Dual-Route Accessible Zeolites: bridging organic- and inorganic-directed syntheses toward greener zeolite production

Abstract

The drive toward sustainable chemistry demands zeolite synthesis strategies that bypass organic structure-directing agents (OSDAs), primarily to circumvent the economic and environmental burdens associated with their use and removal. In this review, we introduce the concept of Dual-Route Accessible Zeolites—zeolite frameworks of identical topology that can be synthesized via both OSDA-assisted and OSDA-free routes. This concept provides a unifying framework for understanding crystallization mechanisms and structure–property relationships. Known zeolites are systematically classified into three categories in terms of synthetic routes: exclusively inorganic, dual-route accessible, and OSDA-dependent. Representative case studies—including Beta, CHA, FAU, LTA, and MFI—are examined to elucidate the distinct roles of inorganic cations, seed engineering, and synthesis conditions in directing zeolite formation. By extracting generalizable principles from these dual-route accessible systems, this review provides strategic guidance for the rational design of OSDA-free synthesis routes. By addressing both economic and environmental considerations, this review connects fundamental insights into zeolite crystallization with practical applications in materials science, catalysis, and sustainable engineering. Together, these perspectives establish a conceptual foundation for green design of zeolite materials.

The drive toward sustainable chemistry demands zeolite synthesis strategies that bypass organic structure-directing agents (OSDAs), primarily to circumvent the economic and environmental burdens associated with their use and removal.

1. Introduction

Zeolites are crystalline microporous materials with well-defined pore architectures and are indispensable in a wide range of industrial applications, including catalytic conversion,^1–3 adsorption and separation,^4–6 and ion exchange.^7–10 Early progress in zeolite science was driven by the hydrothermal synthesis of foundational frameworks such as LTA,¹¹FAU,¹² and MOR,¹³ which relied on inorganic cations as structure-directing agents (SDAs) and closely resembled natural mineralization processes. These materials remain industrially important due to their structural robustness, scalability, and cost-effectiveness.

The introduction of organic structure-directing agents (OSDAs) marked a transformative milestone in zeolite synthesis. OSDA-assisted methods enabled the rational discovery of numerous novel framework topologies with tailored pore structures and catalytic properties.¹⁴ These syntheses often proceed under kinetic control, where organic molecules selectively stabilize metastable intermediates by lowering activation barriers. As a result, crystallization can follow pathways that are inaccessible under purely inorganic conditions.

Despite their synthetic versatility, OSDAs present significant economic and environmental drawbacks. They are often expensive, potentially toxic, and require energy-intensive calcination for removal. Calcination contributes to greenhouse gas emissions and may introduce framework defects or even partial structural collapse. Such limitations are increasingly inconsistent with global efforts toward green and sustainable chemistry, highlighting the urgent need for alternative OSDA-free synthesis strategies.^15–17

In contrast, OSDA-free synthesis generally proceeds under thermodynamic control, favoring phases that minimize the Gibbs free energy of the system. These conditions often yield naturally occurring or low-silica frameworks that reflect intrinsic thermodynamic stability rather than kinetic selectivity. Expanding the structural diversity accessible under these constraints remains a central challenge in zeolite science.

To address this challenge, we propose the concept of Dual-Route Accessible Zeolites–zeolites of identical topology that can be crystallized through both OSDA-assisted and OSDA-free routes. These systems lie at the intersection of thermodynamic feasibility and kinetic accessibility, making them ideal model platforms for investigating crystallization mechanisms. Based on this framework, zeolites within the aluminosilicate family can be classified into three categories in terms of synthetic routes: (i) zeolites formed exclusively through inorganic routes, (ii) Dual-Route Accessible Zeolites via both organic and inorganic strategies, and (iii) structures that currently rely solely on OSDAs for synthesis.

This classification, presented in Section 2 and supported by comprehensive summary tables, establishes a systematic basis for comparative analysis.

Through detailed comparisons of synthesis pathways and resulting material properties in representative dual-route accessible systems, this review elucidates the distinct—and often synergistic—roles of inorganic cations, seed engineering, and synthesis conditions in directing zeolite formation. By distilling these findings into general design principles, we aim to provide practical guidance for the rational development of OSDA-free synthesis routes. In doing so, we bridge fundamental crystallization science with industrial relevance and contribute to the advancement of sustainable zeolite materials.

2. The Dual-Route Accessible Zeolites concept: a framework for mechanistic insights through contrastive synthesis

As introduced above, the concept of Dual-Route Accessible Zeolites provides a powerful comparative framework for understanding zeolite crystallization. In this section, we formalize the concept by establishing a systematic classification of zeolites according to their synthetic accessibility. This framework maps the current landscape of zeolite synthesis and clarifies the distinctive strategic value of Dual-Route Accessible Zeolites for mechanistic investigation.

We define three synthesis-based categories in terms of synthetic routes. Summary tables are provided to guide the discussion throughout this review and to underscore the central role of Dual-Route Accessible Zeolites in bridging inorganic and organic synthesis paradigms.

2.1. The three-tier classification system

This classification system is not intended merely to catalogue known zeolite syntheses. Rather, it serves as an analytical tool to enable meaningful comparisons, identify knowledge gaps, and uncover opportunities for future discovery.

This category includes naturally occurring zeolite minerals as well as several industrially important synthetic frameworks. Their crystallization relies solely on inorganic SDAs and generally proceeds under thermodynamic control (Table 1).

Table 1. Zeolites synthesized exclusively by inorganic routes.

IZA codea	Zeolite name	Syn. type	Dimension	Essential rings	Cations	Si/Al	Ref.
ABW	Li-ABW	Synthetic	1	[81·61·41]	Li+, Na+	1	18
AFG	Afghanite	Natural	0	[66·42]	Na+, K+, Ca2+	1	19
ANA	Analcime; Leucite; Pollucite	Natural	3	[81·61·41]	Na+; Na+, K+; Na+, Cs+	2; 2; 2.43	20–22
ATT	RMA-3	Synthetic	2	[82·61·43]	Rb+	3.62	23
BCT	|Ca–|[Al–Si–O]–BCT; Svyatoslavite	Synthetic	1	[81·61·41]	Ca2+; Na+, Ca2+	1	24
		Natural				1
BEA	Tschernichite	Natural	3	[12 × 12 × 12]b	Na+, Mg2+, Ca2+, Fe3+	3; 3.7	25
					Na+, K+, Mg2+, Ca2+, Fe3+
BIK	Bikitaite	Natural	1	[81·61·51]	Li+	2	26
BOG	Boggsite	Natural	3	[122·102·65·53·45]	Na+, Ca2+	4.23	27
BRE	Brewsterite	Natural	2	[82·61·52·42]	K+, Ba2+, Sr2+	3	28
CAN	Cancrinite	Natural	1	[121·62·41]	Na+, Ca2+	1.5	29
CAS	Cs-aluminosilicate	Synthetic	1	[81·63·51]	Cs+	5	30
CHA	Chabazite; Willhendersonite	Natural	3	[81·61·42]	Na+, Ca2+; K+, Ca2+	2; 1	31 and 32
DAC	Dachiardite; Svetlozarite	Natural	2	[101·81·55·41]	Na+, K+, Mg2+, Ca2+, Fe3+	4.59; 3.24	33
EAB	Bellbergite	Natural	2	[81·63·43]	Na+, K+, Ca2+, Ba2+, Sr2+	1	34
EDI	Edingtonite	Natural	3	[82·42]	Na+, K+, Ca2+, Ba2+	1.50	35
EON	Direnzoite	Natural	2	[121·86·61·58·45]	Na+, K+, Mg2+, Ca2+	3.62	36
EPI	Epistilbite	Natural	2	[82·53·41]	Na+, K+, Ca2+	2.82	37
ERI	Erionite	Natural	3	[81·63·42]	K+, Mg2+, Ca2+	2.46	38
FAR	Farneseite	Natural	0	[68·44]	Na+, K+, Ca2+	1	39
FAU	Faujasite	Natural	3	[121·62·42]	Na+, Ca2+	2.33	40
FER	Ferrierite	Natural	2	[101·81·61·54]	Na+, K+, Mg2+	5	41
FRA	Franzinite	Natural	0	[612·45]	Na+, K+, Ca2+	1	42
GIS	Gismondine; Amicite; Gobbinsite; Garronite	Natural	3	[81·42]	Ca2+; Na+, K+; Na+, K+, Ca2+; Na+, Ca2+	1; 1; 1.67; 1.67	43–45
GIU	Giuseppettite	Natural	0	[610·44]	Na+, K+, Ca2+	1	46
GME	Gmelinite	Natural	3	[121·81·61·43]	Na+, Ca2+	2.16	47
GOO	Goosecreekite	Natural	3	[84·62·43]	Ca2+	3	48
HEU	Clinoptilolite	Natural	2	[101·82·53·42]	Na+, K+, Mg2+, Ca2+	4.35	49 and 50
	Heulandite				K+, Ca2+, Sr2+	2.75
JBW	K,Na-JBW	Synthetic	1	[81·62·41]	Na+, K+	1	51
LAU	Laumontite; Leonhardite	Natural	1	[101·65·43]	Ca2+; Na+, K+, Ca2+	2; 2.16	52 and53
LEV	Levyne	Natural	2	[81·63·42]	Na+, K+, Ca2+	1.86	54
LIO	Liottite	Natural	0	[68·43]	Na+, K+, Ca2+	1	55
LIT	Lithosite	Natural	3	[101·83·62·42]	K+	2	56
LTJ	Linde type J	Synthetic	3	[83·62·41]	K+	1	57
LTN	Linde type N	Synthetic	0	[81·67·45]	Na+	1	58
MAN	Martinandresite	Natural	2	[82·61·43]	Na+, K+, Ba2+	2.82	59
MAR	Marinellite	Natural	0	[68·43]	Na+, K+, Ca2+	1	60
MAZ	Mazzite	Natural	1 and 3	[121·82·61·51·42]	Na+, K+, Mg2+, Ca2+	2.72	61
MER	Merlinoite	Natural	3	[83·43]	K+, Ca2+	2.44	62
	Linde W	Synthetic			K+	2.1
MFI	Mutinaite	Natural	3	[106·68·511·41]	Na+, K+, Mg2+, Ca2+	7.58	63
MON	Montesommaite	Natural	3	[82·51·41]	Na+, K+	2.56	64
MOR	Mordenite	Natural	2	[121·83·55·41]	Na+	5	65
NAT	Natrolite	Natural	3	[91·81·42]	Na+	1.5	66
OFF	Offretite	Natural	3	[121·81·63·43]	K+, Mg2+, Ca2+	2.46	67
PAR	Partheite	Natural	1	[101·81·63·42]	Ca2+	1	68
PAU	Paulingite	Natural	3	[89·62·411]	Na+, K+, Ca2+, Ba2+	3.42	69
PHI	Phillipsite; Harmotome	Natural	3	[83·44]	Na+, K+, Ca2+; Ca2+, Ba2+	2; 2.20	70
SOD	Sodalite	Natural	0	[61·41]	Na+	1	71
STI	Stellerite; Stilbite	Natural	2	[101·82·61·51·42]	Na+, K+, Mg2+, Ca2+, Fe3+, Sr2+; Na+, Ca2+, Mg2+	3.53; 2.50	72 and 73
TER	Terranovaite	Natural	2	[104·65·53·42]	Na+, K+, Mg2+, Ca2+	5.50	74
THO	Thomsonite	Natural	3	[83·43]	Na+, Ca2+, Sr2+	1	75
TOL	Tounkite-like mineral	Natural	0	[614·46]	Na+, K+, Ca2+	1	76
TSC	Tschörtnerite	Natural	3	[82·63·44]	K+, Ca2+, Ba2+, Sr2+, Cu2+	1	77
WEN	Wenkite	Natural	3	[101·81·62·43]	Na+, K+, Ca2+, Ba2+	1.22	78
YUG	Yugawaralite	Natural	2	[83·51·42]	Ca2+	3	79

^aThis table mainly includes aluminosilicate zeolites.

