Interzeolite Transformations as a Sustainable Pathway to Zeolite Design: Structural Drivers, Activation Media, and Phase Selectivity

Abstract

Interzeolite transformation (IZT) has emerged as a versatile strategy for accessing zeolite frameworks through controlled framework reorganization under comparatively simplified synthesis conditions. Unlike traditional synthesis approaches that frequently require organic structure-directing agents (OSDAs), highly alkaline media, and prolonged thermal treatment, IZT converts pre-existing zeolite into a new topology, enabling direct reuse of crystalline matter while reducing synthesis complexity. This review examines how structural drivers, including framework density, structural memory, and building-unit compatibility, govern transformation pathways and phase selectivity across five principal transformation approaches: (i) solution-mediated, (ii) assembly–disassembly–organization–reassembly (ADOR), (iii) mechanically assisted, (iv) steam-assisted, and (v) fully solid-state systems. These approaches promote distinct transformation pathways that govern framework reconstruction, structural inheritance, and phase selectivity. Recent advances in solvent-free, mechanochemical, steam-assisted, and microwave-assisted synthesis demonstrate the potential of IZT to reduce solvent consumption, template usage, and crystallization times. Despite these advances, major challenges remain in predicting transformation outcomes, controlling transient intermediates, and establishing scalable and quantitatively validated sustainability metrics. Collectively, these developments position IZT as a promising platform for the rational and sustainable design of next-generation zeolitic materials.

1. Introduction

Zeolites are crystalline microporous aluminosilicates whose structural diversity, thermal robustness, and precisely defined pore networks determine their central role in catalysis, separations, and environmental remediation [1,2,3,4,5]. Their impact spans both established petrochemical processes and emerging sustainable technologies in green-oxidation chemistry [6,7,8]. Conventional hydrothermal and solvothermal syntheses, however, rely on complex precursor gels, elevated temperatures, long crystallization times, and frequently organic structure-directing agents (OSDAs), resulting in high energy demands, toxic reagent use, and substantial chemical waste [9,10]. These limitations have intensified efforts to develop more resource-efficient routes to zeolite materials [11,12,13].

Interzeolite transformation (IZT) has emerged as one of the most promising alternatives. Although early hydrothermal studies described framework “conversion” primarily as a recrystallization pathway rather than a targeted synthesis route [14], IZT was later recognized as a reproducible transformation process in which one zeolite framework converts into another under specific chemical or thermal conditions [15,16,17,18]. Initially, parent–daughter product relationships could be rationalized through framework topology, building-unit compatibility, and transformation energetics [19,20,21]. Seed-assisted strategies further revealed the importance of heterogeneous nucleation and cross-nucleation in directing these pathways [22,23].

Recent advances have expanded IZT into a versatile platform for alternative and potentially more resource-efficient zeolite synthesis. Low-temperature and even room-temperature transformations [16] and microwave-assisted routes [17] provide rapid, energy-saving pathways for zeolite conversion. Partial or composite interconversions [24,25] further enable the targeted generation of hybrid and hierarchical materials. Across these developments, IZT has diversified into multiple transformation environments and mechanistic pathways spanning solution-mediated to fully solid-state systems. However, despite these advances, a unified perspective that integrates mechanistic principles with sustainability considerations remains underdeveloped.

This review consolidates current knowledge on IZT from both mechanistic and sustainability viewpoints, highlights recent breakthroughs, and outlines opportunities for environmentally responsible design of zeolitic materials. The review is structured to move from mechanism to application. Section 2 reviews the five activation environments and the transformation pathways associated with each. Section 3 discusses how structural features such as framework density, secondary building units, and composite building units influence the evolution from parent to daughter phases and control the degree of structural memory preserved throughout the transformation process. Section 4 and Section 5 examine the chemical and process parameters that govern zeolite formation in practice, including the Si/Al ratio, alkalinity, the identity of charge-balancing cations, seeding strategies, and the use of organic structure-directing agents. Section 6 addresses sustainability considerations, and Section 7 outlines the most critical open questions that remain in the field.

2. Concept and Significance of IZT

IZT is a synthesis strategy in which a pre-existing zeolite framework is reorganized into a different topology under controlled chemical or physical stimuli [26]. Unlike conventional gel-based crystallization, which assembles frameworks from molecular precursors, IZT proceeds through the restructuring of an already-formed aluminosilicate network, allowing partial preservation of structural motifs during framework reconstruction. This process typically involves partial dissolution of the parent phase followed by nucleation and growth of the daughter phase, although the extent of dissolution and the degree of structural inheritance vary widely across systems.

Early studies on the conversion of kaolinite and related aluminosilicates into zeolites [27,28] demonstrated that crystalline frameworks can undergo topological reorganization without complete amorphization, thereby establishing the conceptual basis for IZT. Over the past decades, IZT has evolved into a versatile and controllable methodology capable of accessing metastable or otherwise difficult-to-obtain zeolite frameworks. In contrast to conventional gel-based synthesis, which often relies on complex precursor systems and OSDAs, IZT frequently allows the parent zeolite to act both as a source of reactive species and as a source of locally inherited structural motifs, facilitating rapid nucleation and growth and often resulting in reduced crystallization times and improved phase selectivity [16,29,30,31].

The significance of IZT extends beyond synthetic efficiency. By emphasizing framework reorganization rather than assembly from molecular precursors, IZT provides an alternative perspective on crystallization pathways and highlights the role of structural inheritance in directing phase evolution. From a practical standpoint, the reduced reliance on organic templates and simplified synthesis protocols contributes to improved material efficiency and decreased process complexity. Moreover, the ability to convert existing or end-of-life zeolite materials into new functional frameworks introduces opportunities for circular approaches to zeolite design [32,33,34].

2.1. Synthesis Approaches to IZT

IZTs can be classified according to the degree of molecular mobility in the activation medium, spanning solution-mediated, quasi-solid-state, and fully solid-state regimes. For clarity, the transformation systems discussed here are grouped according to their dominant activation environment or reconstruction mechanism into five commonly reported categories: (i) solution-mediated, (ii) ADOR, (iii) mechanically assisted, (iv) steam-assisted, and (v) fully solid-state pathways. Within each activation environment, multiple structural mechanisms—such as diffusionless rearrangement, competing phase formation, intergrowth development, and recrystallization [26]—may operate depending on the parent topology and chemical conditions. In the following sections, these modes are discussed separately to clarify their mechanistic distinctions and avoid conceptual overlap.

2.1.1. Solution-Mediated Transformations

Solution-mediated IZT—where the parent zeolite dissolves at least partially into the liquid phase, supplying soluble species that feed nucleation and growth of the daughter framework—is the most reported [26,35]. It typically proceeds through one of three scenarios: (1) partial dissolution leading to amorphous or pseudomorphic intermediates [36,37]; (2) partial dissolution without amorphous intermediates leading to zeolite-to-zeolite conversion [38]; or (3) cross-nucleation, in which the daughter zeolite nucleates on the surface of a different parent zeolite despite lacking structural similarity [39,40].

Figure 1 provides a schematic overview of solution-mediated IZTs, illustrating the three mechanistic scenarios—partial dissolution with amorphization, direct zeolite-to-zeolite conversion, and cross-nucleation—introduced above. Amorphous intermediates or partial dissolution have been observed in FAU → EDI [16], FAU → *BEA [41] and many other IZTs, where the parent phase first collapses to an amorphous phase that subsequently crystallizes into the daughter framework. In less common cases, such as the FAU → AEI/AFX or FAU → KFI [42,43] transformation, crystallization proceeds through partial dissolution of FAU followed by rapid consumption of the dissolved species, without formation of an amorphous intermediate [38,42].

Figure 1. Schematic representation of solution-mediated IZTs, in which the parent crystal may be partially dissolved to form an amorphous intermediate, partially dissolved without forming an amorphous phase, or provide a surface that enables cross-nucleation. Based on the literature cited in Section 2.1.1.

