminum forms, all of which can be delaminated to form unilamellar structures [185,186].  [115,193] 12MR Al, Ce --Ti-YNU-1 [194,195] 12MR Al -- -a information reported as pore classification (i.e., mesopore), dimension (in Å) and/or pore ring size (MR); b d-spacing distance.

2D Layers with STI and RRO Framework Topology
Zeolite RRO and STI also contain 10-and 8-MR channels, as shown in Table 4. The layered precursor (named PKU-22) with an STI topology was reported in 2017 [159], which is a silicogermanate and was hydrothermally synthesized under fluoride conditions using tetraethylammonium (TEA + ) cations as the SDA. PKU-22 is constructed of STI layers stacked along the [100] direction, with TEA + cations residing in the interlayer spaces and F − anions existing within the layer and connected to Ge atoms, which also act as charge compensation species. Topotactic condensation was observed upon the heating of PKU-22, and the resulting product, PKU-22a, possessed an STI-type framework.
RRO zeolite has a two-dimensional channel system with intersecting 8-and 10-MR pores. The pore openings determined from structure analysis are 5.8 Å × 4.1 Å (8MR) and 5.9 Å × 4.1 Å (10MR). Similar to 2D layers with STI framework, the layered material with RRO framework only has one defined form (RUB-39) up until now. The synthesis of RUB-39 silicate was done by using dimethyldipropylammonium hydroxide as the SDA [60,196]. RUB-41, framework type code RRO, has been synthesized as a calcination product using the layered silicate, RUB-39, as precursor. The insertion of Al [169,197] and B [158] functional T-atoms into the layered precursor as well as its condensation to 3D framework silicate zeolites has been achieved. Silylation of the layered precursor RUB-39 with dichloro-dimethylsilane (DCDMS) providing layer interconnection the led to the formation of IEZ-RRO [168].
3.4. 2D Layers from 14-and/or 12-MR Zeolite (UTL, IWW, UOV, SAZ-1) Topology All of the 2D layered precursors discussed in the above sections are synthesized by hydrothermal crystallization of zeolite synthesis gels, i.e., the bottom-up synthesis. In this category of 2D precursor layers, the precursor is made from the pre-prepared parent zeolite in a synthesis process called the ADOR mechanism [49,198]. The key feature of the parent zeolite is the presence of a hydrolytically sensitive Ge dopant incorporated within the framework at a specific site (a double-four-ring (D4R) unit), which allows the chemically selective removal of the units containing the dopant. As a result, the germanium bonds such as Si-O-Ge or Ge-O-Ge (preferentially located within the D4R units) are selectively hydrolyzed, whereas the bonds within the layers, predominantly Si-O-Si bonds, are largely unaffected. This leads to the formation of 2D layered zeolite precursor materials, which can be treated with other post-methods to generate new zeolite structures and 2D zeolite derivatives ( Figure 10).
Germanosilicate UTL (IM (Institut Français du Pétrole and University of Mulhouse)-12, Mulhouse (twelve)) is an ideal ADOR starting point because of its chemical composition and stability of the layered units that are formed upon disassembly [199,200]. The resulting layered material was designated IPC-1P (Institute of Physical Chemistry-1 Precursor), which has a thickness of approximately 9 Å and possesses the same x-y projection as that of zeolite FER, although connectivity is more complicated in the z-direction, corresponding to a longer repeat unit, i.e., 12.5 Å vs. 7.5 Å. Similar to the FER precursor (PREFER), IPC-1P consists of rigid, compact layers that possess neither intra-layer zeolite-like channels nor well-defined inter-layer pores. IPC-1P has led to the formation of multiple types of zeolite materials, including IPC-1 (from direct calcination), IPC-4 (for layers connected simply through an oxygen atom), and IPC-2 (for layers connected through a single-four-ring unit (S4R)) [66]. The S4R connections and oxygen bridges produce two different inter-layer spacings, 11 Å and 9 Å, respectively. Furthermore, a medium/large pore zeolite, IPC-6 (12-10-MR and 10-8-MR pore systems), whose unit cell contains one of each of these different types of connections, has also been fabricated. A similar structure, IPC-7, has layer connections consisting of an equal quantity of S4Rs and D4Rs, possessing a unit cell consisting of one of each of these connections to produce a large-pore zeolite containing 14-12 and 12-10-MR pores [201]. IPC-1P can also be reconfigured using choline cations as the SDA to shift the layers with respect to each other to form IPC-9P. The layers can then be reassembled in two ways: by (1) calcination to form IPC-9 and by (2) calcination after intercalation of diethoxydimethylsilane to form IPC-10 [202].
Ge-rich IWW was treated with acidic solution at ambient temperature, leading to a layered material called IPC-5P with the inter-layer distance reduced by 1 Å to 3 Å depending on the applied conditions. IPC-5 is formed after calcination, confirming the successful application of the ADOR mechanism on an additional zeolitic structure-IWW [65]. In contrast to zeolite UTL, where two new zeolite structures (OKO (Oppervlakte en Katalyse One) and PCR (IPC-4 (four))) form from condensing the layered precursors via calcination, the layered structure, IPC-5P, tends to maintain the original IWW framework after calcination. Additionally, the ADOR transformation of a germanosilicate parent zeolite with a UOV topology produces a new zeolite named IPC-12 [207,208]. During this transformation, the pore system shifts from two dimensional, containing 12-MR and 8-MR channels along the [100] direction intersecting with a 10-MR channel, to a one dimensional pore system without the 10-MR intersecting channel. The ADOR method was also applied to a newly-discovered germanosilicate zeolite, SAZ-1, where an acid solution was used to remove the germanium-containing D4R units, producing a layered intermediate called SAZ-1P. SAZ-1P was further manipulated to produce IPC-15, whose layers were connected by O-linkages, and IPC-16, which contained S4R links between layers [209]. The same methodology has been applied to zeolites with ITH (Instituto de Tecnologia Quimica Valencia-thirteen), ITR (Instituto de Tecnologia Quimica Valencia-thirty-four), and IWR (Instituto de Tecnologia Quimica Valencia-twenty-four) topologies, but the resulting materials have not been explored with enough depth to fully elucidate the their structures [210].