^bThe [12 × 12 × 12] notation for beta zeolite denotes its three-dimensional intersecting 12-ring pore system. Detailed enumeration of smaller rings (e.g., 6-, 5-, and 4-rings) is not provided due to the complex intergrown structure, which arises from the disordered stacking of polymorphs A and B.

Zeolites in this category can be synthesized through both OSDA-assisted and OSDA-free routes. They form the central focus of this review. Their dual synthetic accessibility enables controlled, side-by-side comparisons under organic and inorganic conditions. Such comparisons provide a unique opportunity to disentangle kinetic and thermodynamic contributions and to elucidate fundamental crystallization mechanisms (Table 2).

Table 2. Dual-Route Accessible Zeolites: zeolites synthesized through both OSDA-assisted and OSDA-free routes.

IZA codea	Zeolite name	Initial synthesis path	Breakthrough path	OSDA-free synthesis conditions and product properties	Ref.	OSDA-directed synthesis and product properties	Ref.
BEA	Beta	Organic first	OSDA-free breakthrough	40SiO2 : 1Al2O3 : 10Na2O : 570H2O, T = 140 °C, t = 17–19 h, seed: calcined beta; product: crystals (about 100−160 nm); open micropores	80	50SiO2 : 1Al2O3 : 18-tetraethylammonium hydroxide : 2.65 Na2O : 590H2O, T = 140 °C, t = 72 h; product: Si/Al = 24; crystals (about 28 nm); acidic: H+-473 K: 0.161 mmol g; H+-673 K: 0.084 mmol g−1	81
BIK	Bikitaite	Inorganic first	Organic modification	10SiO2 : 1Al2O3 : 0.88Cs2O : 0.22Na2O : 300H2O, T = 165 °C, t = 96 h, tumbling autoclave at 30 rpm; product: Si/Al = 6.4–12.6; crystals (about 1 µm); BET: 45 m2 g−1	82	60SiO2 : 1Al2O3 : 9Cs2O : 15-butane-1,4-diamine : 3000H2O, T = 180 °C, t = 72 h, tumbling autoclave at 60 rpm; product: Si/Al = 15; crystals (about 4 µm); BET: very small micropore volume	83
	EU-7
BOG	Boggsite	Inorganic first	Organic modification and OSDA-free breakthrough	76.9SiO2 : 1Al2O3 : 20Na2O : 1538.5H2O, T = 125 °C, t = 336 h, seed: Si/b-ITQ-47; product: Si/Al = 7.8–10.9; crystals (about 0.5–14 µm); BET: 582 m2 g; acidic: 251.3 µmol g−1	84	5SiO2 : 1GeO2 : 0.222H3BO3 : 2.4-tert-butyliminotris(dimethylamino)phosphorane : 60H2O, T = 150 °C, t = 600 h; product: crystals (about 10 µm)	85
	ITQ-47
	Si/Al-BOG
BPH	Linde Q	Inorganic first	Organic modification	4SiO2 : 1Al2O3 : 16K2O : 192H2O, T = 50 °C, t = 162 h; product: Si/Al = 1–1.3; crystals (about 200–500 nm); basicity: 0.105–0.350 mmol CO2 per g	86	5SiO2 : 1Al2O3 : 8 -tetraethylammonium hydroxide : 2 tetramethylammonium chloride : 1LiCl : 200 H2O, T = 125 °C, t = 72 h; product: Si/Al = 1.5–2.5	87
	UZM-4
CHA	Chabazite	Inorganic first	Organic modification and OSDA-free breakthrough	24SiO2 : 1Al2O3 : 5.4Na2O : 1.8K2O : 560H2O, T = 140 °C, t = 72 h, seed: SAPO-18; product: Si/Al = 3.7; crystals (about 200–400 nm); BET: 563 m2 g−1	88	1.0SiO2 : 0.026 Al2O3 : 0.1 Na2O : 44H2O : 0.1 N,N,N-trimethyl-1-adamantylammonium : (0.02% polydiallyldimethylammonium), T = 180 °C, t = 10 h; product: BET: 647–728 m2 g; acidic: 764–780 µmol g−1	89
	SSZ-13
EMT	EMC-2	Organic first	OSDA-free breakthrough	5.15SiO2 : 1Al2O3 : 18.45Na2O : 240.3H2O, T = 30 °C, t = 36 h; product: Si/Al = 1.14; crystals (about 6–15 nm); BET: 578 m2 g−1	90	10SiO2 : 1Al2O3 : 0.33 1,4,7,10,13,16-hexaoxacycleoctadecane : 2.1Na2O : 0.016–0.1 Na3PO4 : 140H2O, T = 100 °C, t = 6–20 days; product: Si/Al = 3–3.7; crystals (about 3–5 µm)	91
EON	ECR-1	Organic first	OSDA-free breakthrough	10SiO2 : 1Al2O3 : 2.5Na2O : 200H2O, T = 100 °C, t = 14 days; product: Si/Al = 3.5	92	62SiO2 : 1Al2O3 : 0.03-tetramethylammonium : 18.6 Na2O : 1214H2O, T = 130 °C, t = 108 h; product: Si/Al = 4.2; crystals (about 0.5–1 µm)	93
ERI	Erionite UZM-12	Inorganic first	Organic modification and OSDA-free breakthrough	38.46SiO2 : 1Al2O3 : 9.43Na2O : 2.89 K2O : 538.46H2O, T = 150 °C, t = 5 days; product: Si/Al = 3; crystals (about 2–12 µm); BET: 458 m2 g−1	94	20SiO2 : 1Al2O3 : 7.6 K2O : 2-hexamethonium bromide : 8-tetrapropylammonium hydroxide : 322.6H2O, T = 150 °C, t = 5 days; product: Si/Al = 5.91; BET: 439 m2 g−1	95
	Linde T
EUO	EU-1	Organic first	OSDA-free breakthrough	100SiO2 : 1Al2O3 : 28.17Na2O : 3500H2O, T = 160 °C, t = 18 h, seed: EUO; product: Si/Al = 12; BET: 194 m2 g; acidic: 0.75 mmol g−1	96	55SiO2 : 1Al2O3 : 3.3-hexamethonium bromide : 6.6 Na2O : 1650H2O, T = 160 °C, t = 60 h; product: Si/Al = 27–259; BET: 347–353 m2 g; yield: 98 wt%	97
FAU	Faujasite	Inorganic first	Organic modification and OSDA-free breakthrough	12SiO2 : 1Al2O3 : 3.5Na2O : 160H2O, T = 100 °C, t = 13 days; product: Si/Al = 3.5	98	83.33SiO2 : 1Al2O3 : 46.78 -tetrabutylammonium hydroxide : 1.17 -tetramethylammonium hydroxide : 1.375 Na2O : 1716.67 H2O, T = 120 °C, t = 5 days; product: Si/Al = 10.64; crystals (about 100–300 nm)	99
	Zeolite Y
FER	Ferrierite	Inorganic first	Organic modification and OSDA-free breakthrough	31.8SiO2 : 1Al2O3 : 6.49 Na2O : 1113H2O, T = 150 °C, t = 168 h, seed: RUB-37; product: Si/Al = 14.5; crystals (about 2–4 µm); BET: 370 m2 g; acidic: 0.91 mmol g−1	100	25SiO2 : 1Al2O3 : 10 4-(aminomethyl)pyridine : 2.5 Na2O : 450H2O, T = 170 °C, t = 36 h, tumbling autoclave at 20 rpm; product: crystals (about 0.7–2 µm); acidic: 0.79 mmol g−1	101
	ZSM-35
GME	Gmelinite	Inorganic first	Organic modification and OSDA-free breakthrough	5.2SiO2 : 1Al2O3 : 1.18Na2O, T = 120 °C, t = 20 h; product: crystals (about 1–4 µm); BET: 114 m2 g; yield: 90 wt%	102	30SiO2 : 1Al2O3 : 5.4 N,N-dimethyl-3,5-dimethylpiperidinium hydroxide : 7.95 Na2O : 612H2O, T = 140 °C, t = 110 h; product: Si/Al = 4	103
	CIT-9
HEU	Heulandite	Inorganic first	Organic modification	30SiO2 : 1Al2O3 : 9.6Na2O : 1400H2O, T = 150 °C, t = 96 h, seed: natural stellerite; product: Si/Al = 3.7; BET: 376 m2 g; yield: 44 wt%	104	10SiO2 : 1Al2O3 : 1.6 4-carbon diquat : 1.25 Na2O : 300H2O, T = 160 °C, t = 43 days; product: Si/Al = 6.4–9.8	105
	Clinoptilolite
KFI	ZK-5	Organic first	OSDA-free breakthrough	16SiO2 : 1Al2O3 : 0.6Na2O : 11.8K2O : 320H2O, T = 140 °C, t = 15 days, seed: KFI; product: Si/Al = 5–5.8; crystals (about 1.5–2 µm); BET: 406 m2 g−1	106	10SiO2 : 1Al2O3 : 1 1,4,7,10,13,16-hexaoxacycleoctadecane : 2.3 K2O : 0.1SrO : 160H2O, T = 150 °C, t = 5 days; product: Si/Al = 3.45; BET: 90.7 m2 g−1	107
LEV	Levyne	Inorganic first	Organic modification and OSDA-free breakthrough	40SiO2 : 1Al2O3 : 14Na2O : 1400H2O: 40–80 alcohol, T = 120 °C, t = 72 h, seed: RUB-50; product: Si/Al = 4.06; crystals (about 1 µm); BET: 387 m2 g−1	108	36SiO2 : 1Al2O3 : 7.2 N,N-dimethylpiperidinium hydroxide : 5.4 Na2O : 180H2O, T = 170 °C, t = 24 h; product: Si/Al = 11; BET: 664 m2 g; yield: 57 wt%	109
	ZK-20
LTA	Linde type A	Inorganic first	Organic modification	4.62SiO2 : 1Al2O3 : 1.28Na2O : 44.36H2O, T = 100 °C, t = 1 day; product: Si/Al = 2.1; crystals (about 0.1–3 µm)	110	20SiO2 : 1Al2O3 : 10 1,2-dimethyl-3-(4-methylbenzyl)imidazolium hydroxide : 2 tetramethylammonium hydroxide : 9.8 HF : 100H2O, T = 175 °C, t = 7 days; product: Si/Al = 11–23	111
	ZK-21
	UZM-9
LTL	Linde type L	Inorganic first	Organic modification	20SiO2 : 1Al2O3 : 10K2O : 400H2O, T = 170 °C, t = 18 h; product: Si/Al = 2.59; crystals (about 15 nm); BET: 591 m2 g−1	112	20SiO2 : 1Al2O3 : 10.2 K2O : 4 1,4,7,10,13,16,19-heptaoxacyclohenicosane : 1030H2O, T = 180 °C, t = 3 days; product: Si/Al = 3.5; crystals (about 1–4 µm)	113