In dissolution–recrystallization pathways, the parent phase supplies reactive oligomeric species that accelerate nucleation. The final topology is governed by both thermodynamic stability and kinetic factors—solution composition, alkalinity, temperature, nucleation dynamics, the presence of seeds or OSDAs, and Ostwald-type phase evolution. What makes the FAU → EDI system particularly instructive is the competition between two processes usually discussed in isolation: amorphization of the parent and nucleation of the daughter. Ferdov confirmed that FAU passes through a detectable amorphous intermediate before EDI nucleation begins, captured directly by PXRD at both room temperature and 60 °C [16], establishing that structural collapse precedes crystallization of the product. What the data leave open is what that intermediate actually looks like internally. Rather than a fully structureless gel, it may retain some local ordering inherited from the parent—a partially disordered aluminosilicate network that carries enough structural memory to influence how and where EDI nuclei form. The pseudomorphic crystallization of hollow EDI particles reported in the same study points in this direction [16]: nucleation tracked the shape of the amorphous precursor closely enough to reproduce its morphology, which is hard to reconcile with a completely featureless gel. It remains an open question whether the amorphous phase retains enough structural memory to steer nucleation or bias the final topology. This aspect has received far less attention than the thermodynamic arguments that typically dominate discussions of transformation selectivity [26,35].

Beyond the intrinsic mechanistic factors governing dissolution and nucleation, several external parameters can accelerate or redirect solution-mediated IZT. Microwave irradiation accelerates transformation through rapid volumetric heating and enhanced supersaturation dynamics, enabling FAU conversion to MER, LTJ, CAN, and ANA within minutes rather than days [17]. Dealumination acts differently—by generating surface defects and mesopores, it increases the reactive surface area of the parent, lowering the effective activation barrier for framework dissolution [31]. Ultrasound enhances dissolution and mass-transfer rates and has been shown to accelerate LTA → SOD conversion [44,45], though the number of IZT systems studied under sonochemical conditions remains small; its potential for low-temperature radioiodine capture via CH₃I adsorption [46] provides motivation for further investigation. Cross-nucleation represents a mechanistically distinct pathway in which the parent does not fully dissolve but instead provides a heterogeneous nucleation surface for the daughter phase. This has been documented in CHA → AFI [39] and FAU → RHO [47] transformations. Molecular simulations confirm that cross-nucleation does not require shared SBUs or CBUs between parent and daughter—nucleation can proceed through transient interfacial layers that lower the energy barrier to critical-nucleus formation regardless of structural similarity between the two frameworks [40].

Together, these mechanisms show how solution-mediated IZT spans a continuum from dissolution-assisted recrystallization to interfacial nucleation. Partial dissolution enables direct zeolite-to-zeolite conversion, while deeper breakdown leads to reconstructive pathways governed by solution chemistry. Cross-nucleation adds a distinct interfacial route in which the parent phase acts as a heterogeneous nucleation surface rather than a source of dissolved species. The scope of solution-mediated transformation has recently been extended beyond aluminosilicate zeolites: in KOH solution, preformed ETS-4 particles transform into single crystals of GTS-1 through a reversed crystallization pathway initiated at the particle surface, establishing the first interzeolite-type transformation between microporous heteropolyhedral titanosilicates and demonstrating that crystal morphology can be templated through particle self-assembly [48].

2.1.2. Assembly–Disassembly–Organization–Reassembly (ADOR)

Although ADOR is sometimes treated separately from classical IZT due to its layer-selective reconstruction mechanism, it is included here on functional grounds: it begins with a crystalline zeolite and ends with a distinct, IZA-assigned framework, satisfying the simplest operational definition of a zeolite-to-zeolite transformation. Its products—OKO, PCR, and the IPC-series frameworks—are inaccessible by any conventional IZT route, so omitting it would leave a synthetically important family of transformations unaddressed. Where ADOR genuinely departs from the rest of the IZT landscape—in its requirement for germanosilicate parents, its bond-selective hydrolysis step, its layer-by-layer reconstruction mechanism, and its capacity for programmable pore architecture—these distinctions are explicitly noted in the following section.

The ADOR mechanism (Figure 2) is a selective topological reconstruction route that enables access to zeolite frameworks unobtainable by conventional hydrothermal synthesis [49,50,51]. It relies on the hydrolytic weakness of Ge-rich double four-ring (d4r) units in parent germanosilicates such as UTL, where Ge is preferentially located in positions that can be selectively cleaved during disassembly [49,50,51]. Hydrolysis of these Ge–O bonds proceeds rapidly in water or strong acids, showing fast Ge removal while the siliceous layers remain intact [50,51]. The resulting IPC-1P layers retain intralayer connectivity but expose silanols that allow controlled rearrangement governed by temperature, pH, added silicon, and time [52,53].

Figure 2. General scheme of the ADOR mechanism in IZT, in which a germanosilicate framework is selectively disassembled into layers, reorganized, and subsequently reassembled into a new three-dimensional structure. Based on the literature cited in Section 2.1.2.

Reassembly occurs via high-temperature silanol condensation, and the degree of prior organization determines the crystallinity and final topology [54], showing larger rearrangements for fully disconnected IPC-1P than for partially connected IPC-2P or IPC-6P [52]. The ADOR route yields daughter frameworks including IPC-2 (OKO) and IPC-4 (PCR) [55], as well as IPC-6 (*PCS), IPC-7, IPC-9, and IPC-10, which currently have no assigned IZA codes. In contrast to solution-mediated IZTs, ADOR proceeds through bond-selective disassembly followed by layer-based reassembly to generate new zeolite topologies.

2.1.3. Mechanical Activation and Defect-Driven Transformations

Mechanical activation has established itself as a practical route to IZT under reduced-solvent or solvent-free conditions [18,56]. Grinding and milling introduce lattice distortion, raising the internal energy of the parent zeolite and generating a defect-rich, partially amorphous surface that is far more reactive than the intact crystal. The consequences are well-documented across framework types: ZSM-5 loses substantial crystallinity and microporosity upon milling [57], while natural clinoptilolite undergoes fragmentation to nanoscale particles that retain useful adsorption capacity despite partial amorphization [58]. The mechanistic picture emerging from these and related studies [59,60,61,62,63,64,65,66,67] is that milling simultaneously creates strained, undercoordinated sites that lower the barrier to Si–O bond hydrolysis; shifts the Si/Al coordination distribution toward more reactive Q(1Al) and Q(2Al) environments; and in some cases, releases structurally bound water as a local crystallization medium. Taken together, these effects redirect the transformation away from the standard dissolution–reprecipitation sequence and toward pathways in which amorphous or defect-rich intermediates serve as the immediate precursors to the daughter phase. Mechanically pre-activated seeds accelerate this process further by providing heterogeneous nucleation sites and shortening induction periods [59,66].

Building on these principles, recent experimental works show that ball milling of parent zeolites such as FAU-type materials can dramatically promote their transformation into other frameworks including SOD, GIS, ANA, CAN, MER, LTA and CHA type structures [18,56]. These two studies reveal distinct but complementary mechanochemical routes that differ primarily in what governs phase selectivity (Figure 3). In the first route, the identity of the alkali cation is the dominant control variable: NaOH directs FAU toward ANA and CAN, KOH toward MER and CAN, and CsOH selectively toward ANA [18]. In the second route, using KOH throughout, phase selectivity is instead governed by the interplay of temperature, water content, and seeding—with lower water content and moderate temperature favoring MER, while higher water content combined with elevated temperature and CHA seeds enables selective formation of CHA [56]. Importantly, both routes involve mixed crystalline and amorphous intermediates depending on reaction conditions: transient crystalline phases (GIS, SOD, potassium silicate) and amorphous precursors are documented in both studies, with the nature of the intermediate governed by the specific conditions rather than the route identity. MER is accessible via both routes. These findings collectively show that mechanochemical activation can either preserve or erase structural inheritance depending on the extent of disorder introduced during milling and that the interplay of kinetic and thermodynamic factors under non-equilibrium milling conditions makes the contribution of each difficult to disentangle [18,56].