Other Types of 2D Zeolites (MEL, FAU, MOR, MRE, TON)
The successful synthesis of lamellar MFI zeolite inspired the exploration of other 2D zeolite frameworks containing heteroatoms and medium/large micropores. In the past decade, 2D lamellar zeolites with MEL [69], MOR [74], MRE [75], TON [73], and FAU [70,71] topologies have been prepared. It should be noted that these zeolites do not yet have unilamellar, multilamellar, or 2D derivative structures, unlike the 2D layered zeolites discussed in Sections 3.1-3.4 above. Instead, they are nanosheet aggregates or nanosheet plates in which the nanosheets are tens of nanometers thick formed in a one-step hydrothermal crystallization process. Table 9 summarizes the structural and compositional properties of these 2D zeolites as well as the topological features of their 3D counterparts. In addition to the formation of microporous zeolite structures, the 2D layers formed form the ADOR process can be further treated to create swollen and pillared zeolites [31,[203][204][205]. For example, swelling is performed using the cationic surfactant, hexadecyltrimethylammonium (CTMA), yielding IPC-1SW. The pillared derivative of this material was denoted as IPC-1PI. Isomorphically substituted B, Ti, Al and Fe have also been successfully incorporated in the layered precursor materials. It is important to note that these pillared materials do not possess microporosity because they consist of dense layers supported by amorphous silica pillars.

Other Types of 2D Zeolites (MEL, FAU, MOR, MRE, TON)
The successful synthesis of lamellar MFI zeolite inspired the exploration of other 2D zeolite frameworks containing heteroatoms and medium/large micropores. In the past decade, 2D lamellar zeolites with MEL [69], MOR [74], MRE [75], TON [73], and FAU [70,71] topologies have been prepared. It should be noted that these zeolites do not yet have unilamellar, multilamellar, or 2D derivative structures, unlike the 2D layered zeolites discussed in Sections 3.1-3.4 above. Instead, they are nanosheet aggregates or nanosheet plates in which the nanosheets are tens of nanometers thick formed in a one-step hydrothermal crystallization process. Table 9 summarizes the structural and compositional properties of these 2D zeolites as well as the topological features of their 3D counterparts. The 2D MEL-type zeolite consists of titanosilicate (MTS (Multilayered Titanium Silicalite)-2) nanosheets synthesized using binary templates cetyltrimethylammonium tosylate (CTATos) and tetrabutylammonium hydroxide (TBAOH) [69]. The MTS-2 product exhibits a morphology of micro-sized nanosheet aggregates, in which each of the nanosheets exists in the range of 50-100 Å thickness, and it is inclined to orient along one direction. The 2D FAU-type zeolite X has a similar particle morphology to that of MTS-2, but the nanosheets (~70 Å thick) are organized in a house-of-cards-like assembly with wide macroporous interstices between the nanosheet stacks. The initial synthesis in 2D FAU zeolite used 3-(trimethoxysilyl)propyl hexadecyl dimethyl ammonium chloride (TPHAC) as the SDA [70]; later on, inorganic salts such as zinc nitrate and lithium carbonate were shown to produce similar particle morphologies [71]. 2D MOR, MRE and TON zeolites all have a plate-like morphology. The MOR layered zeolite was synthesized in the presence of the C 16 H 33 -N + (CH 3 ) 2 -C 2 H 4 -N(CH 3 ) 2 Br (C 16-2-0 ) template [74], resulting in nanoplates with a wide dimension of about 3 µm on the a-b planes and 200-400 Å thickness along the c-axis direction. Further investigation into the synthesis parameters' effects on developing layered MOR material determined that the structure and charge of the cationic gemini-type SDA played a critical role in determining morphology. As a result, hierarchical MOR displayed nanorod, nanobrick, and nanoplate morphologies under different synthesis conditions [211]. 3D MOR is characterized by a 12-MR main channel (6.5 Å × 7.0 Å) and a parallel 8-MR channel (2.6 Å × 5.7 Å) along the c axis, which are interconnected by 8-MR side-pockets (3.4 Å × 4.8 Å) along the b-axis. The as-produced 2D MOR nanoplate has a reduced thickness along the c-axis, which shortens the length of the 12-and 8-MR channels in the material. Though this material was referred to nanosheets in the original reports, the structural analysis down to the unit-cell level has not yet been assessed, and, therefore, we refer to the morphology as a nanoplate structure to provide this distinction. The 2D TON (i.e., ZSM-22) zeolite nanoplates, with a thickness of about 100 Å, were synthesized through hydrothermal crystallization using a 1-ethylpyridinium bromide template. The mechanism was assumed to be a multi-step crystallization process involving the aggregation and fusion of elementary nanorods, and inhomogeneous Al distribution was considered to be a key factor [73]. TON is known as a 1D channel system with 10-MR pore openings of 4.5 Å × 5.5 Å. The thickness of the nanosheets increased from 80 Å to 500 Å with increasing Al content in the starting mixture. Lastly, nanosheets possessing an MRE zeolite topology (LMZN) were synthesized using a benzophenanthrene template and a ((C 6 H 2 ) 3 -(O-C n H 2n -N + (CH 3 ) 2 -C 6 H 12 -N(CH 3 ) 2 (Br − ))6 (denoted BPT n−6−0 ) surfactant. The nanosheets were composed of alternating ∼30 Å-thick zeolite layers and ∼40 Å -thick surfactant micelles [75].