MAZ	Mazzite	Inorganic first	Organic modification and OSDA-free breakthrough	50SiO2 : 1Al2O3 : 13.125Na2O : 4.375K2O : 1000H2O, T = 140 °C, t = 24 h, seed: calcined omega; product: Si/Al = 3.2; crystals (about 1 µm); BET: 380 m2 g; yield: 33 wt%; acidic: 0.85 mmol g−1	114	15.2SiO2 : 1Al2O3 : 1.4 tetramethylammonium chloride : 7.1 Na2O : 460H2O, T = 100 °C, t = 21 days; product: Si/Al = 4.3	115
	Omega
MEL	ZSM-11	Organic first	OSDA-free breakthrough	100SiO2 : 1Al2O3 : 30Na2O : 2000H2O, T = 140 °C, t = 15 h, seed: calcined ZSM-11; product: Si/Al = 8.8; crystals (about 50–150 nm); BET: 326 m2 g; yield: 18 wt%	116	80SiO2 : 1Al2O3 : 6 tetrabutylammonium hydroxide : 3.2 Na2O : 2000H2O, T = 90 °C, t = 24 h and T = 170 °C, t = 48 h, seed: Silicalite 2; product: Si/Al = 10–30; crystals (about 500 nm); BET: 439 m2 g; acidic: 0.83 mmol g−1	117
MER	Merlinoite	Inorganic first	Organic modification	6.9SiO2 : 1Al2O3 : 5.8K2O : 200H2O, T = 170 °C, t = 8 h; product: Si/Al = 2; crystals (about 6–8 µm); yield: 64.8 wt%	9	30SiO2 : 1Al2O3 : 3 N,N-1,1,2,6-tetramethylpiperidinium hydroxide : 2.25 Na2O : 1.5 K2O : 316.8H2O, T = 160 °C, t = 72 h; product: Si/Al = 6.01; crystals (about 1–3 µm); BET: 200 m2 g−1	118
	Zeolite W
MFI	Mutinaite ZSM-5	Inorganic first	Organic modification	40SiO2 : 1Al2O3 : 5.2Na2O : 1500H2O, T = 180 °C, t = 120 h; product: Si/Al = 14; BET: 223 m2 g−1	119	78.12SiO2 : 1Al2O3 : 28.78 tetrapropylammonium hydroxide : 2643.3H2O : 63.44 urea, T = 180 °C, t = 48 h; product: Si/Al = 26.2; crystals (about 90–100 nm)	120
MOR	Mordenite	Inorganic first	Organic modification	20SiO2 : 1Al2O3 : 1.6Na2O : 300H2O, T = 170 °C, t = 30 h, seed: commercial MOR; product: Si/Al = 9.4; crystals (about 50–200 nm); BET: 428 m2 g; yield: 94 wt%	121	40SiO2 : 1Al2O3 : 8-tetraethylammonium hydroxide : 5 Na2O : 640H2O, T = 170 °C, t = 36 h; product: Si/Al = 15.3; crystals (about 0.5–1.5 µm); BET: 413 m2 g−1	122
MSE	MCM-68	Organic first	OSDA-free breakthrough	100SiO2 : 1Al2O3 : 28Na2O : 2K2O : 2000H2O, T = 140 °C, t = 48 h, seed: calcined MCM-68; product: Si/Al = 6–7	123	20SiO2 : 1Al2O3 : 2 1,1′-(cyclohexane-trans-1,4-diyl)bis(1-methylpiperidi : n-1-ium) diiodide : 3.75 K2O : 600H2O, T = 160 °C, t = 42 h; product: Si/Al = 10.7	124
MTT	ZSM-23	Organic first	OSDA-free breakthrough	120.5SiO2 : 1Al2O3 : 32.5Na2O : 4216.9H2O, T = 170 °C, t = 5 h, seed: calcined ZSM-23, tumbling autoclave at 30 rpm; product: Si/Al = 20; crystals (about 1–2 µm); BET: 117 m2 g; acidic: 0.76 mmol g−1	125	33.33SiO2 : 1Al2O3 : 5 heptamethonium bromide : 5.33 Na2O : 1333.3H2O, T = 160 °C, t = 335 h; product: Si/Al = 14–39.3; crystals (about 1 µm); BET: 150–281 m2 g−1	126
MTW	ZSM-12	Organic first	OSDA-free breakthrough	80SiO2 : 1Al2O3 : 14Na2O : 920H2O, T = 165 °C, t = 36 h,seed: calcined ZSM-12; product: Si/Al = 16.2; crystals (about 0.2–1.5 µm); yield: 47 wt%	127	60SiO2 : 1Al2O3 : 9 1,5-bis(methylpyrrolidine) pentane bromide : 10 Na2O : 2000H2O, T = 170 °C, t = 24 h; crystals (about 10 µm)	128
MWW	MCM-22	Organic first	OSDA-free breakthrough	MCM-22: 100SiO2 : 1Al2O3 : 25 Na2O : 5000H2O, T = 120 °C, t = 5 h and T = 160 °C, t = 96 h, seed: MCM-22; product: Si/Al = 11.1; yield: 36.2 wt%	129 and 130	MCM-22: 30.3SiO2 : 1Al2O3 : 10.61 hexamethyleneimine : 2.73 Na2O : 1363.64H2O, T = 150 °C, t = 7 days; product: Si/Al = 13; BET: 483 m2 g−1	131 and 132
	MCM-49			MCM-49: 40SiO2 : 1Al2O3 : 10.8Na2O : 2080H2O, ultraphonic aging: T = 50 °C, t = 0.5 h, T = 140 °C, t = 32 h, seed: MCM-49; product: Si/Al = 7.6; crystals (about 150 nm); BET: 245 m2 g−1		MCM-49: 40SiO2 : 1Al2O3 : 18 cyclohexanine : 4 Na2O : 800H2O, T = 150 °C, t = 45 h, seed: calcined MWW; product: Si/Al = 12.5; crystals (about 2 µm); BET: 434 m2 g; yield: 75 wt%
NES	NU-87	Organic first	OSDA-free breakthrough	60SiO2 : 1Al2O3 : 9Na2O : 600H2O, T = 160 °C, t = 84 h, seed: calcined EU-1; product: Si/Al = 7–12.9; BET: 410 m2 g; yield: 57.9 wt%	133	40SiO2 : 1Al2O3 : 5-decane-1,10-bis(trimethylammonium bromide) : 6 Na2O : 1.33 NaBr : 2000H2O, T = 170 °C, t = 21 days, seed: NU-87; product: Si/Al = 17.5	134
OFF	Offretite	Inorganic first	Organic modification	21.95SiO2 : 1Al2O3 : 4.88Na2O : 2.32K2O : 0.30Cs2O : 341.5H2O, aging: t = 21 days, T = 90 °C, t = 6 days; product: Si/Al = 3.5; crystals (about 2–15 µm); BET: 380 m2 g−1	135	16SiO2 : 1Al2O3 : 16-tetraethylammonium hydroxide : 1 tetramethylammonium chloride : 3KCl : 480H2O, T = 100 °C, t = 14 days, tumbling autoclave at 60 rpm; product: Si/Al = 3.15; BET: 360 m2 g−1	136
	Linde T
PAU	Paulingite	Inorganic first	Organic modification	2.5SiO2 : 1Al2O3 : 2.75K2O : 50H2O, T = 135 °C, t = 3 days, seed: calcined ECR-18; product: no data	116	10SiO2 : 1Al2O3 : 0.667 N-methyltropinium : 1.33Na2O : 1.33K2O : 266.67H2O, T = 120 °C, t = 7 days, tumbling autoclave at 60 rpm; product: Si/Al = 4.1–4.4; crystals (about 1–2 µm); BET: 420–620 m2 g−1	137
	ECR-18
PHI	Phillipsite	Inorganic first	Organic modification	Phillipsite: 5SiO2 : 1Al2O3 : 1.5Na2O : 0.5K2O : 80H2O, T = 65 °C, t = 120 h; product: Si/Al = 1.34–3.32	138	10SiO2 : 1Al2O3 : 4-choline : 0.6Na2O : 1.4Rb2O : 200H2O, T = 150 °C, t = 7 days; product: Si/Al = 4.8; crystals (about 3 µm)	139
	Harmotome			Harmotome: 2SiO2 : 1Al2O3 : 5.5Na2O : 0.5BaO : 190H2O, T = 65 °C, t = 24 h; product: Si/Al = 2.2
RHO	RHO	Inorganic first	Organic modification	12.5SiO2 : 1Al2O3 : 10Na2O : 0.725Cs2O : 125H2O, T = 90 °C, t = 1 h; product: Si/Al = 1.35–1.46; crystals (about 50 nm); BET: 650 m2 g−1	140	10SiO2 : 1Al2O3 : 0.5 1,4,7,10,13,16-hexaoxacycleoctadecane : 1.8Na2O : 0.3 Cs2O : 100H2O, T = 110 °C, t = 4 days; product: Si/Al = 3.5	141 and 142
RTH	[Al]-TTZ-1	Organic first	OSDA-free breakthrough	65SiO2 : 1Al2O3 : 26Na2O : 13 000H2O, T = 170 °C, t = 7 days, seed: deboronated RUB-13; product: Si/Al = 41; crystals (about 1–5 µm); acidic: 0.25 mmol g−1	143	25SiO2 : 1Al2O3 : 5.25 2,6-methyl-N-methylpyridinium : 2.75 Na2O : 450H2O, T = 130 °C, t = 12 h; product: Si/Al = 8.8; crystals (about 0.1–0.3 µm); BET: 576 m2 g−1	144
STF	Al-SSZ-35	Organic first	OSDA-free breakthrough	80SiO2 : 1Al2O3 : 20Na2O : 7600H2O, T = 150 °C, t = 40 h, seed: STF; product: Si/Al = 11–20; yield: 26–47 wt%	145	46SiO2 : 1Al2O3 : 23 1,2,3-triethyl-4-methylimidazolium hydroxide : 23HF : 552H2O, T = 185 °C, t = 11 days; product: Si/Al = 45.9	146
STI	Stilbite	Inorganic first	Organic modification	Stilbite: 6SiO2 : 1Al2O3 : 0.11Na2O : 0.89CaO; product: Si/Al = 2.87	147	30SiO2 : 1Al2O3 : 4.5 1,4-bis(N-methylpyrrolidinium)-butane : 15Na2O : 1200H2O, T = 160 °C, t = 7 days, tumbling autoclave at 100 rpm; product: Si/Al = 7.1	148
	Stellerite TNU-10			Stellerite: 7SiO2 : 1Al2O3 : 1CaO, hydrothermal conversion of artificial water-free precursor glasses; product: Si/Al = 3.55
SZR	SUZ-4	Organic first	OSDA-free breakthrough	21SiO2 : 1Al2O3 : 3.95K2O : 500H2O, T = 160 °C, t = 6 days, seed: calcined SUZ-4; product: crystals (about 5 µm)	149	21SiO2 : 1Al2O3 : 2.6 tetraethylammonium hydroxide : 4.5 K2O : 520H2O, T = 150 °C, t = 4 days, tumbling autoclave at 60 rpm; product: Si/Al = 6; BET: 236 m2 g; yield: 56–84 wt%	150
TON	ZSM-22	Organic first	OSDA-free breakthrough	100SiO2 : 1Al2O3 : 9.5K2O : 4545H2O, T = 140 °C, t = 2 days, seed: calcined ZSM-22, tumbling autoclave at 45 rpm; product: Si/Al = 44; crystals (about 2–4 µm); BET: 176 m2 g; yield: 88 wt%	151	50SiO2 : 1Al2O3 : 2.8 1,4-bis(3-methylimidazolyl)butane hydroxide: 6.3Na2O : 950H2O, T = 170 °C, t = 2 days; product: Si/Al = 22.3; crystals (about 60 nm); BET: 185.7 m2 g; yield: 84.5 wt%	152

^aThis table mainly includes aluminosilicate zeolites.