Figure 3. Comparison of two mechanochemical interzeolite conversion routes from FAU-type zeolite. In the cation-directed route [18], phase selectivity is governed by the identity of the alkali cation (Na⁺, K⁺, Cs⁺), yielding ANA, CAN, SOD, and MER as daughter phases. In the parameter-directed route [56], phase selectivity is controlled by the interplay of temperature, water content, and seeding, affording MER, CHA, and LTA. Both routes proceed through mixed crystalline and amorphous intermediates depending on reaction conditions. MER is accessible via both routes. Based on refs. [18,56].

2.1.4. Steam-Assisted Transformations

Steam-assisted conversion represents an intermediate regime between fully solution-mediated and fully solid-state transformation. In these systems, water is present primarily as vapor, allowing localized hydrolysis and short-range bond mobility while limiting long-range transport of dissolved species. As a result, structural inheritance effects are often enhanced relative to conventional hydrothermal synthesis. The FAU → CHA transformation exemplifies this behavior [68], as steam enables partial bond rearrangement and framework reorganization without fully dissolving the parent structure, allowing CHA to emerge through a combination of localized hydrolysis, defect migration, and short-range reconstruction. Steam-assisted conversion combined with mechanochemical activation is treated as a distinct category because mechanochemistry generates reactive defects, whereas steam provides the chemical driving force for crystallization—two fundamentally different mechanisms that act sequentially rather than as a single unified process. Mechanochemical activation coupled with steam enables rapid conversion of FAU to GME (20 h) and other frameworks (MOR, MFI, ANA) [69]. The approach extends to complex and waste-derived systems, as demonstrated by the conversion of coke-containing spent MFI into hierarchical CHA via steam-assisted dry-gel conversion, where residual carbon directs mesoporosity and limits crystal growth [33]. More broadly, vapor-phase crystallization pathways—such as the steam-driven evolution of coal fly ash from sodalite to cancrinite/nosean—highlight the ability of water vapor alone to mediate sequential framework transformations under mild conditions [70]. Figure 4 summarizes the main steam-assisted and mechano-steam-assisted IZTs discussed above, highlighting the distinct mechanistic regimes and representative FAU- and MFI-derived pathways. Overall, steam-assisted pathways show that IZT can proceed efficiently in highly confined environments, where limited water availability suppresses extensive dissolution but still enables framework rearrangement and crystallization.

Figure 4. Main steam- and mechano-steam-assisted IZTs. Based on the literature cited in Section 2.1.4.

One persistent challenge that cuts across all five categories is the blurring of category boundaries in practice. Many systems reported as “steam-assisted” involve a non-trivial amount of residual liquid water, and “solvent-free” conditions often rely on adsorbed surface moisture that is difficult to quantify. More rigorous operando measurement of actual water activity during transformation—rather than nominal addition amounts—would sharpen mechanistic distinctions and improve cross-laboratory reproducibility. More broadly, virtually all mechanistic descriptions of IZT currently rely on ex situ snapshots; real-time in situ PXRD, total scattering, and solid-state NMR studies are needed to observe transient intermediates directly and establish whether competing pathways operate sequentially or in parallel.

2.1.5. Pressure- and Thermally Driven Transformations

A smaller but mechanistically important class of solid-state transformations involves direct framework rearrangement induced by pressure or thermal activation—collectively referred to as “fully solid-state systems” in the Abstract, and distinct from both the ball-milling-based mechanically assisted IZT of Section 2.1.3 and the steam-assisted IZT of Section 2.1.4. In these systems, bond breaking and reconstruction occur without conventional dissolution–reprecipitation processes. The pressure-induced transformation of ITQ-29 into ITQ-50 provides a representative example [71]. Rather than undergoing irreversible amorphization, the framework experiences a reconstructive crystalline-to-crystalline transition under pressure, preserving long-range order while reorganizing connectivity. Related behavior has also been reported in zeolite-like titanosilicates (R-GTS-1), where reversible thermally induced symmetry changes occur through framework distortion and reorganization [72], and in LTA-SOD transformation [73]. These observations demonstrate that structural collapse in porous frameworks does not necessarily lead to irreversible amorphization and may instead provide access to alternative crystalline states. Comparable pressure-induced disordering phenomena have also been reported in related porous crystalline materials, including zeolitic imidazolate frameworks (ZIFs), although these systems fall outside the strict definition of IZT [74,75,76]. Their inclusion here serves only to illustrate broader principles of framework flexibility, defect generation, and reconstructive behavior in porous solids. A complementary expression of this continuum emerges under thermal collapse regimes. In zeolite-like transition-metal silicates such as ETS-4 [72] and CuSH-1Na [77], thermal amorphization proceeds along an expansive trajectory, generating ultralight macroporous foams through progressively lower-density bulk amorphous states rather than densification as described for LTA and FAU zeolites [78,79]. Together, these observations illustrate that framework collapse and reconstruction in porous solids may proceed through multiple crystalline or amorphous intermediates under non-hydrothermal conditions (Figure 5).

Figure 5. Schematic presentation of scenarios of pressure- and thermally driven framework rearrangements in solid state synthesis conditions. Based on the literature cited in Section 2.1.5.

3. Structural Relationships Between Parent and Daughter Zeolites in IZTs

Note on dataset assembly: The dataset underlying Table 1 and Figure 6 and Figure 7 was assembled by systematic survey of peer-reviewed publications reporting crystallographically confirmed IZTs. Cases were included where the parent was a crystalline zeolite framework, the product matched a confirmed IZA framework type, and the synthesis conditions (temperature, time, and activation medium) were clearly reported. Cases with amorphous or non-zeolitic parents or with unidentified products were excluded. Framework density values come from the IZA Structure Commission database; Secondary Building Unit (SBU) and Composite Building Unit (CBU) assignments follow IZA conventions. Where the same parent–daughter pair was reported by multiple independent groups or under distinct conditions, each entry was counted separately so that the full accessible parameter space is represented. The dataset is therefore representative rather than exhaustive, and readers wanting the most current inventory should consult the IZA Synthesis Commission database directly.

Figure 6. IZT behavior of zeolite parent phases and temperature distribution of conversion processes. (a) Number of IZTs observed for each parent zeolite phase, categorized by framework density (FD) change. (b) Histogram of most frequent IZT temperatures. The data underlying this figure were compiled and analyzed by using the literature summarized in Table 1.

Figure 7. Structural inheritance and synthesis effects of IZTs. (a) Relative prevalence of transformations preserving common SBUs and CBUs (SOLU—solution, MECHA—mechanical (includes water-deficient conditions, see Table 1 footnote), STEMA—steam, SOLFRE—solvent-free, SOLSTA—solid-state). (b) Frequency distribution of preserved SBUs across reported IZTs. (c) Relationship between framework density (FD) evolution and preservation of common SBUs and CBUs. (d) Number of IZTs obtained by different methods. Data compiled and analyzed from the literature summarized in Table 1.

3.1. Framework Density Evolution and Thermodynamic Trends

IZTs are shaped not only by chemical environment and reaction kinetics but also by intrinsic structural relationships between parent and daughter frameworks. Two descriptors—framework density (FD) and structural building units—provide a basis for rationalizing transformation pathways. As shown in Table 1 and Figure 6a, most IZTs produce daughter frameworks with higher FD, suggesting a frequent tendency toward densification across reported systems. However, this trend is partially influenced by the prevalence of low-FD FAU-type parents. Importantly, deviations such as LEV → CHA [80], LTL → CHA [81,82], MFI → CHA [33,83], FER → CHA [84], and others demonstrate that FD alone does not uniquely determine transformation outcomes: kinetic factors, structural memory, or specific intermediates can override the thermodynamic drive and stabilize lower-density topologies, particularly for ANA, FER, LTL, MFI, and MOR-based parents. The temperature distribution provides additional mechanistic insight (Figure 6b). Most reported transformations occur between 120 and 180 °C, conditions commonly associated with hydrothermal dissolution–reprecipitation pathways. Lower-temperature events correspond predominantly to mechanochemical or ADOR processes, where activation barriers are reduced through defect generation, bond-selective hydrolysis, or preorganized layered intermediates. Conversely, the high-temperature tail is consistent with highly reconstructive transformation conditions involving extensive framework rearrangement.