Acidity Properties of 2D Zeolites
The catalytic advantages of 2D zeolite materials, compared to their 3D analogues, are derived from the presence of active sites with appropriate acidity and improved accessibility. Acid sites are formed when heteroatoms are incorporated into zeolites via the isomorphous substitution of Si with other elements during direct hydrothermal synthesis or post-modification. The composition and coordination of heteroatoms determine the acidity type (i.e., Brønsted and Lewis) and strength, as shown in Figure 11. Enhanced acid site accessibility indicates the capability of 2D zeolites to process bulky molecules in catalysis. Relative to the multilamellar 2D zeolite precursors, increased acid site accessibility can be realized by the delamination and pillarization of 2D zeolite precursors; delamination increases external surface area and exposes the acid sites along the outward surface, and pillarization maintains and/or expands the gallery space between two adjacent zeolite layers, thus enhancing the accessibility of external acid sites. Though many studies have shown that higher conversions are achieved in catalytic reactions over 2D zeolites than their 3D microporous analogues, studies on effects of reduction in zeolite dimension on acidity properties have lagged behind. Therefore, very few reviews have been published on the acidity properties of 2D zeolites in past few decades [8,43]. In this section, we summarize the recent progress made on acidity characterizations of 2D zeolites, with focuses on the delaminated and pillared structures. It should be noted that many acidity measurement tools and protocols developed for 3D zeolites are equally applicable to 2D zeolite materials. Given the excellent review papers on these methods for 3D zeolites [4,[212][213][214], we only focus on the introduction of techniques and acidity results that are obtained from the characterization of 2D zeolites. For visual significance, Figure 11 shows the representative structures of acidity in zeolite materials.
decades [8,43]. In this section, we summarize the recent progress made on acidity characterizations of 2D zeolites, with focuses on the delaminated and pillared structures. It should be noted that many acidity measurement tools and protocols developed for 3D zeolites are equally applicable to 2D zeolite materials. Given the excellent review papers on these methods for 3D zeolites [4,[212][213][214], we only focus on the introduction of techniques and acidity results that are obtained from the characterization of 2D zeolites. For visual significance, Figure 11 shows the representative structures of acidity in zeolite materials.

Techniques for Acidity Characterization of 2D Zeolite Materials
The acid site type and strength influence the activity and selectivity of 2D zeolite materials in catalysis, which are correlated to the coordination environment of heteroatoms. The most widely studied heteroatom in 2D zeolites is Al, therefore 27 Al magic-angle spinning nuclear magnetic resonance ( 27 Al-MAS-NMR) spectroscopy is commonly used to probe its coordination structures. In this method, the tetrahedrally coordinated framework Al atoms exhibit a signal in the range 51.5-65

Techniques for Acidity Characterization of 2D Zeolite Materials
The acid site type and strength influence the activity and selectivity of 2D zeolite materials in catalysis, which are correlated to the coordination environment of heteroatoms. The most widely studied heteroatom in 2D zeolites is Al, therefore 27 Al magic-angle spinning nuclear magnetic resonance ( 27 Al-MAS-NMR) spectroscopy is commonly used to probe its coordination structures. In this method, the tetrahedrally coordinated framework Al atoms exhibit a signal in the range 51.5-65 ppm (depending on the type of zeolite and Si/Al ratio), while the octahedrally coordinated Al species (denoted as extra-framework Al) exhibit a chemical shift of 0 ppm [215,216]. In addition, a weak signal around 30 ppm is sometimes reported and ascribed to penta-coordinated Al species [43,217,218]. It is said that the shape and signal position of the 27 Al-MAS-NMR spectra are independent of the crystal size, shape, and dimensionality of a particular zeolite [79,219]. The 2D zeolites, whether in delamination and pillarization form, however, often have higher fractions of extra-framework Al compared to their 3D analogues.
The FTIR spectroscopy of the hydroxyl (-OH) groups or adsorbate probe molecules is a well-established technique for evaluating the nature and strength of acid sites in zeolite materials. Depending on the zeolite framework and dimension, there are several types of -OH groups, including (i) lattice termination silanol (-SiOH) groups, (ii) -OH groups occurring at defect sites (hydroxyl nests), (iii) -OH groups attached to extra-framework heteroatoms, and (iv) bridging -OH groups (i.e., Brønsted (Si-OH-Al) acid sites). These -OH groups give rise to bands in the fundamental stretch region at~3740, 3720,~3680,~3600-3650 cm −1 , in sequence [220][221][222][223][224][225]. The precise position of the -OH group in the Brønsted acid site varies with the zeolite's framework structure and dimensionality [226,227]. The FTIR spectra of adsorbed probe molecules on acid sites in zeolites can provide information on the acid type and strength. These probe molecules are often organic bases and should be small enough to access all acid sites enclosed within the zeolite micropores. Pyridine, with a kinetic dimeter of 5.4 Å [45,228], is a commonly used organic probe molecule. The FTIR spectrum of adsorbed pyridine produces one band at around 1545 cm −1 due to the formation of pyridinium ions on Brønsted acid sites and another band at 1454 cm −1 due to pyridine ions on Lewis acid sites. The FTIR spectra of adsorbed pyridine can quantify the acid site concentration by combining the band intensity with the corresponding molar extinction coefficients by using the Lambert-Beer equation. The capability of retaining pyridine at different temperatures reflects the different strength of acid sites in zeolites. In general, the higher retaining temperature corresponds to stronger acid sites in the examined zeolites.
Similarly, the acid site quantity and strength in zeolites can be probed by the temperature-programmed desorption (TPD) of amines such as ammonia (NH 3 ). In the measurement, the catalyst is firstly saturated with NH 3 at a lower temperature. A linear ramping of the temperature of the zeolite sample in a flowing inert gas stream is then applied, which results in a profile of TPD-NH 3 . The acid site concentration is determined from the amount of ammonia desorbing above some characteristic temperature in the profile, while the acidity strength is reflected by the peak desorption temperatures. The applications of the TPD-NH 3 technique has been well described in previous review articles [4,213,229].