This category comprises many complex and recently discovered zeolite topologies whose formation remains strongly OSDA-directed. For these materials, organic species appear essential for stabilizing key intermediates or lowering crystallization barriers. Identifying the kinetic, thermodynamic, or compositional constraints that hinder their OSDA-free synthesis represents a major scientific challenge and an important frontier in zeolite research (Table 3).

Table 3. Zeolites currently dependent on OSDAs.

IZA codea	Zeolite name	OSDA name	Temp. (°C)	Time (h)	Seeds	OSDA/Si	Si/Al	Ref.
AEI	SSZ-39	Cis,trans-N,N-dimethyl-3,5-dimethylpipperidinium hydroxide or 2,4,4,6-tetramethylmorpholinium hydroxide	160	48	SSZ-39	0.2	8.7 or 9.4	153
AFT	SSZ-112	1-Methyl-1-propylpiperidinium hydroxide and hexamethonium hydroxide	150	72	—	0.15–0.2 and 0.01–0.08	6.4–6.8	154
AFX	SSZ-16	N,N-(1,4-Phenylenebis(methylene))bis(N,N,N-triethylammonium)hydroxide	150	120	—	0.225	7.8	155
ATS	SCM-39	4-Pyrrolidinopyridine hydroxide	170	336	—	0.5	11.6	156
BEC	Al-BEC	p-Xylylene-bis(N-methylpiperidinium hydroxide)	150	240	—	0.35	17.9	157
CDO	ECNU-57	1,4-Bis(N-methylpyrrolidinium)	170	168	—	0.25	22	158
CON	SSZ-26	1,4-Bis(N-cyclohexylpyrrolidinium)butane dications	150	45	SSZ-26	0.5	—	159
DDR	ZSM-58	Methyltropinium iodide	160	120	ZSM-58	0.25	48.0	160
EAB	Na-TMA-EAB	Tetramethylammonium hydroxide	80	168	EAB	0.5	4.6	161
ESV	ERS-7	Choline hydroxide	175	336	—	0.5	14.0	162
ETL	EU-12	Choline chloride	150	168	—	0.4	9.0	163
ETV	EMM-37	1,1,1′,1′-Tetramethyl-3,3′-bipyrrolidine-1,1′-dium	160	168	—	0.15	5.7	164
IFR	MCM-58	Benzylchinuklidinium bromide	170	192	—	0.2	19.0	165
IFW	Al-ITQ-52	1,4-Butanediylbis[tris(dimethylamino)]phosphonium hydroxide	150	120	[B]-ITQ-52	0.214	17.8	166
IHW	ITQ-32	N,N,N′,N′-tetramethyldecahydrocyclobuta[1,2-c;3,4-c′]dipyrrolidinium	175	336	—	0.25	13.0–32.0	167
IMF	IM-5	1,5-Dibromopentane and 1-methylpyrrolidine	160	264	—	0.167 and 0.5	14.0–16.0	168
ION	Al-IDM-1	p-Phenylene-dimethylene-bis(tripropylammonium)	160	144	—	0.25	97.0	169
ITE	Mu-14	1-Azoniatricyclo[4.4.4.01, 6]tetradecane hydroxide	175	140–210	—	0.1	18.2–30.5	170
ITH	Al-ITH	Cationic oligomer	160	240	ITH	0.005	70.0	171
ITR	Al-COE-12	N,N′-Diethyl-N,N,N′,N′-tetramethylhexamethylene-diammonium hydroxide	175	144	ITR	0.25	52.0–175.0	172
IWR	Al-ITQ-24	1,1′-[1,4-Phenylenebis(methylene)]bis(1-methylpyrrolidin-1-ium)	160	216	—	0.5	32.5 and 65.0	173
IWV	ITQ-27	Diphenyldimethylphosphonium hydroxide	150	1416	—	0.5	29.4	174
JZO	ZEO-1	Tricyclohexylmethylphosphonium	190	360	—	0.5	14.5	175
LOS	Losod	Bispyrrolidinium	100	96	—	3.93	1.0	176
LTF	LZ-135	Tetramethylammonium and tetraethylammonium	100	240	—	1 and 1	2.4	177
MEI	UZM-22	Choline hydroxide	100	336	—	0.8	4.0–4.7	136
MFS	ZSM-57	N,N,N,N′,N′,N′-Hexaethylpentanediammonium	160	168	—	0.1	24.7	178
MOZ	ZSM-10	1,4-Dimethyl-1,4-diazoniabicyclo[2.2.2]octane	140	96	—	0.06	3.5	179
MRT	ZSM-43	Choline hydroxide	150	480	—	0.3–0.54	5.2–6.5	180
MSO	MCM-61	1,4,7,10,13,16-Hexaoxacyclooctadecane	175	168	—	0.3	13.2	181
MTF	Al-MCM-35	Hexamethyleneimine	175	288	—	0.3	36	182
MTN	Al-ZSM-39	Benzyltrimethylammonium hydroxide	170	168–336	—	0.2	21.0–23.0	183
MWF	ZSM-25	Tetraethylammonium bromide	140	168	—	0.72	3.4	184
NON	Nonasil	Tetramethylethylenediamine	130	96	—	0.1	36.0	185
PTO	PST-31	1,1′-(1,4-Butanediyl)bis(2,4-dimethyl-1H-pyrazol-2-ium)	175	168	—	0.125	16	186
PTT	PST-33	5-Azonia-spiro[4.4]nonane	175	24	—	0.13	4.1	187
PTY	PST-30	1,1′-(1,4-Butanediyl)bis(2,5-dimethyl-1H-pyrazol-2-ium)	175	336	—	0.25	11.5	188
PWN	PST-29	N,N′-Dimethyl-1,4-diazabicyclo[2.2.2]octane	120	336	—	0.067	4.5	189
PWO	PST-21	1,2,3-Trimethylimidazolium	175	336	—	0.5	10.1	190
PWW	PST-22	1,3,4-Trimethylimidazolium or 1,2,3,4-tetramethylimidazolium	175	336	—	0.5	11.1	190
RFE	ZSM-48	Hexamethyldiammonium bromide	160	36	ZSM-48	0.03	29.0	191
RRO	RUB-41	Dimethyldipropylammonium hydroxide	140 or 150	48–96	RUB-39	0.5	19–163	192
RUT	Nu-1	Tetramethylammonium hydroxide	170	168	—	0.6	11.5	193
SBT	PST-32	N,N′-Dimethyl-1,4-diazabicyclo[2.2.2]octane	120	168–336	PST-32	0.025	4.0	194
SEW	Al-SSZ-82	1,6-Bis(N-cyclohexylpyrrolidinium)hexane dications	160	168	B-SSZ-82	0.1	23.9	195
SFE	Al-SFE	4-Dimethylaminopyridine	170	72	B-SFE	0.6	57.0	196
SFW	SSZ-52	N-Ethyl-N-(3,3,5-trimethylcyclohexyl)pyrrolidinium hydroxide	165	48	—	0.15–0.21	4.0–7.0	197
STT	Al-SSZ-23	N,N,N-Trimethyl-1-adamantanaminium hydroxide	160	72	—	0.75	40	198
SWY	STA-30	1,8-(1,4-Diazabicyclo[2.2.2]octane)octyl dication	140	120	—	0.10–0.13	5.8–7.3	199
TUN	TNU-9	1,4-Bis(N-methylpyrrolidinium) butane	160	336	TNU-9	0.15	19.6	200
UFI	UZM-5	Tetramethylammonium hydroxide and tetraethylammonium chloride	150	96	—	0.125 and 1	6.9	87
YFI	YNU-5	Dimethyldipropylammonium	160	165	—	0.17	9.0	201

^aThis table mainly includes aluminosilicate zeolites.

3. Dual-Route Accessible Zeolites: case studies and comparisons

3.1. Beta zeolites

Beta zeolite is a three-dimensional 12-ring framework known for its outstanding hydroisomerization activity and excellent thermal and hydrothermal stability. These properties have established Beta zeolite as a cornerstone material in both industrial catalysis and fundamental zeolite research. Conventionally, highly crystalline Beta zeolites with Si/Al ratios above 12 are synthesized using OSDAs, most commonly tetraethylammonium (TEA⁺).^202,203

Despite their effectiveness, OSDA-assisted routes have several intrinsic limitations. OSDAs are costly and require energy-intensive calcination for removal. Calcination can generate framework defects, including terminal silanol groups, which compromise long-term thermal stability. These drawbacks have motivated the development of OSDA-free alternatives.

Seed-assisted, OSDA-free synthesis has emerged as a promising strategy for preparing low-silica Beta zeolites. This approach enables direct crystallization without the need for post-synthesis calcination to generate porosity. The resulting materials typically exhibit enhanced hydrophilicity and a high density of acid sites, making them attractive for environmentally benign and industrially scalable applications.

A seminal advance was reported by Xie et al. in 2008, who achieved the first rapid OSDA-free synthesis of Beta zeolite using calcined Beta seeds. Crystallization was completed at 140 °C within 17–19 h, yielding nanocrystalline Beta zeolite with fully accessible pore channels. Type-I N₂ adsorption–desorption isotherms confirmed intrinsic microporosity without calcination.⁸⁰ This study demonstrated that seed-directed crystallization can effectively replace OSDAs.

Majano et al. subsequently developed a nano-Beta seed-mediated route using Na⁺ as an inorganic SDA. This strategy stabilized highly aluminous Beta frameworks with Si/Al ratios below 4, approaching the composition of the natural mineral tschernichite.²⁰⁴ The resulting submicron crystals exhibited exceptional hydrophilicity and high acid-site density, making them promising for adsorption, ion exchange, and acid–base catalysis. In addition, both synthesis cost and environmental impact were reduced. Nevertheless, the method suffered from limited crystallization yields and mordenite impurities at elevated temperatures.

In 2010, Kamimura et al. established a reproducible OSDA-free route to high-crystallinity Beta zeolite by systematically optimizing gel composition, seed loading, and seed Si/Al ratio.²⁰⁵ A subsequent mechanistic study in 2011 employed X-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy to probe crystallization pathways. The results revealed partial dissolution of Beta seeds, followed by crystal growth primarily on amorphous aluminosilicate surfaces rather than through independent nucleation (Fig. 1a).²⁰⁶ Intermediate seed addition accelerated crystallization and reduced crystal size, confirming that seeds function predominantly as growth templates. These findings provided a mechanistic foundation for rationally seed-assisted design.

Fig. 1. (a) Schematic illustration of the seed-assisted crystallization in OSDA-free Beta zeolite synthesis.²⁰⁶ Copyright 2011, American Chemical Society. (b and c) Comparative catalytic performance as a function of Si/Al ratio.²⁰⁸ Copyright 2022, Elsevier.