3.2. Structural Inheritance and Building-Unit Compatibility

Structural relationships between zeolite frameworks are commonly described using secondary building units (SBUs) and composite building units (CBUs) [85]. These descriptors capture the degree of structural inheritance between parent and daughter frameworks and influence transformation mechanisms. Figure 7a shows that transformations involving shared SBUs occur more frequently than those involving CBUs. This suggests that local secondary structural motifs are more readily preserved during framework reorganization. Although daughter phases most commonly exhibit higher framework density than their parent structures, the comparatively low frequency of CBU inheritance indicates limited preservation of long-range framework connectivity during densification.

Figure 7b shows that 4-membered and 6-membered ring SBUs are the most frequently preserved structural motifs across reported IZT systems. Their high prevalence suggests that these geometrically simple and energetically stable units function as persistent reconstruction intermediates during framework evolution. In contrast, more complex SBUs such as 6-2 and 6-6 motifs occur far less frequently and are generally associated with specific transformation families, particularly FAU-derived systems.

Table 1 also shows that the majority of transformations lack common SBUs or CBUs, indicating that large-scale framework reconstruction is common in IZT, even though local structural motifs may still influence nucleation behavior and intermediate stability. Representative examples include FAU → MFI [30], FAU → EDI [16], FAU → LTJ [17] and *BEA → AFI [86], all of which proceed without structural inheritance. In contrast, the minority of transformations retain common SBUs or CBUs and proceed through topotactic or pseudotopotactic pathways, including FAU → CHA, FAU → AEI, FAU → AFX, LEV → CHA, and LTL → CHA. In these cases, shared structural motifs such as 4-rings and 6-rings act as structural memory elements that reduce nucleation barriers and accelerate crystallization.

Beyond motif preservation, framework density evolution provides insight into the thermodynamic direction of IZT processes. The predominance of higher-FD daughter phases indicates that densification frequently correlates with observed transformation pathways and may reflect an underlying thermodynamic preference across both reconstructive and partially inherited transformation pathways (Figure 7c), suggesting that framework density primarily reflects thermodynamic driving forces rather than the degree of structural inheritance. In contrast, transformations toward lower FD frameworks are comparatively rare and exhibit limited motif preservation, consistent with more extensive reconstructive rearrangement.

3.3. Influence of Activation Medium on Transformation Pathways

Beyond structural inheritance, the activation medium strongly influences the mechanistic pathway of IZT. Solution-mediated systems generally promote dissolution–reprecipitation processes and therefore favor reconstructive transformations [35,36]. In solution-mediated IZT, few variables matter more than the molar concentration of the alkaline activator. Higher OH⁻ concentrations accelerate Si–O–T bond hydrolysis across the FAU framework, raising the pool of soluble silicate and aluminate species available for renucleation [26,35]. At low alkalinity, dissolution proceeds slowly and the system remains close to thermodynamic equilibrium, typically leading to the most stable framework accessible under these conditions [87]. At intermediate alkalinity, as in FAU → CHA [29,88] and FAU → EDI [16] syntheses—where the reaction window is set by a narrow near-saturation KOH concentration rather than a broad alkalinity range—the kinetics are sufficiently fast to stabilize metastable products that would not form at higher OH⁻ concentrations. At high alkalinity, rapid and extensive dissolution delivers a highly reactive gel from which dense frameworks such as ANA and SOD nucleate preferentially [87,89]. The identity of the alkali cation matters independently of concentration: Na⁺ strongly favors sodalite- and cancrinite-cage architectures [87]; K⁺ promotes larger-cage products such as LTL and MAZ [90], as well as OFF under supplemental NaOH conditions [91], while Cs⁺ and mixed-cation systems can stabilize RHO or KFI frameworks whose cage dimensions match the cation radius more closely [42,43]. Achieving a target framework therefore requires attention to both the concentration and the identity of the activator simultaneously. The influence of synthesis temperature deserves separate consideration. Room-temperature IZTs—the FAU → EDI conversion being the clearest example [16]—operate through an unusual dissolution–renucleation pathway that entirely bypasses the thermal activation required for conventional hydrothermal synthesis, and they tend to yield metastable, Al-rich products that would collapse to denser phases (ANA, SOD) at higher temperatures. Conventional hydrothermal treatment (80–180 °C, autogenous pressure) remains the most widely studied regime and generally favors dissolution–reprecipitation with progressively denser daughter frameworks as temperature increases [35,90]. Sol–gel preconditioned routes—in which the parent zeolite is selectively dealuminated or desilicated before transformation—offer additional compositional leverage by tuning the Si/Al ratio of the reactive medium prior to conversion [92,93,94]. Microwave-assisted synthesis compresses crystallization from days to minutes through volumetric heating, without fundamentally altering the underlying dissolution–renucleation mechanism [17]. Taken together, these regimes span a wide continuum from room temperature to >200 °C and from aqueous solution to solvent-free conditions, each imposing distinct constraints on dissolution rate, nucleation kinetics, and the degree of structural inheritance retained in the product; representative examples spanning this continuum include FAU → MOR [90], FAU → STF [29], and *BEA → CHA [95]. Figure 7d shows that solution-mediated synthesis is the dominant approach for transformations. Mechanical, steam-assisted, and solvent-free systems also exhibit partial preservation of SBUs, although less frequently, indicating that short-range structural inheritance can persist even under reduced-solvent or solid-state conditions. These trends demonstrate that the degree of structural preservation is strongly influenced by the synthesis route. Unlike most reconstructive IZT pathways, ADOR-type processes preserve larger framework fragments and therefore exhibit enhanced retention of structural connectivity. In contrast, quasi-solid-state environments such as steam-assisted conversion enable limited hydrolysis and short-range bond mobility while suppressing long-range transport. These conditions amplify the influence of inherited SBUs and CBUs, as observed in FAU → CHA [68]. Fully solid-state and mechanochemical activation introduce defects, strain, and local disorder that lower nucleation barriers and can either enhance or erase structural memory depending on the energy-input pathway, as demonstrated in FAU → ANA [17,18,87], FAU → GIS [18,90], FAU → SOD [18,89], and LTA → IFR [71]. Thus, activation mode strongly influences the balance between amorphization, reconstructive reassembly, and topotactic rearrangement and must be considered alongside FD and structural inheritance when rationalizing IZT behavior.

3.4. Crystallization Kinetics and Energy Input

In addition to influencing transformation mechanisms, synthesis methodology also strongly affects crystallization kinetics, as evidenced by the temperature–time data summarized in Table 1. Conventional hydrothermal synthesis (80–180 °C) typically requires hours to days, particularly for reconstructive transformations lacking structural inheritance. Microwave-assisted synthesis reduces crystallization times to minutes (e.g., FAU → ANA, FAU → MER, FAU → LTJ) [17], reflecting rapid heating and supersaturation effects. Mechanochemical activation provides an intermediate regime, where defect generation and enhanced reactivity enable phase formation within hours under reduced thermal input (e.g., FAU → ANA, FAU → GIS, FAU → SOD, FAU → MER, FAU → CHA, FAU → LTA) [18,56]. All these trends show that synthesis methodology strongly influences crystallization kinetics and the extent of structural inheritance.

The framework-density and building-unit analyses presented here are useful for rationalizing known transformations, but their predictive power is limited. Given a known transformation, FD change and SBU/CBU overlap can explain why it occurred and why a particular structural pathway was favored—but they cannot predict this in advance. What they cannot yet do is predict, before the experiment, which daughter frameworks will form from an untested parent under new conditions. The apparent prevalence of densification in the dataset also likely carries a publication bias: well-crystallized, phase-pure products are far more commonly reported than failed transformations and amorphous residues, and the latter are systematically underrepresented. Future data compilations that explicitly log null results and failed attempts would substantially improve the statistical reliability of any trend extracted from this kind of survey.