The quantitative analysis of the external acid sites in 2D zeolites is important to understand these materials' catalytic performances in processing bulky reactant molecules. The concentration of total acid sites obtained from using a small organic base probe molecule, which can access all of the acid sites, combined with the concentration of acid sites determined using a bulky analogue that cannot enter the system of micropores, provides information about the distribution of acid sites in distinct locations of the 2D zeolites. A range of organic base molecules, as summarized in Table 10, have been paired together for this purpose. In the measurement, the uptake of small organic base molecules determines the density of total acid sites in the material, while the uptake of the large molecules measures the concentration of external acid sites. The ratio of adsorbed large base molecules to the small ones is the accessibility (or fraction) of the external acid sites in the analyzed 2D zeolites. This technique is often combined with FTIR spectroscopy, GC (gas chromatography) or MS measurement to quantify the adsorbed organic bases. In addition, the 31 P MAS NMR spectroscopy of adsorbed phosphine oxides is a powerful tool for quantifying the external acid sites and acid site strength [67,79,230]. The method takes advantage of the strong adsorption of basic phosphine oxides (TMPO (tetramethylphosphonium) or TBPO (tributylphosphine oxide)) on Brønsted acid sites in zeolite framework, resulting in a one-to-one ratio. The interaction between a Brønsted (Si-OH-Al) acid site and an oxygen atom from phosphine oxide results in a 31 P MAS NMR chemical shift that increases with the acid strength. Therefore, the chemical shifts of 31 P MAS NMR signals indicate the acid strength, while the peak area suggests the quantity of the acid sites with the identified acid strength. TMPO and TBPO probe the total and external surface acid sites in zeolites, respectively, and the ratio of adsorbed TBPO to TMPO defines the accessibility of acid sites in the measured 2D zeolite.

2D Zeolites Explored as Acid Catalysts
The structural and compositional properties of 2D zeolites and their derivative structures are summarized in Tables 1-9. The elements Al, Ti, Sn, Ge, and B are currently the dominant heteroatoms present in the available 2D zeolites, and they offer the Brønsted and/or Lewis acidity required for catalysis. The 6-MR 2D zeolites (e.g., AST and SOD) are only available in the siliceous form, and their pore sizes within the layers are too small to accommodate any hydrocarbon molecules for reaction. Therefore, no acidity studies have been conducted on this class of 2D zeolites. In the category of 8-MR 2D zeolites, NSI and RWR have isomorphously substituted heteroatoms and derivative (i.e., delamination and pillarization) structures, so acidity characterization has mainly focused on these two zeolite materials. For the 10-MR 2D zeolites, STI is a siliceous zeolite, while HEU and RRO do not yet have a delaminated or pillared structure. Hence, acidity characterization has been centered on the derivative structures of FER, MFI, and MWW zeolites. Due to the small pore sizes in the 2D layers of NSI, RWR, and FER, the acidity characterization has not been studied in great detail compared to 2D MFI and MWW zeolites. Lastly, the 2D layered IPC-1P, generated from the 14-and 12-MR parent zeolite, has B, Ge, and Al heteroatoms within its framework, as well as within that of its 2D derivative structures. The lack of microporosity in the 2D layers and structural stability limits the current research on the synthesis and structural elucidation of this class of materials. The acidity characterization and catalysis applications of these materials have not been explored extensively [240]. Given these considerations, the acidity characterizations for 2D zeolites have mainly focused on the derivatives of NSI, RWR, FER, MFI, and MWW zeolites, as sequentially introduced below.

Acidity Characterization for 2D NSI, RWR and FER Zeolites
Among the derivative structures of 2D NSI zeolites, the delaminated form (i.e., ITQ-18) has been explored for its acidity properties and compared with those of the 3D NSI structure (i.e., Nu-6(2)). The coordination structure of Al in ITQ-18 was characterized using 27 Al MAS NMR spectroscopy, where the resulting spectrum shows a resonance at~52 ppm for Al in tetrahedral coordination and an additional weak peak at~0 ppm for extra-framework octahedral Al [15,41,106,178]. The presence of some extra-framework Al is attributed to the calcination step, which occurs immediately after the delamination treatment of its 2D layered precursor, Nu-6(1), since the latter does not show an obvious peak at~0 ppm in its spectrum. Nu-6(2) has a comparable 27 Al MAS NMR spectrum to that of ITQ-18, confirming the co-existence of framework and extra-framework Al after the direct calcination of Nu-6(1). FTIR spectra in the -OH region show that the Brønsted (Si-OH-Al) acid group has a band at 3610 cm −1 in ITQ-18, a similar position to that of Nu-6(2) [15]. In addition, FTIR spectra of adsorbed pyridine illustrate that the acid sites in ITQ-18 are accessible to pyridine, while Nu-6(2) has a negligible pyridine adsorption [15]. This suggests that the delaminated 2D NSI should have higher activity when carrying out acid catalyzed reactions than the 3D Nu-6(2) zeolite.