Zhang et al. further refined this approach in 2013 by introducing trace amounts of seeds and lowering the crystallization temperature to 120 °C. The resulting high-purity Beta exhibited a surface area of 655 m² g⁻¹, approximately 45% higher than that of conventional OSDA-assisted Beta.²⁰⁷ The combination of large surface area, substantial pore volume, and low defect density translated into superior adsorption and catalytic performance.

Subsequent studies focused on improving yield and introducing structural hierarchy. In 2016, Otomo et al. increased crystallization yields to ∼80% by employing aluminum-rich precursors and invoking a solution-mediated growth mechanism.²⁰⁹

In 2018, Zhang et al. reported an OSDA-free alkaline hydrothermal strategy using commercial Beta as the silicon source. This approach induced simultaneous framework recrystallization and mesopore formation at 150 °C, increasing mesopore volume by 50–80%.²¹⁰ The proposed hierarchical formation–recrystallization (H–R) mechanism preserved the intrinsic microporous structure and acid strength. The resulting hierarchical Beta catalyst exhibited excellent performance in benzyl alcohol/mesitylene alkylation, establishing a green route to high-performance catalysts.

More recently, Minnebruggen et al. synthesized aluminum-rich Beta with low Si/Al ratios, high crystallinity, and large pore volumes by fine-tuning seed characteristics and silicon sources.²⁰⁸ The catalyst showed outstanding activity in isobutane/butene alkylation, achieving C₈ selectivities above 85% (Fig. 1b and c). Its performance surpassed that of the industrial La–HY benchmark while offering a more cost-effective alternative.²¹¹

The Si/Al ratio is a key structural parameter governing the physicochemical and catalytic properties of Beta zeolite. OSDA-free routes typically produce low-silica Beta with Si/Al ratios of 4–6 via seed-induced crystallization. In contrast, OSDA-assisted methods generally yield materials with higher Si/Al ratios (10–30).

Lower Si/Al ratios increase framework aluminum content and thus Brønsted acid-site density. Ammonia temperature-programmed desorption (NH₃-TPD) and pyridine-infrared spectroscopy measurements indicate that total acid density increases by 20–30% relative to high-silica counterparts. Higher aluminum content also enhances hydrophilicity through increased hydroxyl groups and silanol nests, which benefits aqueous-phase reactions such as biomass conversion.²¹²

Low-silica Beta zeolites also exhibit improved thermal robustness. Owing to a reduced density of framework defects, crystallinity loss after calcination is limited to 8–15%, compared with ∼33% for conventional OSDA-assisted samples. The combination of high acid density, enhanced hydrophilicity, and improved structural stability leads to superior catalytic performance.

For instance, in isobutane alkylation, the high density of Brønsted acid sites promotes efficient hydrogen transfer while suppressing oligomerization. Productivities exceeding 30 g_prod g_cat⁻¹ and C₈ selectivities above 85% have been reported. Similar improvements are observed in isomerization and transalkylation reactions, where enhanced diffusion and improved acid-site accessibility increase yields and reduce coke formation.

In dual-route accessible Beta systems, the Si/Al ratio plays a central role in determining catalytic behavior by simultaneously modulating acid-site density and the hydrophilic–hydrophobic balance of the framework. Comparative analysis of OSDA-assisted and OSDA-free routes reveals how synthetic strategy directly shapes framework composition, defect density, and catalytic performance. As such, Beta zeolite serves as a prototypical example of how Dual-Route Accessible Zeolites enable mechanistic insight through contrastive synthesis.

3.2. CHA zeolites

The CHA framework was first identified in the natural mineral chabazite. In 1963, Smith et al. resolved its crystal structure by X-ray diffraction, revealing a rhombohedral lattice (space group R3̄m) built from double six-ring (d6r) that assemble into a three-dimensional cage-based channel system.

The first synthetic low-silica CHA zeolite was reported in 1981 by Barrer et al. using an all-inorganic hydrothermal system containing silica sol, aluminum hydroxide, and potassium hydroxide. Crystallization at 150 °C yielded materials with Si/Al ratios below 3.²¹³ However, these highly aluminous frameworks showed poor thermal stability and frequently collapsed during ion-exchange, limiting their practical applicability.

A major advance was achieved when Zones and co-workers introduced N,N,N-trimethyl-1-adamant ammonium hydroxide (TMAda⁺) as an OSDA. This strategy enabled the synthesis of CHA zeolites with Si/Al ratios exceeding 12 and markedly improved thermal stability.²¹⁴ These materials, particularly SSZ-13, became benchmarks for catalysis. However, the high cost, toxicity, and environmental burden associated with TMAda⁺ are their drawbacks. Consequently, developing stable and scalable OSDA-free routes has become a central objective in CHA research.

In 2014, Hiroyuki et al. reported a seed-assisted route that enabled direct crystallization of CHA with intermediate Si/Al ratios within 24 h.²¹⁵ The method leveraged the synergistic effect of Na⁺, K⁺, and Cs⁺ in combination with oriented CHA seeds. The resulting materials exhibited high surface areas and tunable compositions. By adjusting alkali-metal ratios or incorporating boron, the Si/Al ratio could be increased to 6.1.

In 2018, Bing et al. developed a seed-directed, OSDA-free synthesis of hierarchical SSZ-13 microspheres with high Si/Al ratios (∼17).²¹⁶ The mesopore-enriched microspherical morphology significantly enhanced mass transport. In methanol-to-olefins (MTO) reactions, these catalysts achieved combined ethylene and propylene selectivities above 90% and lifetimes exceeding 550 min. This study demonstrated that OSDA-free synthesis can simultaneously deliver hierarchical structure and high catalytic performance.

Kim et al. later reported a rapid strontium-assisted hydrothermal route for low-silica K-CHA (Si/Al = 2.5), achieving crystallization at 150 °C within 10 h.²¹⁷ The material exhibited a hierarchical coil-like microsphere morphology integrating micro- and mesoporosity. Sr²⁺ accelerated crystallization and enhanced CO₂ affinity, while K⁺ tuned pore-window selectivity. The resulting adsorbent achieved a high water vapor adsorption capacity of 316 cm³ g⁻¹, highlighting its potential for dehydration applications.

Mechanistic understanding advanced significantly through work by Muraoka et al., who elucidated the FAU-to-CHA interzeolite conversion pathway.²¹⁸ Using FAU as the sole Si and Al source under mild alkaline conditions, they obtained phase-pure CHA without amorphous intermediates. Combined ²⁹Si magic-angle spinning nuclear magnetic resonance (²⁹Si MAS NMR) and computational analysis revealed that crystallization proceeds via selective silicate–aluminate rearrangement rather than direct transfer of d6r units (Fig. 2f). The resulting CHA inherited a biased Al distribution from the FAU precursor, enhancing thermodynamic stability. This study provided an atomic-level blueprint for tailoring active-site distributions.

Fig. 2. Representative OSDA-free synthesis strategies for CHA zeolites, highlighting the roles of cationic systems, mechanistic pathways, and processing conditions: (a) hydration-radius-driven inorganic SDA crystallization.²²³ Copyright 2024, The Royal Society of Chemistry. (b) Aging-assisted synthesis.²²¹ Copyright 2021, Elsevier. (c) SAPO-seed-mediated transcription.⁸⁸ Copyright 2023, Wiley-VCH. (d) K⁺/Sr²⁺-directed phase formation.²²⁰ Copyright 2021, American Chemical Society. (e) Morphology control via gel composition.²²⁴ Copyright 2024, Springer Nature. (f) Topotactic FAU-to-CHA transformation.²¹⁸ Copyright 2019, The Royal Society of Chemistry.

Debost et al. synthesized nanosized CHA crystals (Si/Al = 2.0) with particle sizes below 200 nm by precisely controlling Na⁺/K⁺/Cs⁺ mixtures.²¹⁹ Compared with OSDAs-assisted analogues, these materials exhibited lower Si/Al ratios and preferential Cs⁺ occupation at 8-ring SIII' sites. This configuration produced high CO₂ adsorption capacities (3.8 mmol g⁻¹) and effectively infinite CO₂/CH₄ selectivity. The behavior was attributed to a cation-induced “molecular trap-gate” mechanism combined with enhanced diffusion and cyclic stability.

Parallel work by Liang et al. demonstrated that Sr²⁺ can function both as a crystallization promoter and as an inorganic SDA.²²⁰ Low-silica CHA (Si/Al = 2) crystallized within 24 h at 100–140 °C without OSDAs or seeds. Sr²⁺ incorporation accelerated kinetics and improved hydrothermal stability by modulating Al distribution (Fig. 2d).

Deng et al. reported a K⁺-directed synthesis of low-silica CHA (Si/Al = 1.5–2.5) that substantially reduced cost and environmental impact (Fig. 2b).²²¹ Pore blocking by K⁺ restricted N₂ diffusion, but ion exchange reversibly reopened the pores. This system provided a valuable experimental platform for studying trapdoor adsorption mechanisms.

Hao et al. further demonstrated direct synthesis of low-silica CHA (Si/Al = 2.06) for Cs⁺ removal from radioactive wastewater.²²² The 8-ring apertures closely matched the ionic radius of Cs⁺, while Na⁺ sites remained exchangeable. In real wastewater, the material achieved residual activities below 7 Bq L⁻¹, uptake times under 5 min, and excellent radiation stability, outperforming conventional adsorbents at significantly lower cost.

Wang et al. introduced a transcription-inspired strategy using SAPO-18 seeds synthesized with N,N-diisopropylethylamine (DIPEA) to direct OSDA-free CHA (Si/Al = 3.7) crystallization (Fig. 2c).⁸⁸ The method enabled precise control of Al pairing, improving hydrothermal stability and ammonia selective catalytic reduction (NH₃-SCR) performance.

In 2024, Mallette et al. reported an inorganic SDA strategy based on hydration-radius-mismatched ion pairs (e.g., Na⁺/Li⁺, K⁺/Sr²⁺).²²³ This approach enabled pure-phase CHA crystallization at temperatures as low as 65 °C without seeds or K⁺, reducing synthesis times from weeks to hours (Fig. 2a). The resulting materials exhibited low Si/Al ratios (∼1.5–2.1), high acid-site densities, and tunable stacking-fault densities, highlighting the importance of ionic hydration cooperativity in nucleation.

Chu et al. subsequently reported an ultra-fast, environmentally benign synthesis of K-CHA nano-aggregates using only colloidal silica and aluminum hydroxide.²²⁴ Tunable morphologies—including hamburger-, disk-, and walnut-like structures—were obtained within 6 h at 433 K by adjusting the gel Si/Al ratio (Fig. 2e). The walnut-shaped sample (Si/Al = 7) exhibited the highest pore volume (0.297 cm³ g⁻¹), surface area (340 m² g⁻¹), and methane uptake (25.74 cm³ g⁻¹), achieving CH₄/N₂ selectivity greater than 3 at low pressures (p/p₀ < 0.6).

Most recently, Wang et al. reported a low-temperature OSDA-free synthesis of low-silica CHA (Si/Al = 1.7) using SAPO-35 seeds.¹⁰ The material exhibited enhanced divalent cation uptake and strong pH stability, achieving Sr²⁺ uptake capacities of 112–186 mg g⁻¹ for nuclear wastewater treatment.