Table 1. Qualitative comparison of framework-density (FD) changes, the presence of common composite building units (CBUs) in the parent and daughter phases, activation media, temperature and time of IZTs.

Parent	Daughter	FD (Daughter)	Common SBUs	Common CBUs	Synthesis	Temp.(°C)	Time	Ref.
ANA	CAN	lower	no	no	Solution	410–460	2–4 d	[14]
ANA	LEV	lower	6	no	Solution	280	1 d	[14]
ANA	SOD	lower	no	no	Solution	350–450	4 d	[14]
AFI	CHA	lower	no	no	Solution	200	16 h	[96]
*BEA	AEI	no change	4	no	Solution	140	24–30 h	[97]
*BEA	AFI	higher	no	no	Solution	150–180	1–3 d	[86]
*BEA	CHA	no change	no	no	Solution	150	6 d	[95]
*BEA	MFI	higher	no	mor	Solution	150	24–40 h	[29]
CHA	AFI	higher	4	no	Solution	150–180	22–72 h	[39,40]
FAU	ABW	higher	4	no	Solution	95	96 h	[87]
FAU	AEI	higher	6, 4	no	Solution	150–170	1–3 d	[38,98]
FAU	AFX	higher	4	d6r	Solution	140–210	10 min–4 d	[38,99,100]
FAU	ANA	higher	6-2, 6, 4	no	Mechanical/ Steam/ Solution	95–170	15–120 min	[17,18,69,87]
FAU	*BEA	higher	no	no	Solution	140	6 h	[41]
FAU	CAN	higher	6, 4	no	Solution/ Mechanical	110–160	15–120 min	[17,18]
FAU	CHA	higher	6, 4	d6r	Solution/ Steam/ Mechanical/ Solvent-free	120–180	12–48 h	[29,47,56,68,88,101] *
FAU	GME	higher	6, 4	no	Solution/ Mechanical/ Steam	120–240	20 h–14 d	[69,102]
FAU	EDI	higher	no	no	Solution	RT-160	10 min–35 d	[16,17]
FAU	EMT	no change	6-6, 6-2, 6, 4	d6r, sod	Solution	140	6 h	[100]
FAU	ERI	higher	4	d6r	Solution	150	8 h	[103]
FAU	FER	higher	no	no	Solution	150	1 d	[90]
FAU	GIS	higher	4	no	Solution/ Mechanical	110–125	2–24 h	[18,90]
FAU	HEU	higher	no	no	Solution	240	14 d	[102]
FAU	KFI	higher	6-6, 6-2, 4-2, 6	d6r	Solution	140	3–4 d	[42,43]
FAU	LEV	higher	6	d6r	Solution	125	3 d	[90]
FAU	LTA	higher	6-2, 6	sod	Solution/ Mechanical	80–130	1–48 h	[56,104] *
FAU	LTL	higher	no	d6r	Solution	125	7 d	[90]
FAU	LTJ	higher	no	no	Solution	160	15 min	[17]
FAU	MAZ	higher	no	no	Solution	70	14 d	[90]
FAU	MER	higher	no	no	Solution/ Mechanical	95–160	15 min–96 h	[17,18,56,87] *
FAU	MFI	higher	no	no	Solution	190	32 h	[30]
FAU	MOR	higher	no	no	Solution/ Mechanical/ Steam	125	7 d	[69,90]
FAU	MSE	higher	no	no	Solution	160	3 d	[105]
FAU	MTN	higher	no	no	Solution	170	10 d	[21]
FAU	MTW	higher	no	no	Solution	150	40 h	[29]
FAU	OFF	higher	6, 4	d6r	Solution	125	1 d	[91]
FAU	PHI	higher	4	no	Solution	240	14 d	[102]
FAU	RUT	higher	no	no	Solution	140–170	2–6 d	[20]
FAU	RHO	higher	4	no	Solution	150	0.5–5 d	[47]
FAU	SOD	higher	6	sod	Solution/ Mechanical	110–160	2 h–3 d	[18,89]
FAU	STF	higher	no	no	Solution	150	40 h	[29]
FAU	YFI	higher	no	no	Solution	160	165 h	[106]
FER	CHA	lower	no	no	Solution	160	6 d	[84]
GIS	ANA	higher	4	no	Solution	140	3 d	[107]
GIS	CHA	lower	4	no	Solution	135	40–168 h	[108]
LEV	ANA	higher	6	no	Solution	200	24 h	[80]
LEV	CHA	lower	6	d6r	Solution	125	1.5 h	[80]
LTA	GIS	higher	4	no	Solution	82	13 d	[109]
LTA	IFR	higher	6-2	no	Solid-state	RT	N/A	[71]
LTA	SOD	higher	6	sod	Solution/ Mechanical/ Solid-state	50–800	30 min–15 h	[18,44,45,73]
LTL	CHA	lower	no	d6r	Solution	155–170	24–120 h	[81,82]
MFI	AEI	lower	no	no	Solution	140	1–2 d	[97]
MFI	CHA	lower	no	no	Solution/Steam	160	6 d	[33,83]
MOR	ANA	higher	no	no	Solution	180	1–5 d	[37]
MOR	*BEA	lower	no	mor	Solvent-free	160	1 d	[110]
MOR	CAN	lower	no	no	Solution	180	5 d	[37]
MWW	*BEA	higher	no	no	Solvent-free	190	2 h	[111]
STI	FAU	lower	no	no	Solution	82	4 h	[112]
UTL	OKO	higher	no	mor, mtt, cas, non, fer, ton	ADOR	95	16 h	[51,52]
UTL	PCR	higher	no	mor, mtt, cas, fer, ton	ADOR	90	16 h	[51,52]
UTL	*PCS	higher	no	mor, mtt, cas, fer, ton	ADOR	RT-95	16 h	[52,113]

* Water-deficient (near-dry) mechanochemical conditions; no bulk solvent added [56]; RT—room temperature.

Table 1. Qualitative comparison of framework-density (FD) changes, the presence of common composite building units (CBUs) in the parent and daughter phases, activation media, temperature and time of IZTs.