For 2D zeolites with an RWR topology, only the pillared derivative structures have been studied for acidity properties because the heteroatoms, such as Al, were introduced during the post-treatment (i.e., pillarization) of the 2D layered materials. The 27 Al MAS NMR spectra confirm the presence of tetrahedral framework Al in the pillared RWR zeolite [112,241]. When the Al content is high and the product was calcined, a small amount of Al exists as an octahedrally coordinated structure. The presence of Brønsted (Si-OH-Al) acid groups was characterized by the FTIR spectra of -OH groups in the pillared RUB-18 material, which showed a tail in the range of 3687-3413 cm −1 [241]. Compared to 2D zeolites with NSI and RWR frameworks, 2D zeolites with an FER topology have been studied more often in catalytic reactions [14,161,[242][243][244]. The acidity properties of 2D FER zeolite in delamination and pillarization forms have been studied and compared to the 3D FER analogue. ITQ-6, the representative delaminated FER zeolite, was studied using both the 27 Al MAS NMR and FTIR of -OH groups to determine the presence of tetrahedral framework Al, and the absolute acid site concentration was lower than that of 3D FER due to the dealumination during the exfoliation process [14]. The adsorption-desorption of pyridine at different temperatures traced by FTIR spectra has shown that ITQ-6 has a higher portion of Lewis acid sites and thus a lower portion of Brønsted aid sites than FER zeolite. Additionally, the fraction of strong acid sites is higher in the exfoliated form (i.e., ITQ-6) than that of the 3D FER zeolite [14]. The acid sites in both 2D ITQ-6 and 3D FER are accessible to pyridine, and the adsorption of di-tert-butyl peroxide (DTBP), which cannot access the acid sites within the 10-MR micropores of 3D FER, shows that the fraction of external acid sites is <5% in 3D FER, in contrast to~90% in ITQ-6 zeolite [14]. The pillaring of 2D layered FER precursors has been extensively explored (Tables 4 and 5). ITQ-36 is the earliest version of pillared FER zeolite [34], but the acidity characterization of this material is not available in open literature. The acidity characterization of another pillared 2D FER precursor (i.e., PLS-3) by both FTIR of -OH groups and 27 Al MAS NMR spectra shows that the acidic Si-OH-Al groups are present in the FER framework, and the acid sites have higher accessibility to DTBP compared to the direct calcined PLS-3 zeolite [244].

Acidity Characterization for 2D MFI Zeolites
Corresponding to the success of synthesizing 2D multilamellar MFI aluminosilicate and its derivative structures (Tables 4 and 6), the acidity characterization for these 2D MFI zeolites has been intensively conducted. Acidity measurements have been done on the unilamellar, multilamellar, and pillared MFI structures and have been compared to the acidity properties of conventional 3D MFI zeolite (Table 11). As seen in the characterizations for other 2D zeolites discussed above, the 27 Al MAS NMR technique has been used to examine the coordination environment of the Al element in 2D MFI zeolites [245][246][247][248][249]. The results have shown that, in randomly stacked (i.e., unilamellar) MFI nanosheets, the Al element is found not only in tetrahedral environments but also in octahedral and penta-coordinated/distorted four-coordinated positions [246]. Multilamellar MFI and its pillared form contain tetrahedral framework Al and a small amount of extra-framework Al species [48,245,248,249]. The quantitative analysis for fractions of extra-framework Al in these 2D MFI zeolites has been done by comparing the peak area of each species visible in the 27 Al MAS NMR spectra. As shown in Table 11, multilamellar MFI contains more extra-framework Al than pillared MFI, while the 3D MFI has much less extra-framework Al compared to these two derivatives [245,249]. Overall, the fraction of extra-framework Al is indicative of the defective nature of 2D MFI zeolites, which is expected to follow the order of unilamellar MFI > multilamellar MFI > pillared MFI > 3D MFI. The FTIR spectra of -OH groups are used to recognize the Brønsted (Si-OH-Al) acid sites in these MFI zeolites. For all 2D and 3D MFI zeolites, the -OH vibration in Brønsted acid sites is located at~3615 cm −1 [246,250]. The 2D MFI nanosheet, however, is dominated by -Si-OH groups on the external surfaces, which differs from the low peak intensity for this group in 3D MFI zeolite. The acid site type, quantity, and accessibility in 2D MFI zeolites have been measured by different techniques, including the FTIR of adsorbed small and bulky organic base molecules, organic base uptake measurement by GC/MS instruments, and 31 P MAS NMR spectra (Table 11). For example, the FTIR spectra of adsorbed pyridine, collidine, and DTBP molecules have shown that the 2D unilamellar and multilamellar MFI zeolites have a higher fraction of Lewis acidity than that of 3D MFI, while the accessibility of acid sites is improved due to the open structure [247][248][249][250]. The acid site accessibility of pillared MFI and SPP zeolites has been measured by direct DTBP uptake measurements, which maintain the characteristics of the high acid site accessibility in 2D MFI zeolites [173,245,251,252]. In particular, the concentrations of external surface and pore mouth Brønsted acid sites in the pillared MFI zeolite was quantified by a combined dimethyl ether (DME) titration and methanol dehydration in the presence of DTBP or triphenylphosphine (TPP) titrants, respectively [232]. DME can access all Brønsted acid sites in pillared MFI zeolites and thus determine the total number of acid sites. DTBP cannot access acid sites in micropores, but it can access the external surface and at the pore mouth regions. In contrast, TPP can only access acid sites on the external surface. The degree of decrease in methanol dehydration rate in the presence of DTBP or TPP indicates the fraction of the sum of external surface and pore mouth acid sites and the sole fraction of external surface acid sites, respectively. It shows that pillared MFI contains ∼32% external surface and ∼6% pore mouth acid sites.