OSDA-free CHA zeolites typically exhibit Si/Al ratios below 3. This composition results in high Brønsted acid-site density, strong hydrophilicity, and exceptional ion-exchange capacity. These features underpin their superior performance in CO₂ capture, radionuclide remediation, and catalysis.

For example, low-silica CHA can achieve CO₂ adsorption capacities up to 3.8 mmol g⁻¹ with near-infinite CO₂/CH₄ selectivity through cation-induced trapdoor effects at 8-ring windows.²¹⁹ In nuclear wastewater treatment, removal efficiencies above 99% for ¹³⁷Cs⁺ have been achieved under realistic conditions, even at extremely low mass concentrations (e.g., ¹³⁷Cs⁺: 1587.22 Bq L⁻¹, corresponding to ∼0.496 ppt) and minimal adsorbent dosages (solid-to-liquid ratio of 1/2000–1/500 g mL⁻¹).^10,222

In NH₃-SCR catalysis, Cu-exchanged OSDA-free CHA delivers NO_x conversions above 90% over a wide temperature window (225–550 °C).⁸⁸ Although high-temperature durability remains slightly lower than that of high-silica SSZ-13, optimization of Al pairing and distribution offers a clear pathway toward enhanced stability.

OSDA-free synthesis of CHA zeolites—enabled by inorganic cation direction, seed engineering, and cooperative ion-pair effects—provides a green and cost-effective route to low-silica materials with outstanding adsorption and catalytic performance. Future efforts should focus on precise control of Al distribution, cation siting, and hierarchical morphology to further expand the industrial application of CHA zeolite–based materials.

3.3. FAU zeolites

FAU-type zeolites, primarily X and Y, have played a central role in industrial catalysis and adsorption since the 1950s. Early syntheses relied exclusively on inorganic hydrothermal crystallization of sodium aluminosilicate gels. These routes produced low-silica FAU zeolites with Si/Al ratios typically below 2.5.¹² Although such materials possess high acid-site densities, their limited hydrothermal stability restricts their performance under harsh industrial conditions.

The demand for more stable FAU zeolites, particularly for petroleum refining, drove efforts in the 1970s to increase framework Si content. However, purely inorganic routes were unable to overcome the intrinsic Si/Al limit of ∼2.5.

A major breakthrough occurred in 1990, when Delprato et al. synthesized high-silica FAU (Si/Al ≈ 4.5) using crown ethers such as 1,4,7,10,13-pentaoxacyclopentadecane (15-crown-5) as OSDAs.²²⁵ These organic species stabilized the FAU supercage through synergistic charge-matching and space-filling effects, thereby promoting silicon incorporation. Despite its effectiveness, this approach suffers from high OSDA cost, energy-intensive OSDA removal, and significant environmental burden.

In response, OSDA-free synthesis has reemerged as a major research direction. Recent advances leverage seed-assisted crystallization, additive modulation, kinetic control, and data-driven optimization to extend the attainable Si/Al ratio while adhering to green chemistry principles.

In 2015, Awala et al. reported OSDA-free FAU nanocrystals (10–400 nm) synthesized via low-temperature homogeneous nucleation.²²⁶ The material exhibited high crystallinity, tunable Si/Al ratios (1.1–2.1), micropore volume (∼0.30 cm³ g⁻¹), and crystallization yields above 80%. Their hierarchical porosity and abundant external active sites enhanced macromolecular catalysis, as demonstrated in 1,3,5-triisopropylbenzene conversion. Excellent colloidal stability also suggested potential in coatings and biomedical carriers, underscoring the scalability of OSDA-free nano-FAU synthesis.

In 2017, Oleksiak et al. achieved the first one-step OSDA-free synthesis of HOU-3 (FAU topology) with Si/Al = 3.0 through precise kinetic control.²²⁷ Crystallization was conducted under low-alkalinity conditions (Si/OH⁻ ≥ 1.5), reduced Na⁺ concentration, and prolonged heating at 100 °C for at least 3 days. Structural analysis revealed a non-random Si–Al distribution, with d6r units preferentially containing three aluminum atoms (Fig. 3a). This arrangement lowered the framework energy relative to a random distribution at the same composition. Compared with commercial zeolite Y, HOU-3 retained crystallinity after hydrothermal treatment at 230 °C for 6 days and exhibited a 28% increase in catalytic lifetime during cumene cracking.

Fig. 3. Schematic summary of recent OSDA-free strategies for synthesizing high-silica FAU zeolites (Si/Al > 3): (a) alkalinity and kinetic control (Si/Al = 3.0).²²⁷ Copyright 2017, Wiley-VCH. (b) ZnO-assisted synthesis (Si/Al = 3.4).²³⁰ Copyright 2023, American Chemical Society. (c) Co²⁺-assisted synthesis (Si/Al = 3.08).²²⁹ Copyright 2021, Elsevier. (d) ·OH radical-assisted crystallization (Si/Al = 3.17).²²⁸ Copyright 2020, Wiley-VCH. (e) Machine-learning-guided optimization (Si/Al = 3.5).⁹⁸ Copyright 2023, Nature Portfolio (Springer Nature). (f) Drug-delivery-inspired slow-release Al incorporation (Si/Al = 3.44).²³¹ Copyright 2025, Wiley-VCH.

In 2020, Wang et al. introduced a synergistic OSDA-free strategy combining ·OH radical-assisted in situ crystallization with mild post-synthesis dealumination.²²⁸ The resulting Y zeolite exhibited high crystallinity and Si/Al = 3.17. Compared with OSDA-assisted methods, this route eliminated OSDA-related costs. Relative to conventional steam dealumination, it minimized crystallinity loss and improved overall yield (Fig. 3d).

In 2021, Meng et al. developed a Co²⁺-assisted synthesis route.²²⁹ Co²⁺ formed metastable Co–O–Si pentacoordinate intermediates that were weaker than Al–O–Si bonds, thereby promoting localized silicon enrichment (Fig. 3c). Simultaneously, microcrystalline Co(OH)₂ reduced the nucleation energy barrier. This dual effect extended the induction period while accelerating crystal growth, enabling simultaneous enhancement of Si/Al ratio and reduction of crystal size under OSDA-free conditions.

In 2023, Parmar et al. achieved Si/Al = 3.4 using ZnO-assisted hydrothermal synthesis under mild conditions (Fig. 3b).²³⁰ In parallel, Li et al. applied machine-learning-guided optimization of sodium aluminosilicate gel compositions.⁹⁸ By identifying low Na₂O content and extended crystallization times as critical parameters, they achieved high-purity FAU zeolite with Si/Al = 3.5 (Fig. 3e)—the highest reported for OSDA-free synthesis at that time. Compared with conventional FAU (Si/Al ≤ 2.5), this material exhibited enhanced proton activity in propane dehydrogenation and cracking, attributable to optimized Si–Al distribution and proton microenvironments, while preserving microporosity and structural stability.

Most recently, Zhao et al. (2025) introduced a drug-delivery-inspired slow-release strategy for OSDA-free zeolite Y synthesis, achieving Si/Al = 3.44 (Fig. 3f).²³¹ Aluminophosphates (AlPOs) were employed as controlled aluminum sources, enabling precise regulation of Al incorporation kinetics. The resulting material exhibited a randomized Al distribution characteristic of kinetically controlled crystallization and showed markedly improved hydrothermal stability. In furfuryl alcohol etherification, it delivered nearly threefold higher turnover frequencies than conventional zeolite Y.

A clear contrast exists between OSDA-free and OSDA-assisted routes to high-silica FAU.

OSDA-free strategies rely on inorganic cations (e.g., Na⁺), mineralizers (e.g., NaOH), additives, and careful kinetic regulation under relatively mild hydrothermal conditions. These approaches align strongly with green chemistry principles and have extended the Si/Al ratios to ∼3.5.⁹⁸ However, they often require long crystallization times and precise compositional control, and may suffer from impurity formation or uneven acid-site distributions.

In contrast, OSDA-assisted synthesis has evolved toward increasingly sophisticated strategies. Early methods focused on direct silicon enrichment using crown ethers. Later developments introduced the NOA-co strategy, where FAU nuclei, a bulky OSDA, and a gel system with low alkalinity work synergistically to drive the crystallization of a high-silica FAU zeolite.^232–234 More recently, dual low-charge-density OSDAs (i.e., tetramethylammonium and tetrabutylammonium) with minimized Na⁺ content have enabled Si/Al ratios as high as 10.64.⁹⁹ These approaches provide efficient and controllable access to highly siliceous zeolite Y but remain constrained by cost and environmental considerations.

Further progress in OSDA-free FAU synthesis should build on mechanistic insights derived from OSDA-assisted systems. Strategies that optimize crystallization pathways, regulate aluminum distribution, and integrate computational modeling with data-driven design offer promising avenues for further increasing Si/Al ratios and catalytic performance.

Equally important is the development of scalable and economically viable synthesis protocols. Bridging laboratory innovation with industrial implementation will be essential to fully realize the potential of OSDA-free FAU zeolites within a sustainable materials framework.

3.4. LTA zeolites

LTA zeolite (Linde type A) was the first zeolite produced on an industrial scale. It was originally reported by Breck and co-workers in 1956.¹¹ Sodium LTA is synthesized under purely inorganic conditions via the hydrothermal crystallization of sodium silicate and sodium aluminate. The resulting material typically exhibits an Si/Al ratio of approximately 1 and crystallizes in the cubic system (space group Pm3̄m). Its structure consists of well-defined α-cages interconnected by eight-ring windows. These structural characteristics endow LTA zeolites with excellent adsorption and ion-exchange properties, making them widely used in separation processes and water-softening.

Despite the simplicity and low cost of inorganic synthesis, the fixed Si/Al ratio (∼1) significantly limits the thermal stability and catalytic versatility of LTA. To overcome this intrinsic constraint, OSDAs were introduced to enable higher framework Si contents. In 2004, Corma et al. reported the synthesis of pure-silica LTA (ITQ-29) in a fluoride medium using 4-methyl-2,3,6,7-tetrahydro-1H,5H-pyrido-[3.2.1-ij]quinolinium hydroxide as the OSDA.²³⁵ ITQ-29 possesses an effectively infinite Si/Al ratio, representing a milestone in high-silica LTA synthesis. This material exhibits markedly enhanced hydrophobicity and improved structural stability.

Subsequent studies further refined OSDA-assisted strategies. In 2015, Boal and co-workers synthesized high-silica LTA zeolites with Si/Al ratios ranging from 12 to 42 using imidazole-based OSDAs in combination with tetramethylammonium (TMA⁺) as a co-directing agent.²³⁶ This approach significantly expanded the catalytic applicability of LTA. Using a related strategy, Hong's group synthesized high-silica LTA zeolites for NH₃-SCR reactions.¹¹¹ Although highly effective, OSDA-assisted routes require calcination to remove the OSDA, increasing cost, energy consumption, and environmental burden.

In parallel, OSDA-free synthesis strategies have attracted growing attention due to their economic and environmental advantages. In 2014, Conato et al. reported the synthesis of a Si-rich LTA zeolite (HOU-2) without OSDAs by carefully adjusting the Si/Al and Si/OH⁻ ratios under low-temperature hydrothermal conditions (Fig. 4).¹¹⁰ The achieved Si/Al ratio of 2.1 was the highest reported for OSDA-free LTA at that time. In 2017, Ji et al. developed a wet-gel crystallization method in which the precursor gel was filtered prior to heating.²³⁷ This approach enabled efficient production of LTA microspheres with high yield and reduced energy consumption. Anbia et al. subsequently prepared hierarchical LTA microspheres by precisely controlling hydrothermal parameters.²³⁸ The resulting materials exhibited high surface areas and improved diffusion properties, broadening their potential for sustainable adsorbent and catalyst design.