Parent	Daughter	FD (Daughter)	Common SBUs	Common CBUs	Synthesis	Temp.(°C)	Time	Ref.
ANA	CAN	lower	no	no	Solution	410–460	2–4 d	[14]
ANA	LEV	lower	6	no	Solution	280	1 d	[14]
ANA	SOD	lower	no	no	Solution	350–450	4 d	[14]
AFI	CHA	lower	no	no	Solution	200	16 h	[96]
*BEA	AEI	no change	4	no	Solution	140	24–30 h	[97]
*BEA	AFI	higher	no	no	Solution	150–180	1–3 d	[86]
*BEA	CHA	no change	no	no	Solution	150	6 d	[95]
*BEA	MFI	higher	no	mor	Solution	150	24–40 h	[29]
CHA	AFI	higher	4	no	Solution	150–180	22–72 h	[39,40]
FAU	ABW	higher	4	no	Solution	95	96 h	[87]
FAU	AEI	higher	6, 4	no	Solution	150–170	1–3 d	[38,98]
FAU	AFX	higher	4	d6r	Solution	140–210	10 min–4 d	[38,99,100]
FAU	ANA	higher	6-2, 6, 4	no	Mechanical/ Steam/ Solution	95–170	15–120 min	[17,18,69,87]
FAU	*BEA	higher	no	no	Solution	140	6 h	[41]
FAU	CAN	higher	6, 4	no	Solution/ Mechanical	110–160	15–120 min	[17,18]
FAU	CHA	higher	6, 4	d6r	Solution/ Steam/ Mechanical/ Solvent-free	120–180	12–48 h	[29,47,56,68,88,101] *
FAU	GME	higher	6, 4	no	Solution/ Mechanical/ Steam	120–240	20 h–14 d	[69,102]
FAU	EDI	higher	no	no	Solution	RT-160	10 min–35 d	[16,17]
FAU	EMT	no change	6-6, 6-2, 6, 4	d6r, sod	Solution	140	6 h	[100]
FAU	ERI	higher	4	d6r	Solution	150	8 h	[103]
FAU	FER	higher	no	no	Solution	150	1 d	[90]
FAU	GIS	higher	4	no	Solution/ Mechanical	110–125	2–24 h	[18,90]
FAU	HEU	higher	no	no	Solution	240	14 d	[102]
FAU	KFI	higher	6-6, 6-2, 4-2, 6	d6r	Solution	140	3–4 d	[42,43]
FAU	LEV	higher	6	d6r	Solution	125	3 d	[90]
FAU	LTA	higher	6-2, 6	sod	Solution/ Mechanical	80–130	1–48 h	[56,104] *
FAU	LTL	higher	no	d6r	Solution	125	7 d	[90]
FAU	LTJ	higher	no	no	Solution	160	15 min	[17]
FAU	MAZ	higher	no	no	Solution	70	14 d	[90]
FAU	MER	higher	no	no	Solution/ Mechanical	95–160	15 min–96 h	[17,18,56,87] *
FAU	MFI	higher	no	no	Solution	190	32 h	[30]
FAU	MOR	higher	no	no	Solution/ Mechanical/ Steam	125	7 d	[69,90]
FAU	MSE	higher	no	no	Solution	160	3 d	[105]
FAU	MTN	higher	no	no	Solution	170	10 d	[21]
FAU	MTW	higher	no	no	Solution	150	40 h	[29]
FAU	OFF	higher	6, 4	d6r	Solution	125	1 d	[91]
FAU	PHI	higher	4	no	Solution	240	14 d	[102]
FAU	RUT	higher	no	no	Solution	140–170	2–6 d	[20]
FAU	RHO	higher	4	no	Solution	150	0.5–5 d	[47]
FAU	SOD	higher	6	sod	Solution/ Mechanical	110–160	2 h–3 d	[18,89]
FAU	STF	higher	no	no	Solution	150	40 h	[29]
FAU	YFI	higher	no	no	Solution	160	165 h	[106]
FER	CHA	lower	no	no	Solution	160	6 d	[84]
GIS	ANA	higher	4	no	Solution	140	3 d	[107]
GIS	CHA	lower	4	no	Solution	135	40–168 h	[108]
LEV	ANA	higher	6	no	Solution	200	24 h	[80]
LEV	CHA	lower	6	d6r	Solution	125	1.5 h	[80]
LTA	GIS	higher	4	no	Solution	82	13 d	[109]
LTA	IFR	higher	6-2	no	Solid-state	RT	N/A	[71]
LTA	SOD	higher	6	sod	Solution/ Mechanical/ Solid-state	50–800	30 min–15 h	[18,44,45,73]
LTL	CHA	lower	no	d6r	Solution	155–170	24–120 h	[81,82]
MFI	AEI	lower	no	no	Solution	140	1–2 d	[97]
MFI	CHA	lower	no	no	Solution/Steam	160	6 d	[33,83]
MOR	ANA	higher	no	no	Solution	180	1–5 d	[37]
MOR	*BEA	lower	no	mor	Solvent-free	160	1 d	[110]
MOR	CAN	lower	no	no	Solution	180	5 d	[37]
MWW	*BEA	higher	no	no	Solvent-free	190	2 h	[111]
STI	FAU	lower	no	no	Solution	82	4 h	[112]
UTL	OKO	higher	no	mor, mtt, cas, non, fer, ton	ADOR	95	16 h	[51,52]
UTL	PCR	higher	no	mor, mtt, cas, fer, ton	ADOR	90	16 h	[51,52]
UTL	*PCS	higher	no	mor, mtt, cas, fer, ton	ADOR	RT-95	16 h	[52,113]

* Water-deficient (near-dry) mechanochemical conditions; no bulk solvent added [56]; RT—room temperature.

4. FAU-Derived Transformation Pathways and Reaction Environment

IZTs from FAU zeolites are governed mainly by the composition of the parent framework, particularly the Si/Al ratio, as well as the alkalinity and type of cations present in the reaction medium. The Si/Al ratio strongly influences the composition and reactivity of the FAU-derived aluminosilicate oligomers that mediate restructuring, thereby determining both the feasibility and direction of conversion.

Beyond the type of daughter framework, the Si/Al ratio of the parent FAU has a direct bearing on the physicochemical properties of the product [93]. The aluminosilicate oligomers released during FAU dissolution carry a composition that closely mirrors the parent ratio [26,35], so a low-silica FAU (Si/Al ≈ 2.6–3) generates predominantly Al-rich species that template small-pore frameworks with dense Al distributions and high ion-exchange capacity. High-silica FAU (Si/Al > 10) tends to release more siliceous oligomers during transformation, which often leads to the formation of silica-rich daughter phases such as CHA or AEI [29,38]. These frameworks are generally more hydrothermally stable and therefore better suited for applications such as NH₃-SCR in automotive exhaust treatment [114,115]. At intermediate Si/Al ratios, the oligomer population is heterogeneous, and the transformation is correspondingly less selective, often yielding phase mixtures or frameworks with broad Al distributions [93,94]. Dealuminated FAU precursors have been used to target catalyst-grade SSZ-13 with Si/Al ratios appropriate for SCR, demonstrating that precursor composition is a practical lever for tuning product performance rather than merely a synthetic variable [116,117].

Low-silica FAU (Si/Al ≈ 2.6–3) consistently yields Al-rich small-pore frameworks, most commonly CHA or EDI, particularly under strongly basic or Al-supplemented conditions [16,17,87]. In contrast, high-silica FAU (Si/Al > 10) can generate more siliceous CHA or OFF, provided that OH⁻ concentration, alkali cation (Na⁺, K⁺, TMA⁺, Li⁺), and OSDA choice are tuned to match the Si/Al ratio of the parent and the crystallizing product [91,92,116,117,118,119]. Under strongly basic conditions, extensive Si dissolution can partially decouple product Si/Al from the parent, enabling “unusual” Si/Al values in CHA and OFF when oligomer reactivity is appropriately modulated [91,116,118]. Structural compatibility, especially the presence of shared D6R and 4- and 6-ring motifs between FAU and CHA/OFF frameworks, may promote nucleation and growth by improving the compatibility of released oligomers during reconstruction [68]. Although ZK-5 (KFI) also contains D6R units, the FAU → ZK-5 transformation does not proceed through D6R inheritance because FAU loses all crystallinity before ZK-5 nucleation. In this system, the parent framework dissolves completely, and ZK-5 crystallizes through an anion-templated, OSDA-free pathway directed by K⁺ and NO₃⁻. As a consequence of this solution-mediated mechanism, the Si/Al ratio of ZK-5 reflects the composition of the dissolved aluminosilicate species rather than direct transfer of the parent FAU Si/Al ratio [42].

Figure 8 illustrates the general effect of the initial FAU Si/Al ratio and reaction environment on IZT outcomes, summarizing the pathways described above. These observations establish that FAU Si/Al ratio and reaction environment strongly influence the likelihood of specific transformation pathways. The patterns described here are useful, but predicting transformation outcomes from FAU Si/Al ratio alone remains out of reach. Most studies change one precursor variable at a time, which makes it nearly impossible to separate the intertwined effects of Si/Al ratio, alkalinity, and cation identity on how oligomers speciate and where nucleation begins. Running factorial experiments across these variables—backed by in situ ²⁹Si NMR of the reactive medium—would put the framework on much stronger predictive footing.

Figure 8. Generalized influence of initial FAU Si/Al ratio and reaction environment on likely IZT pathways. Based on analysis of the data compiled in Table 1 and literature cited therein.