The acid site strength in 2D MFI has been measured by FTIR spectra of pyridine adsorption after desorption at different temperatures [247,249,253]. In comparison to 3D MFI zeolites, the unilamellar 2D MFI nanosheets have weaker acid strength. The TPD-NH 3 profiles show that the MFI nanosheet exhibits a lower percentage of strong acid sites over total acid sites [254,255], consistent with the results from adsorption-desorption of pyridine measurement. In addition, the acid site location, strength, and accessibility in 2D MFI nanosheets were evaluated by Ryoo's group using the 31 P MAS NMR spectra of TMPO and TBPO probe molecules [79]. TMPO has a kinetic diameter of 5.5 Å [230], so it can access all external and internal acid sites. On the other hand, the TBPO molecule (∼8.2 Å) can be adsorbed exclusively on external surfaces for titration of external acid sites [44]. The spectra for the MFI zeolites were deconvoluted into multiple peaks with different chemical shifts, which were assigned to the chemisorption of probe molecules on Brønsted acid sites with four different strengths. Different from the acidity strength results obtained from FTIR spectra of adsorbed pyridine, the 31 P MAS NMR spectra of TMPO show that 2D MFI nanosheets have high fractions of weak, medium, and strong acid sites, while the fraction of acid sites with medium-high strength is low compared to 3D commercial MFI zeolites. The external acid sites with three different (weak, medium, and high) strengths were determined by 31 P MAS NMR spectra of TBPO in both 2D and 3D MFI zeolites. Similarly, 2D MFI nanosheets have high proportions of weak and strong acid sites, while the concentration of acid sites with medium strength is low. The external acid sites were calculated to be 32.0% of the total concentration of framework acid sites. The variation in fractions of external acid site accessibility in 2D MFI zeolites in Table 11 should be attributed to different measurement methods and variations in precise material structures produced from different research groups. Overall, the 2D MFI materials consistently show higher acid site accessibility than that of 3D MFI zeolite.

Acidity Characterization for 2D MWW Zeolites
Due to the aluminosilicate composition and medium micropore sizes, the acidity of 2D MWW zeolites has been extensively studied. Among many types of delaminated and pillared derivative structures, the ones generated from MCM-22(P) [57,145], MCM-56 [62,150,181], and SSZ-70 [153,154] have been the most widely studied. Over the past four decades, different protocols have been developed for the delamination of MCM-22(P) to form derivative structures. The earliest technique required a high-pH (in the range of 13.5-13.8) aqueous medium at an elevated temperature (e.g., 353 K) during MCM-22(P) swelling [104,256,257]. The swollen material was subsequently pillared to produce MCM-36 or exfoliated by ultrasonication to form ITQ-2 [13,258]. The 27 Al MAS NMR spectra show a decrease in tetrahedral framework Al and a small increase in extra-framework hexa-or penta-coordinated Al when transiting from MCM-22 to ITQ-2 or MCM-36 [28,62,187,219,259], which confirms the dealumination and partial amorphization of the 2D zeolite layers during this approach. In 2010, Tspatasis and co-workers swelled MCM-22(P) at room temperature conditions [21,260], which preserved the layer and pore structure, as well as the zeolite composition, since both MCM-22 and MCM-36 showed similar peaks in the 27 Al MAS NMR spectra. In 2011, Katz's group exfoliated MCM-22(P) using a combination of tetrabutylammonium fluoride and chloride surfactants at pH 9 in an aqueous solution. The resultant product, UCB-1, showed the retention of tetrahedral framework Al at~55 ppm and did not lead to the formation of extra-framework Al at~0 ppm [22]. Similar to MCM-22(P), MCM-56 has been delaminated [182] and pillared [182,261] using the original protocol practiced for MCM-22(P). The 27 Al MAS NMR spectra for the swollen and exfoliated samples contain signals for both framework and extra-framework Al. The intensity of the framework Al signal diminishes after swelling and sonication, in accordance with the decrease in the absolute concentration of framework Al and increase in extra-framework Al in this material. UCB-3 was synthesized by delamination of Al-SSZ-70 under non-aqueous conditions using fluoride anions [185]. The mild delamination conditions did not lead to leaching of framework Al to form extra-framework Al, which was consistent with UCB-1 synthesis.
Changes in acidity between the 3D and 2D MWW derivative structures have been further studied by monitoring the -OH vibration region in the FTIR spectra [28,62,258,[262][263][264][265]. The results have shown that a larger number of -SiOH groups (bands at 3745 cm −1 ) are present on ITQ-2 and MCM-36 than those in 3D MWW, as is expected from the delamination process. Meanwhile, spectroscopic results in the Si-OH-Al region (~3615 cm −1 ) have shown a decrease after delamination, which has been caused by a certain degree of dealumination. Further analyses for the distinct positions of Si-OH-Al groups in the supercages (3621 cm −1 ) and in the sinusoidal channels (3608 cm −1 ) of MWW zeolites were also performed to provide a basis for comparison between 3D MWW (MCM-22) and 2D MWW (MCM-36) zeolites [226]. The signal belonging to Brønsted sites in the MWW supercages was about seven times weaker for 3D MWW than that of 2D MWW, indicating that the supercages are not re-formed when the zeolite does not condense into a 3D connected structure.