Fig. 4. Ternary phase diagram and topological structure of LTA zeolites illustrating the influence of gel composition on the Si/Al ratios.¹¹⁰ Copyright 2015, The Royal Society of Chemistry.

More recently, Wang et al. synthesized commercial-grade LTA zeolites with excellent Co²⁺ adsorption capacity using mild activation strategies, including low-temperature alkali fusion of spent fluid catalytic cracking (FCC) catalysts.⁸ Novembre et al. prepared NaA zeolite from rice husk ash at only 60 °C, substantially reducing energy consumption.²³⁹ In 2025, Zheng and co-workers introduced a solid-phase activation strategy to synthesize LTA zeolite from kaolin-enriched soil.²⁴⁰ This approach requires only room-temperature aging followed by heating at 100 °C, greatly reducing NaOH consumption and simplifying equipment requirements. These advances represent important progress in scalable, low-energy OSDA-free LTA synthesis.

Overall, OSDA-assisted and OSDA-free routes to LTA exhibit distinct advantages and limitations. OSDA-assisted methods—using TMA⁺ or imidazole-based OSDAs—enable access to high-silica and pure-silica LTA with superior thermal stability, hydrophobicity, and catalytic performance, particularly for reactions such as NH₃-SCR. However, these benefits come at the expense of higher cost and reduced environmental compatibility.

In contrast, OSDA-free approaches frequently utilize natural resources (e.g., kaolin and rice husk ash)²⁴¹ or industrial wastes (e.g., spent FCC catalysts),²⁴² offering advantages in cost, scalability, and sustainability. Their principal limitation remains the relatively low attainable Si/Al ratios, although steady progress has been made, as exemplified by HOU-2 (Si/Al = 2.1). By optimizing activation strategies—such as solid-phase activation and wet-gel crystallization—OSDA-free synthesis has become particularly suitable for large-scale adsorption and water-treatment applications. Nevertheless, achieving very high framework Si contents under purely inorganic conditions remains challenging.²⁴³

Taken together, OSDA-free synthesis provides a sustainable and industrially scalable route for LTA production, whereas OSDA-assisted methods remain essential for preparing high-performance catalytic variants. Future research should aim to integrate the advantages of both strategies—for example, by developing recyclable or environmentally benign OSDAs—to balance catalytic performance with sustainability.

3.5. MFI (ZSM-5) zeolites

The MFI zeolite, commercially known as ZSM-5, is one of the most iconic and versatile zeolites in heterogeneous catalysis and separation processes. Its unique pore architecture and tunable acidity have made it indispensable in key industrial reactions, such as fluid catalytic cracking and xylene isomerization. The conventional synthesis of ZSM-5, first reported by Mobil researchers in 1972, uses tetrapropylammonium (TPA⁺) cations as the OSDA. This approach typically produces materials with high silica content and excellent catalytic stability. The MFI topology features a three-dimensional pore system of intersecting straight and sinusoidal channels, which underpins its renowned shape selectivity.

Efforts to develop OSDA-free routes for ZSM-5 synthesis have intensified due to the desire to reduce costs and environmental impact. Early attempts were limited by the high kinetic barrier for MFI nucleation in purely inorganic systems. A significant breakthrough came with seed-directed synthesis (SDS), in which pre-formed MFI nanocrystals guide the crystallization toward the MFI phase without organic templates. This method not only achieves high crystallinity but also enables the formation of unique morphologies and properties. For example, Zhang et al. reported an OSDA-free synthesis of mesoporous, Al-rich MFI crystals (Si/Al = 9.5) using MFI seeds.¹¹⁹ The resulting crystals were assembled from aligned nanorods, generating intracrystalline mesoporosity that enhanced mass transfer and catalytic performance in cumene cracking compared to conventional ZSM-5. Beyond seeding, interzeolite transformation (IZT) has emerged as a powerful OSDA-free strategy. Goel et al. demonstrated that high-silica MFI can be synthesized from FAU or BEA precursors under suitable conditions. This transformation relies on the structural memory of the parent zeolite, facilitated by common structural building units or seeds, and allows access to frameworks that typically require OSDA.¹⁴⁵

Recent advances have further expanded the scope and sustainability of OSDA-free ZSM-5 synthesis. Researchers have successfully employed diverse silica–alumina sources, including natural minerals such as diatomite and rectorite, as well as industrial wastes like iron ore tailings and potassium feldspar residues.^244–246 Innovative solvent-free and solid-state methods have also been developed to enhance the green credentials of the process. These approaches involve mechanically mixing solid reactants followed by steam-assisted crystallization (SAC), which eliminates liquid waste, improves energy efficiency, and enhances product uniformity.²⁴⁷ Mechanistic studies on these solvent-free, seed-directed routes indicate that nucleation is often the rate-determining step. Moreover, the scalability of these eco-friendly methods has been demonstrated, with successful syntheses from gram to hundred-gram scales, confirming their industrial potential.²⁴⁸ Beyond catalysis, the resulting ZSM-5 materials have been applied in adsorption processes, such as the removal of volatile organic compounds like methanol.²⁴⁹

The MFI system exemplifies the Dual-Route Accessible Zeolites concept. The OSDA-assisted route (using TPA⁺) provides precise control over phase purity, crystal size, and Si/Al ratios, which are critical for reactions requiring strong acidity and hydrophobicity.^250,251 In contrast, OSDA-free routes, primarily via seeding or IZT, produce materials with distinct characteristics: higher aluminum incorporation (lower Si/Al), interconnected mesoporosity, and hierarchical morphologies. These differences arise from their distinct crystallization pathways. The OSDA-assisted route is a direct templating process that favors specific nucleation and growth, whereas the seed-directed route involves growth and assembly around inorganic seeds. Together, these two approaches provide a comparative platform to understand how the absence of an organic template alters synthesis thermodynamics and kinetics, ultimately yielding materials with complementary properties suited to different applications, from acid-catalyzed reactions to adsorption processes. The MFI case study thus exemplifies the power of the Dual-Route Accessible Zeolites concept to rationalize and guide the development of complementary synthesis strategies.

4. The Dual-Route Accessible Zeolites framework: a comparative lens for decoupling crystallization mechanisms

4.1. Inorganic cations: from thermodynamic balancers to biomimetic structure-directing agents

The role of inorganic cations (e.g., Li⁺, Na⁺, K⁺, Cs⁺ and Ca²⁺) differs dramatically between OSDA-free and OSDA-assisted pathways. This contrast provides a clear perspective on the changing drivers of zeolite crystallization.^252,253

4.1.1. In OSDA-free routes: primary structure-directing and space-filling agents

In the absence of OSDAs, inorganic cations serve as the primary structure-directing agents. Their effectiveness depends on a precise combination of charge, size, and hydration energy, which allows them to cooperatively stabilize specific building units or voids within the target framework.

A notable example is the synthesis of low-silica CHA, where Na⁺, K⁺, and sometimes Li⁺ or Cs⁺ act synergistically. These cations function not only as charge balancers but also as biomimetic templates, occupying CHA cages and replicating the space-filling and electrostatic role of an OSDA such as TMAda⁺ (Fig. 5). This process guides the assembly of aluminosilicate precursors from solution or gel and stabilizes specific intermediates, consistent with broader crystallization theories.²⁵⁴

Fig. 5. Schematic illustrations and correlative analyses highlighting the roles of hydrated cation radii and Collins' hydration-affinity matching in regulating cooperative structure direction, crystallization kinetics, and crystal size in CHA zeolites synthesized using paired inorganic cations.²²³ Copyright 2024, The Royal Society of Chemistry.

Recent studies have shifted the interpretation of cation effects from a traditional “charge–radius” perspective to a “hydration–effect” paradigm. Increasing evidence indicates that hydration shells and hydration energies critically govern nucleation pathways and energy barriers. For instance, in the green synthesis of CHA zeolites, paired cations with mismatched hydration radii (e.g., K⁺/Li⁺, Na⁺/Li⁺) exhibit pronounced synergistic effects. A small fraction of strongly hydrated Li⁺, introduced as a minor component, can suppress impurity phase and markedly accelerate CHA crystallization.²²³

Mechanistically, a cation together with its hydration shell may be regarded as an “inorganic structure-directing super-ion”.²⁵⁵ The hydrated radius of this super-ion can spatially match specific cage-like building units, thereby stabilizing critical nucleation intermediates. This behavior can be rationalized using Collins' law of matching water affinities. According to this principle, cation pairs with dissimilar hydration characteristics—such as chaotropic K⁺ interacting with framework anionic sites—experience hydration-shell repulsion.²⁵⁶ This repulsion weakens electrostatic binding, enhances cation mobility at the solid–liquid interface, and lowers the nucleation energy barrier.

This hydration-based framework provides a molecular-level explanation for selective zeolite crystallization, particularly in Dual-Route Accessible Zeolites systems. It marks a shift from empirical cation screening toward rational synthesis design guided by hydration thermodynamics.

4.1.2. In OSDA-assisted routes: charge compensators and gel-chemistry modulators

When a potent OSDA is present, inorganic cations play a secondary, supportive role. They mainly balance the framework's negative charge and modulate the solubility, gelation, and rheology of the synthesis mixture.

For example, in high-silica SSZ-13 (CHA-type) synthesized with TMAda⁺, Na⁺ fine-tunes the alkalinity and kinetics but does not dictate the CHA topology, which is exclusively controlled by the OSDA. In this scenario, crystallization is under strong kinetic control imposed by the organic species,²⁵⁷ with inorganic cations influencing rate and phase selectivity rather than fundamental structure direction.²⁵⁸

This stark contrast highlights that successful OSDA-free syntheses of complex frameworks require multi-cation inorganic cocktails. These combinations must collectively mimic the geometric and electronic properties of the target OSDA, guiding nucleation through specific thermodynamic or kinetic pathways accessible to inorganic species.

4.2. Seed engineering: from essential hetero-structure-directing to epitaxial growth modifiers

The function and necessity of seeds differ substantially between OSDA-free and OSDA-assisted pathways, reflecting distinct nucleation and growth barriers.

4.2.1. In OSDA-free routes: indispensable heterogeneous structure-directing

For many Dual-Route Accessible Zeolites, OSDA-free synthesis is strictly seed-assisted. Seeds act as obligatory structural directing agents, not merely accelerants.

In OSDA-free Beta or ZSM-5, for example, introduced seed crystals provide surfaces with atomic-level registry to the target framework. These surfaces catalyze the conversion of amorphous gel through mechanisms akin to epitaxial growth or solid-phase transformation, bypassing the high nucleation barrier typical of inorganic systems. This aligns with nonclassical crystallization pathways, where pre-existing crystals direct assembly.

Compared with stochastic homogeneous nucleation, seed-assisted synthesis offers greater controllability and reproducibility. Highly crystalline and phase-pure zeolites can therefore be obtained under milder conditions, including lower temperatures and shorter reaction times.

Mechanistically, seed-assisted synthesis primarily proceeds through heterogeneous nucleation (Fig. 6a). Seed surfaces provide low-energy interfaces for solute adsorption, thereby bypassing random homogeneous nucleation. In many cases, epitaxial growth occurs, and newly formed crystals inherit structural motifs from the seed. Crystallization may proceed via solution-mediated transport—where partial seed dissolution releases secondary building units that subsequently recrystallize—or via solid-state transformation involving direct structural rearrangement. In practice, these pathways often operate concurrently.