5. Chemical Modifiers and Process Control in IZT

5.1. OSDAs Versus Inorganic Cations: Insights from Bruter et al. [35] Dataset

Bruter et al. [35] compiled a systematic dataset of IZT cases that allows direct comparison between OSDA-directed and inorganic-cation-directed transformation pathways. The dataset shows that inorganic cations can direct the formation of dense frameworks from FAU without any organic template, while OSDAs are generally required when the target is a low-density or metastable phase [35]. Within the dataset, cesium promotes the formation of ANA from FAU, while Li⁺, Rb⁺, and K⁺ direct crystallization of ABW, MER, and CHA without the need for organic templates [87]. Switching from organic to inorganic directing agents carries real process advantages. Replacing TMAda or related quaternary ammonium templates with inorganic cations removes a major cost and waste driver: the FAU → MSE synthesis time drops from 16 days to 3 days when the framework itself supplies the reactive aluminate and silicate species, and the calcination step needed to burn out the organic is avoided entirely [105]. Atom economy improves for the same reason—heteroatoms already present in the parent lattice carry over into the product rather than being resupplied from an external source [93]. Where partial densification accompanies the transformation, secondary mesoporosity can also develop in situ, without a separate leaching or steaming step [32]. Overall, these observations show that OSDAs are essential for stabilizing low-density or metastable frameworks, whereas inorganic cations favor simpler, more sustainable routes toward dense zeolite structures [35,87]. IZT therefore provides a potential route toward reduced template usage and simplified synthesis conditions [29,88].

5.2. Seeding and Template-Free Strategies

Three classes of chemical modifiers shape IZT outcomes: heteroatoms incorporated into the framework, organic or inorganic structure-directing agents, and seeds. Each operates through a distinct mechanism and they are not always separable in practice. Heteroatom substitution alters both framework stability and the reactivity of dissolved species during transformation. Zn incorporation into FAU, for instance, stabilizes the metastable parent over a wider temperature window and suppresses the FAU → GIS conversion that would otherwise dominate under mildly alkaline conditions [120]. Al distribution in the parent framework influences oligomer speciation during dissolution, which in turn controls nucleation selectivity and the acid site density of the daughter phase [93]. The same principle extends to isomorphous substitution with Sn and Ti: IZT routes to Sn-Beta and Ti-Beta are feasible and yield materials with Lewis acidity comparable to those obtained by conventional synthesis, provided that seeds and structural compatibility between parent and target are accounted for [110,121]. On the organic template side, the central tension is between selectivity and sustainability. OSDAs such as TMAda⁺ and piperidine derivatives reliably stabilize low-density frameworks like CHA, AEI, and LEV by filling cage voids and suppressing competing nucleation, but they add cost, generate calcination waste, and require handling of corrosive quaternary ammonium solutions [11,12,13]. Template-free and low-template IZT routes—FAU → MFI, MOR → Ti-Beta, USY → ZK-5, among others—demonstrate that the structural motifs inherited from the parent zeolite can substitute for the OSDA’s directing function in many cases, particularly when the parent–daughter pair shares secondary building units [15,29,42,43,110]. Tuning the NaOH/SiO₂ ratio and Si/Al content of the parent typically suffices to steer nucleation without organic additives, though the product is sometimes less phase-pure than the OSDA-directed analogue [29,94]. Seeding introduces a complementary handle on nucleation that is compatible with both OSDA-containing and template-free conditions. Seeds accelerate crystallization and narrow the particle size distribution, and in several systems, they make IZT feasible where it would otherwise stall [42]. What makes seeding in IZT distinct from conventional seed-assisted synthesis is that seeds frequently dissolve and contribute their structural fragments to the reactive pool rather than acting as passive growth templates—the structural memory effect [15]. Molecular simulations of cross-nucleation systems support this picture: even seeds lacking shared composite building units with the target framework can accelerate nucleation through transient interfacial layers that lower the energy barrier to critical-nucleus formation [40]. Microwave-assisted IZT and the deliberate use of partially converted interzeolite intermediates extend the toolbox further, compressing crystallization times and enabling access to products that are difficult to obtain under conventional thermal conditions [16,17]. Figure 9 summarizes how these modifier classes interact during a typical IZT sequence.

Figure 9. Chemical modifiers and process control in IZTs, illustrating how heteroatoms, organic and inorganic structure-directing agents, and seeds collectively influence nucleation and framework evolution during IZT. Based on the literature cited in Section 5.

A recurring shortcoming in the IZT literature on chemical modifiers is that most studies vary only one parameter at a time—one cation type, one OSDA species, one seeding concentration—while everything else is held constant. This makes it genuinely difficult to disentangle coupled effects. Changing the alkali cation, for instance, does not simply alter the “directing” species; it simultaneously shifts the solution pH, the ionic strength, and the solvation environment of aluminosilicate oligomers in the reactive gel. Systematic factorial design experiments, backed by in situ ²⁹Si NMR speciation of the reaction mixture, would yield clearer mechanistic insight and more transferable synthesis guidance than is currently available in the literature.

6. Sustainability Benefits of IZTs

Many reported IZT systems exhibit improved atom economy through framework reuse and reduced dependence on external nutrient sources. For example, FAU- and *BEA-derived routes to high-silica frameworks such as MFI, CHA, STF, and MTW proceed entirely without OSDAs, instead leveraging structural memory in the parent zeolite and inorganic cations, thereby avoiding the costly and waste-intensive templates required in conventional hydrothermal synthesis [29,30,122]. Accordingly, organic-free or low-OSDA IZT protocols are widely recognized as efficient routes to high-silica frameworks while minimizing the use of organic additives [29,30]. Solvent minimization represents a second major sustainability benefit. IZT-based syntheses of CHA from FAU and Ti-Beta from MOR or Ti-MWW can be conducted under solvent-free or very low-template conditions, reducing water consumption and simplifying work-up [110,111]. More broadly, solvent-free zeolite synthesis is recognized as a greener alternative to conventional hydrothermal methods, as it reduces wastewater generation and increases space–time yield by eliminating bulk solvent [123]. Consistent with this, many IZT and related zeolite syntheses are performed in highly alkaline, low-solvent or solvent-free media, improving yield and autoclave utilization while adhering to green chemistry principles of solvent minimization and efficient raw material use. Several reported IZT systems exhibit reduced crystallization times and lower thermal requirements relative to conventional hydrothermal synthesis. Microwave-assisted transformations of FAU significantly shorten crystallization times and reduce energy input relative to conventional hydrothermal treatment, enabling rapid formation of MER, LTJ, CAN, and ANA at 60–170 °C instead of multi-day syntheses near 95 °C [17]. This acceleration is commonly attributed to rapid volumetric heating and enhanced dissolution–recrystallization dynamics [17]. Waste minimization and circular economy principles are also intrinsic to IZT strategies. By converting existing zeolites—including widely available FAU, CHA, and *BEA, often produced at industrial scale without OSDAs—into new frameworks such as CHA, AEI, AFX, MFI, MWW, FER, STF, and MTW, IZT promotes material reuse rather than fresh raw material synthesis [29,68,97,124]. Consequently, commercial zeolites (e.g., FAU, *BEA, CHA, Ti-MWW, MOR) as well as spent or partially transformed materials can serve as nutrient sources or precursors for new catalysts, sometimes yielding hybrid or hierarchical structures instead of being discarded. Reviews on zeolites derived from waste and fly ash further emphasize the importance of such valorization strategies, highlighting the conversion of low-value residues into high-value zeolitic catalysts and sorbents within a circular economy framework [125,126,127]. Thus, the reported IZT systems suggest that framework reuse, reduced template dependence, and process intensification can improve synthesis efficiency relative to conventional hydrothermal routes. These advantages arise from the ability of IZTs to exploit structural inheritance while operating under tunable and resource-efficient conditions.

Table 2 summarizes the principal sustainability advantages of IZT with representative examples and, where primary data are available, indicative quantitative benchmarks. Direct head-to-head comparisons are still rare in the IZT literature—most studies do not report mass balances or E-factor estimates alongside a conventional synthesis benchmark—so several entries remain qualitative. The FAU → CHA route without TMAda⁺ [29,88] illustrates what is achievable: OSDA loading drops from roughly 0.3–0.5 g TMAda⁺ per gram of product to zero, crystallization time falls from several days to 12–48 h, and the calcination step needed to burn out the template is eliminated entirely. An alternative cheaper SDA, DMCHABr, similarly enables rapid FAU → CHA transformation with comparable NH₃-SCR activity [101]. Solvent-free routes such as Ti-MWW → Ti-Beta [110] operate with H₂O/SiO₂ below 5, versus 20–40 in typical hydrothermal synthesis, and room-temperature FAU → EDI [16] requires no external heating at all. These data points are noted in the table where the primary literature provides them; the remaining entries identify directions where future studies should supply the missing numbers.