To quantify the concentration and strength of acid sites in the MWW structures, the FTIR of adsorbed pyridine at different temperature conditions has commonly been enlisted (Table 12). The absolute quantity of both Brønsed and Lewis acid sites in MCM-22, ITQ-2, and MCM-36 has varied across different research groups [182]. Overall, the data have shown that the pillarization and exfoliation processes result in a decrease in the Brønsted acid sites, while the Lewis acid sites are either not affected or increased slightly [182,266]. Except for the very early work [187,258] that showed that pillared MCM-36 and exfoliated ITQ-2 had higher percentages of strong acid sites than that of 3D MCM-22 zeolite, most studies have drawn the opposite conclusion, in which the 2D MCM-36 or ITQ-2 has lower fractions of strong acid sites (Table 12). The synthesis of MCM-36 under mild conditions leads to a much higher concentration of Brønsted acid sites and a higher proportion of sites with medium-to-strong acidity. The MCM-36 prepared under high pH and high temperature conditions has a lower number of total Brønsted acid sites, in agreement with its lower Al content, but the fraction of stronger acid sites is high [260]. The acid site strength of 2D and 3D MWW zeolites is also characterized by profiles of TPD-NH 3 , and similar conclusions to those from FTIR of adsorbed pyridine have been drawn. For MCM-36 or ITQ-2 that are prepared under harsh (high pH and high temperature) conditions, three peaks are often observed in the profiles that consist of a maximum peak at~608 K, a shoulder at 498 K, and a characteristic shoulder in the range of 720-850 K. These peaks are sequentially assigned to the physiosorbed NH 3 and NH 4 + ions formed on strong Brønsted acid sites and NH 3 adsorbed on the strong Lewis acid sites. In comparison to the MCM-22 parent material, MCM-36 and ITQ-2 have more strong Lewis acid sites, consistent with a partial dealumination during the swelling or pillaring processes [26,28,219,265]. Opposite to this trend, the MCM-36 or ITQ-2 prepared under mild conditions show lower acid site strength than MCM-22 [259,261,266]. The accessibility of acid sites to large molecules has been probed by FTIR of adsorbed bulky organic molecules such as DTBP [28,62,156,187,235,238,258,264,269], dimethylquinoline (DMQ) [226,262], 2,2,4-trimethylpentane [26], and pivalonitrile [182]. Along with the total number of acid sites that are determined from pyridine adsorption, the fraction of acid sites accessible to the bulky molecules have also been assessed, as shown in Table 13. It can be seen that there is a much larger concentration of external acid sites in ITQ-2 and MCM-36 than in MCM-22, a much larger concentration of external acid sites in pillared and delaminated MCM-56 than in directly calcined MCM-56, and a much larger concentration of external acid sites in IPC-3PI (pillared IPC-3) than IPC-3. This again supports the conclusion that the delamination and pillarization of 2D layered zeolites increase the number of Brønsted acid sites accessible to large molecules. Due to a higher degree of disorder along the c-axis in MCM-56 than in MCM-22(P) [62], MCM-56 has a higher fraction of external acid sites than that of MCM-22 (Table 13). DTBP titration, in combination with DME titration or the ethanol dehydration reaction, was used to evaluate the accessibility of acid sites in the MCM-36 prepared under mild conditions [173,245,251,252]. The ethanol dehydration rates as a function of cumulative DTBP or pyridine titrant addition on the MCM-22 and MCM-36 zeolites, respectively, were measured. Titration with DTBP over all zeolites initially resulted in a linear decrease in dehydration rates with the increasing addition of the titrant, consistent with the stoichiometric titration of the active sites along the catalyst bed. At saturation, DTBP titration maintained different residual rates over the zeolites with different pore structures. The loss in ethanol dehydration rates reflects the degree of accessibility of bulky DTBP molecules to Brønsted acid sites in zeolites with different pore structures and hence, the number of active sites accessible from the mesoporous environment. The calculation using results from this technique indicated that 8% and 67% Brønsted acid sites in MCM-22 and MCM-36, respectively, are accessible to the DTBP molecule. Compared to DTBP, the TPP molecule is sterically bulkier with a diameter at 9.4 Å [239], but it has a weaker base strength [270]. It can solely access the acid sites on the external surface [271]. The decrease in methanol dehydration rate in the presence of TPP titrant reflects the fractions of external surface and/or pore mouth acid sites in MCM-36 zeolite, where it consists of ∼33% external surface and ∼31% pore mouth acid sites [232].
The measurements were conducted for MCM-22 and MCM-56 zeolites for comparison. Similar to the results from the 2D MFI zeolite spectra, the 31 P MAS NMR spectra of probe molecules (TMPO and TBPO) show peaks with different chemical shifts caused by the Brønsted acid sites with different acid strengths. In particular, MCM-22, MCM-56, and MIT-1 all have peak signals at 85, 72, 68, 63, and 53 ppm, of which the first four peaks are associated with strong Brønsted acid sites and the last one is due to TMPO adsorbed onto Lewis acidic extra-framework Al. Both MCM-56 and MIT-1 have much lower percentages of strong Brønsted acid sites than that of MCM-22. The total number of acid sites was quantified using spectra integration coupled with elemental analysis, which showed that MIT-1 has 64% external acid sites, higher than 41% external acid sites for MCM-56 and 13% for MCM-22. In 31 P MAS NMR spectroscopy characterization for 2D MFI nanosheets [79] and MWW (MIT-1) nanosheets [67], it is important to note that both materials display one fewer type of Brønsted acid site on the external surface compared to inside the micropores. The Lewis acid site concentration in 2D MWW nanosheets seems to be high compared to that of 2D MFI nanosheets since the 31 P MAS NMR spectra of TMPO adsorbed on MFI does not have the clear peak assigned to the Lewis acid sites; this is different from those of TMPO on MIT-1 nanosheets. This phenomenon is consistent with acid site titration results when using DME molecules reported by Liu et al. [245]. MCM-36 and MCM-22 both have a much lower concentration of Brønsted acid sites than Al concentrations, while MFI and PMFI (pillared MFI) zeolites have consistent concentrations of Brønsted acid sites and Al content.

Acidity Characterization for 2D Materials Generated from UTL Zeolite
Presently, research has been focused on the synthesis of 2D zeolites and 3D zeolite structures derived from the 14-and/or 12-MR parent zeolites in the ADOR process. Thus far, acidity characterization has only been conducted on the 2D zeolites (e.g., IPC-1P and IPC-1PI) generated from UTL zeolites. IPC-1P contains individual ultrathin 2D layers with the UTL structure, while these layers of thickness 9 Å possess neither intra-layer zeolite-like channels nor inter-layer pores. The 2D layers are separated by amorphous silica pillars in IPC-1PI. Therefore, the pillared IPC-1PI material does not exhibit any microporosity but possesses large mesopore voids created by the amorphous SiO 2 pillars. The FTIR of -OH vibration regions and adsorbed CD 3 CN was employed to probe the type and concentration of acid sites in the IPC-1PI catalyst [272]. The most intense bands in the spectra, at around 3745 cm −1 , were observed in the case of IPC-1PI, indicating a high concentration of silanol groups, which is characteristic of pillared materials with amorphous silica pillars. In contrast, the intensity of the band representing bridging -OH groups was very low compared to that of the UTL parent zeolite. The Brønsted acidity was greatly decreased, while Lewis acidity decreased slightly, according to the IR measurement of adsorbed CD 3 CN.