Table 2. Sustainability advantages of IZT with representative examples.

Sustainability Aspect	Conventional Route	IZT Example	Benefit
OSDA usage	High (e.g., ZSM-5 with TPA+)	FAU → MFI (Na+/K+, OSDA-free) [30]	Eliminates organic waste, lowers cost
Atom economy	External silica/alumina sources	FAU → AEI [115,128]	Framework reuse improves efficiency
Solvent use	Large water volumes	Ti-MWW → Ti-Beta (solvent-free) [110]	Reduces wastewater
Energy demand	Prolonged heating (typically 100–180 °C, days)	FAU → EDI (room temperature); microwave FAU → ANA, EDI, MER, LTJ [16,17]	Ultra-low energy input; faster synthesis
Waste valorization	Fresh synthesis	Spent FAU → MFI [129]	Circular use of materials
Process intensification	Batch hydrothermal	Thermomechanical FAU → CHA/LTA [56]	No/low solvent, shorter times

Beyond synthesis metrics, IZT-derived materials have been benchmarked against their conventionally prepared counterparts in several catalytic contexts. FAU → CHA-derived SSZ-13 reaches NH₃-SCR activity and hydrothermal stability comparable to materials made with TMAda⁺, a template whose cost per kilogram is roughly an order of magnitude higher than the inorganic reagents used in the IZT route [114]. AEI zeolites obtained from FAU parents via IZT show competitive methanol-to-olefins performance with a remarkably high catalyst lifetime compared to SSZ-13 [128]; Cu-exchanged FAU-derived AEI also demonstrates high NH₃-SCR activity and hydrothermal stability retained after treatment at 900 °C [115]. MOR and GIS phases have been synthesized by IZT from FAU precursors [90], and these materials are expected to retain the acid site densities and ion-exchange capacities needed for FCC co-catalyst and separation applications, though direct head-to-head performance benchmarks against conventionally prepared analogues remain scarce. When IZT generates additional mesoporosity through partial framework densification, it can enhance intracrystalline diffusion without requiring separate post-synthetic treatments such as leaching or steaming. This offers a practical advantage, even if its direct contribution to reducing overall process complexity is difficult to isolate. However, claims regarding the sustainability of IZT are still supported largely by individual case studies rather than systematic evidence. Few reports provide quantitative metrics such as E-factors, water consumption, or energy requirements alongside those of conventional synthesis routes. As a result, despite the apparent environmental benefits of IZT, their true magnitude remains difficult to assess. More consistent, side-by-side evaluations will be needed before these advantages can be convincingly demonstrated and translated into industrial practice.

7. Conclusions

7.1. Challenges, Knowledge Gaps, and Future Perspectives

IZT is a versatile strategy, but it is not without boundaries, and understanding where it falls short is as important as cataloguing its successes. High-silica MFI frameworks—ZSM-5, Silicalite-1, and TS-1—remain difficult targets. FAU and MFI share no common SBUs or CBUs, so any conversion must proceed through essentially complete dissolution and renucleation rather than a structurally guided transformation. FAU → MFI has been reported under forcing conditions (190 °C, 32 h) [30], but yields and phase purity are generally inferior to TPA⁺-directed synthesis, and reproducibility between groups has been inconsistent. TS-1 presents the additional complication that isomorphous Ti incorporation requires precise control over Si/Ti ratios and hydrolysis kinetics that current IZT conditions do not deliver reliably—Ti tends to segregate to external surfaces or form anatase rather than substituting uniformly into framework positions [130]. Silicalite-1 demands near-zero Al content that is incompatible with the aluminosilicate chemistry of typical FAU precursors. These cases illustrate a broader point: IZT is most effective when parent and daughter frameworks share structural motifs or when the target is an Al-rich small-to-medium-pore aluminosilicate. As a general synthesis platform for the full topology space of zeolites, it remains complementary to, rather than a replacement for, OSDA-directed hydrothermal synthesis.

Despite its advantages, IZT faces several challenges that limit its broader implementation. Mechanistic understanding remains incomplete, with multiple competing pathways—such as dissolution–recrystallization, cross-nucleation, and solid-state or solution-mediated rearrangement—contributing to transformation behavior [16,40,131]. This complexity limits predictive control over phase selectivity and transformation outcomes. Control over intermediate phases is also limited. Zeolite syntheses can proceed via multiple transient crystalline or amorphous intermediates whose nucleation and transformation pathways are kinetically controlled and difficult to direct, leading to mixed phases or variable reproducibility [132,133,134]. From a sustainability standpoint, the lack of standardized, quantitative metrics for comparing zeolite syntheses (energy, water, waste) is noted more broadly for zeolite formation from minerals and wastes, where mechanistic studies are still scarce and are needed to optimize conditions and control crystallinity [135]. Addressing these gaps will be essential for rigorously validating IZT as a sustainable technology. Future research directions focus on integrating advanced tools and methodologies. Data-driven and machine learning approaches based on large zeolite synthesis datasets and OSDA–zeolite relationships offer ways to predict synthesis outcomes and phase selectivity, including intergrowths and competing phases [136,137,138,139]. Operando and in situ characterization, especially advanced solid-state NMR and high-energy X-ray total scattering with pair distribution function analysis, are expected to yield deeper insights into nucleation, amorphous-to-crystalline transitions, and intermediate structures [133,140,141,142]. In parallel, upcycling of spent FCC catalysts and other low-value feeds into zeolites is emerging as a promising circular strategy, demonstrating nearly closed recycling loops and conversion of leached residues into fresh zeolites such as ZSM-5, FAU, and others [143,144,145,146]. Overall, the future development of IZT will depend on achieving a more predictive understanding of transformation mechanisms while simultaneously establishing robust and quantitatively validated sustainability metrics. Integrating mechanistic insight with scalable green synthesis strategies will be essential for positioning IZT as a mature platform for next-generation zeolite production.

7.2. Toward Predictive Design of IZTs

Future progress in IZT research will increasingly depend on predictive control of transformation pathways rather than empirical screening. The collective evidence reviewed here suggests that transformation outcomes are governed by a coupled interplay between framework density, local structural inheritance, framework mobility, oligomer reactivity, and activation environment. Integrating these descriptors with data-driven synthesis databases, operando characterization, and machine learning approaches may enable predictive identification of favorable parent–daughter relationships and metastable transformation windows. Such approaches could ultimately transform IZT from a largely empirical methodology into a rationally designed synthesis platform for next-generation zeolitic materials.

The datasets needed to take this seriously are now available. The OSDA–zeolite and ZeoSyn datasets compiled by Jensen et al. [138] and Pan et al. [139] have already been used to model phase selectivity and design OSDAs for specific framework targets in conventional hydrothermal synthesis. The transformation inventory in Table 1 is a more modest but directly relevant starting point for IZT-specific models: each row encodes a parent framework, a set of synthesis conditions, and an observed daughter phase, which is precisely the input–output structure that a classifier or regression model requires. Graph neural network representations of framework topology offer a natural way to encode structural compatibility between parent and daughter before any experiment is run. The more immediate bottleneck is not algorithmic but experimental: models trained on ex situ snapshots cannot capture the transient intermediates that actually control nucleation. Integrating operando total-scattering and NMR measurements with surrogate models—so that the model sees the evolving pair distribution function or ²⁹Si spectrum in real time—would close this gap and is a realistic near-term target given current instrument capabilities. One concrete test of such an approach would be predicting the daughter phase from FAU across a grid of hydroxide concentrations, alkali cation identities, and temperatures. Incorrect predictions from such a model would be at least as informative as correct ones, since they would expose the variables that current structural descriptors fail to capture.