Summary and Outlook
The alteration of conventional 3D microporous zeolites into layered 2D zeolite structures has great potential to diversify zeolites and tailor their structures and acid site environments for catalysis applications. In 2D zeolites, the extremely thin nanosheet slices of crystals produce high external surface areas, up to 50% of total surface area, compared to~2% in micron-sized 3D zeolite, and acid site accessibility is increased by exposing the majority of active sites on external surfaces. The implications of these unique properties of 2D zeolites-including the feasibility of converting bulky compounds that are incapable of entering the zeolite micropores; transport enhancement in handling bulky reactants, intermediates, and products; and less pronounced deactivation effects resulting from the formation of coke deposits-have been demonstrated in many catalysis studies. Overall, 2D zeolites show a significantly better catalytic performance than that of conventional 3D analogues. Despite the demonstrated success of 2D zeolites in advancing catalysis, there are still many challenges that can be addressed to improve and expand their future applications. This review summarized the types and structures, as well as the acidity characterization, of 2D zeolite materials reported in the literature. To better understand the acidity properties of 2D zeolites, the structural and compositional properties of 2D zeolite precursors, as well as their derivatives, were discussed in parallel. The research attempts in the area of 2D zeolites in the past few decades suggest certain future directions.
The most prominent topic in the development of 2D zeolites remains the synthesis of these 2D zeolite structures. Though a great number of zeolite structures have been synthesized in practice, a small fraction has been successfully made into 2D structures. Among the available 2D zeolite materials, the majority have been found by accidental synthesis. The templated synthesis of 2D MFI and ADOR methodology for the synthesis of IPC-1P inspire the feasibility of designed synthesis of 2D zeolite materials. To increase the number of 2D zeolite structures, innovative and creative synthesis methods are required. Accompanying the synthesis of 2D zeolite precursor materials, the post-modification of these precursors to produce diversified 2D derivative structures is also needed. Swelling, delamination, pillaring, and interlayer expansion are the four major methods practiced in formation of 2D zeolite derivatives. Among nearly twenty 2D zeolite precursors, six of them (FER, MWW, MFI, NSI, RWR, and IPC) have been explored in the formation of 2D zeolite derivatives, but only two of them (FER and MWW) have been extensively studied in terms of these four post-modification methods. The application of these post-modification methods to all the existing 2D zeolite precursors would increase the number of members of the 2D zeolite family.
For the catalysis applications of 2D zeolite materials, the inclusion of heteroatoms into the zeolite precursor layers and derivative structures is vital to produce catalytic active sites. The chemical compositions of 2D zeolites, as shown by the statistics in Section 3, indicate that majority of current 2D zeolite precursors solely exist in the siliceous form. Only very few zeolites can be directly synthesized across the entire range of Si/Al ratios from 1 to infinity. The incorporation of heteroatoms such as Al, B, Ti, and Sn allows Brønsted and/or Lewis acidity to exist in 2D zeolite structures. Tactical synthesis by modifying the current recipes for producing 2D siliceous zeolites or by creating new procedures in the presence of heteroatoms in zeolite synthesis gels are needed in order to increase the compositional diversity in 2D zeolites. The post-modification of 2D zeolites can also provide a pathway to the inclusion of heteroatoms into their structures. The pillarization and inter-layer expansion methods have been used to bring heteroatoms such as Ti, B, and Sn into the zeolite pillars or frameworks, but this technique has been limited to only a few 2D zeolites. The diversification of heteroatom precursors and/or 2D zeolite precursors in post-modification processes will provide a great opportunity to create 2D zeolite derivatives with multiple types of acid sites and strengths.
Thirdly, an understanding of the fundamental catalytic acidity properties of 2D zeolites is required. Since zeolite micropores present diameters very close to the size of many molecules, the shape selectivity and molecular sieving are unique properties of 3D microporous zeolites in catalysis. As the characteristic length of the micropore domain shrinks at the single-or near single-unit-cell level in 2D zeolites and the fraction of external zeolite surface area becomes comparable to that of micropore surface area as a consequence, the catalytic properties of the former (if not deactivated) become important or dominant contributors. Though the textural properties of layered 2D zeolite structures and their catalytic applications have been studied, the quantitative evaluation of acidity and acid site locations has not been extensively studied. The discrimination of acid site location and strength, especially in comparison to those of 3D zeolite analogues, will build a fundamental connection to the performance in product selectivity and activity in catalysis. More importantly, the understanding of location of active sites in zeolite frameworks-inside of the channels versus on the external surface-could assist in the rational design of 2D zeolites with specific acid locations, pushing capabilities beyond those of accidental discovery.
Lastly, the advantages sustained by these 2D zeolites have been clearly demonstrated when compared to the activity seen in their 3D counterparts for certain catalytic reactions. However, there is still a lack of understanding regarding long-term, thermal, and hydrothermal stability of the more delicate layered or delaminated 2D zeolite structures. Hydrothermal stability has been investigated for micro-/mesoporous zeolites designed via organic structure-directing agents, resulting in the discovery of significantly lowered stability in these materials relative to their 3D equivalents [273]. This has been attributed to the amorphous structure developing around pore walls due to the SDA [274], but this phenomenon cannot be applied to the crystalline layered and single-sheet zeolite materials. The investigation into the hydrothermal stability of 2D zeolites under reaction conditions could determine their best-suited applications in catalysis and beyond to avoid structural breakdown. With extensive studies and achievements in the areas noted above, it is expected that 2D zeolite could substantially enhance and expand the possibilities of catalysis applications and further increase the widespread use of zeolites as catalysts in both laboratory studies and industry practice.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.