Fig. 6. Crystallization pathways and nucleation mechanisms of zeolites: (a) comparison of homogeneous nucleation via amorphous intermediates with heterogeneous pathways enabled by seeds; (b) reaction network and free–energy profiles for AFI nucleation on CHA seeds, illustrating ITL1-, ITL2-, and precursor-mediated pathways and their corresponding nucleation barriers; (c) two-step nucleation of AFI at 620 K, showing the evolution from monomer aggregation to crystallization via a ∼7 nm amorphous precursor.²⁵⁹ Copyright 2025, American Chemical Society.

Recent molecular dynamics simulations reveal that seeds influence not only nucleation rates but also nucleation mechanisms. Under seed-free conditions, zeolites such as AFI typically crystallize through a nonclassical, two-step pathway involving amorphous intermediate formation followed by ordering (Fig. 6c).²⁵⁹ In contrast, when crystalline seeds (e.g., CHA) are present, ordered interfacial transition layers (ITLs), such as ITL1 and ITL2, form on the seed surface.

These ITLs act as structural scaffolds that promote direct monomer attachment, bypass amorphous intermediates, and enable classical nucleation pathways (Fig. 6b). This transition arises from seed-induced reduction of interfacial energy, which decreases the critical nucleus size and significantly lowers nucleation barriers.

Competition between different ITL polymorphs further determines crystallization outcomes. For example, ITL1 may be kinetically favored but thermodynamically less stable, whereas ITL2 exhibits the opposite behavior. Supersaturation and temperature critically govern pathway selection.

Moderate supersaturation favors classical nucleation, whereas high supersaturation or elevated temperatures can reintroduce nonclassical pathways—even in the presence of seeds. Simulations show that classical nucleation dominates at 620 K under moderate flux, whereas amorphous intermediates reemerge at 660 K or under high flux conditions.

Overall, seed-assisted synthesis functions as a “mechanism switches,” enabling precise control over nucleation pathways, polymorph selection, and crystal growth. It therefore provides a powerful platform for rational zeolite design in catalysis, separation, and related applications. Future studies integrating in situ characterization with theoretical modeling will be essential to fully elucidate seed dynamics in complex reaction environments.

4.2.2. In OSDA-assisted routes: nucleation promoters and morphology regulators

In OSDA-mediated syntheses, seeds accelerate crystallization, improve phase purity, or control crystal size and morphology. However, they are not essential for phase formation. The OSDA itself provides the primary driving force for nucleation. Seeds in this context act as preferential nucleation sites to shorten induction periods or to guide oriented attachment, influencing crystal habit without altering the topology dictated by the OSDA.

This dichotomy emphasizes that seed design in OSDA-free synthesis must prioritize structural compatibility and surface chemistry. In contrast, seed design in OSDA-assisted synthesis can target functional outcomes, such as generating meso-porosity or creating core–shell structures.

4.3. Mineralizers and reaction environment: pathway divergence mediated by OH⁻ and F⁻

High OH⁻ concentrations promote rapid dissolution of silica and alumina, creating abundant soluble aluminosilicate species. This environment favors both solution-mediated growth (monomer or oligomer addition) and nonclassical pathways involving nanoparticle aggregation. OH⁻ systems are effective in both OSDA-free and OSDA-assisted syntheses but can sometimes produce higher defect densities or dense phases. They primarily establish a dynamic, high-supersaturation environment that drives rapid nucleation and growth.

Fluoride acts as a milder mineralizer and a structure-directing anion. It reduces silicate solubility, slowing crystallization and enabling the formation of highly crystalline, low-defect products. In OSDA-assisted synthesis, F⁻ stabilizes specific framework intermediates, especially in high-silica or all-silica zeolites. In OSDA-free systems, fluoride may induce slow reorganization of a dense, less soluble precursor, a pathway distinct from hydroxide-mediated rapid dynamics.

Thus, mineralizers do more than pH adjustment—they select the primary crystallization mechanism. OH⁻ favors faster, solution-mediated dynamics, whereas F⁻ promotes slower solid-phase rearrangements or subunit stabilization. The success of a Dual-Route Accessible Zeolites synthesis often depends on matching the stability of building units to the chemical environment imposed by the mineralizer.

4.4. Synthesized comparative analysis

Applying the Dual-Route Accessible Zeolites framework reveals a clear mechanistic decoupling between OSDA-free and OSDA-assisted routes.

OSDA-free routes: predominantly governed by the synergistic interplay of inorganic cations and seeds. Crystallization tends toward thermodynamically favorable or epitaxially matched products, often via precursor assembly or solid-phase transformation.

OSDA-assisted routes: dominated by kinetic control. The OSDA stabilizes specific transition states or metastable intermediates, enabling formation of frameworks that are more siliceous and less thermodynamically accessible.

This comparative insight moves beyond observation to provide a rational design framework. Structural and compositional features of OSDA-dependent zeolites can be deconstructed to identify the necessary inorganic chemical environment or composite template required to replicate their formation via sustainable, green synthesis. In this way, the Dual-Route Accessible Zeolites framework shifts the paradigm from empirical discovery to predictive engineering.

5. Exploring design rules and forward guidance from Dual-Route Accessible Zeolites for future OSDA-free synthesis

5.1. Natural zeolites: insights from dual-route accessible cases and unresolved challenges

The concept of Dual-Route Accessible Zeolites provides a powerful framework for elucidating crystallization mechanisms by directly contrasting OSDA-assisted and OSDA-free pathways. A representative example is the CHA zeolite, which occurs naturally as the mineral chabazite. In laboratory synthesis, CHA can be obtained either through purely inorganic, cation-mediated hydrothermal routes—closely resembling geological processes—or through OSDA-assisted strategies (e.g., using TMAda⁺), which enable pure-phase, high-silica products.

The coexistence of these pathways indicates that CHA crystallization is accessible under both thermodynamic and kinetic control. This dual accessibility makes CHA an ideal model system for mechanistic investigation.

However, systematic comparison across Dual-Route Accessible Zeolites systems reveals important exceptions. The EAB zeolite is particularly intriguing. EAB exists as a natural mineral and has been successfully synthesized using OSDAs.^161,260 Yet no purely inorganic laboratory synthesis has been reported to date. This discrepancy raises a fundamental question: how does EAB crystallize in geological environments in the apparent absence of OSDAs?

This paradox suggests that EAB formation may depend on specific geochemical conditions or inorganic structure-directing mechanisms that have not yet been reproduced experimentally. EAB therefore represents a critical unresolved case. Clarifying its formation mechanism could uncover previously unrecognized inorganic guidance principles and help bridge natural mineral genesis with synthetic zeolite chemistry.

5.2. Guidance for OSDA-free synthesis of traditionally OSDA-dependent zeolites

In recent years, significant progress has been made in extending OSDA-free synthesis to complex frameworks long considered strictly OSDA-dependent. The successful synthesis of ITQ-47 (BOG) and ITQ-58 provides instructive examples.

ITQ-47 was synthesized directly in aluminosilicate form using a seed-assisted strategy, with Si/B-ITQ-47 serving as the seed. Crystallization proceeded in an aluminosilicate gel without OSDAs, thereby eliminating the need for costly and environmentally burdensome phosphate ester OSDA.⁸⁴ Similarly, ITQ-58 was obtained under OSDA-free conditions using Si/B-ITQ-58 seeds, demonstrating the broader applicability of this approach.²⁶¹

These studies highlight a central principle: crystalline seeds can induce framework formation through core–shell growth or secondary nucleation. In doing so, they effectively replace the kinetic role traditionally played by OSDAs and enable greener, more economical synthesis routes.

Structural features appear to contribute to this feasibility. ITQ-47 contains intersecting 10- and 12-ring channels, whereas ITQ-58 is constructed from multiple cage-like building units. Such structural complexity and intrinsic porosity may reduce nucleation barriers, facilitating crystallization under inorganic, seed-directed conditions.

A broader survey of 36 Dual-Route Accessible Zeolites systems further supports this hypothesis. Several composite building units (CBUs) recur with high frequency. The double six-ring (d6r) unit appears in 11 systems; cas and mor each appear in 7; bik, gme, and lta each in 4; and bre, can, mtt, mtw, jbw, ton, pau, and sod each appear three times. Notably, among the 24 known aluminosilicate zeolites containing d6r units, 11 have already been synthesized via OSDA-free routes.

This strong correlation suggests that certain CBUs intrinsically lower nucleation barriers in inorganic systems, either through favorable thermodynamics or accessible kinetic pathways. The presence of such “facilitative CBUs” may therefore serve as a practical indicator for assessing OSDA-free feasibility. When combined with computational evaluation of CBU stability and seed–framework compatibility, this criterion provides a rational strategy for identifying promising targets among currently OSDA-dependent zeolites.

6. Conclusion and outlook

This review highlights the conceptual and practical significance of the Dual-Route Accessible Zeolites framework as a comparative platform for understanding zeolite crystallization. By systematically contrasting OSDA-assisted and OSDA-free routes, key insights into nucleation, growth, and structure–property relationships have emerged. Case studies of representative systems—including Beta, CHA, FAU, LTA and MFI—show that OSDA-free synthesis, enabled by inorganic cation regulation, seed engineering, and mineralizer control, offers not only economic and environmental advantages but also distinctive mechanistic insights.

OSDA-free zeolites typically exhibit low-to-medium Si/Al ratios. These compositions confer high acid-site densities, strong hydrophilicity, and excellent ion-exchange capacity. Such intrinsic features underpin their superior performance in gas adsorption, aqueous-phase catalysis, and radionuclide sequestration. Importantly, these properties arise directly from the synthesis pathway itself, positioning OSDA-free zeolites as sustainable, application-oriented materials rather than inferior substitutes for OSDA-derived counterparts.

More broadly, the Dual-Route Accessible Zeolites concept aligns with the growing demand for green chemistry while providing a unifying framework for understanding the thermodynamic and kinetic principles governing zeolite formation. It establishes a robust theoretical basis for the rational design of environmentally benign synthesis strategies.

Future research should address the remaining limitations of OSDA-free synthesis through interdisciplinary approaches, as outlined below.

6.1. Rational design guided by computational modeling

The Dual-Route Accessible Zeolites concept provides a powerful framework for leveraging computational modeling—including machine learning, density functional theory, and molecular dynamics—as both a mechanistic lens and a predictive engine. This dual-purpose approach deciphers the energetic and geometric rules governing dual-pathways topology formation and guides the design of novel synthesis routes. For strictly OSDA-dependent topologies (e.g., AEI,^262,263AFX,²⁶⁴ and ITH²⁶⁵), computational screening can identify inorganic agents or simpler organic mimics that replicate critical structure-directing functions. By translating insights from known dual-route accessible systems into design rules, this framework shifts zeolite discovery from empirical screening to rational design.^266–268 It also accelerates the development of sustainable, OSDA-free synthesis protocols while conserving experimental resources.

6.2. Mechanistic elucidation via in situ characterization

Advanced in situ techniques—such as synchrotron X-ray diffraction, solid-state NMR spectroscopy, and cryo-electron microscopy—are essential for capturing early-stage nucleation events under both OSDA-assisted and OSDA-free conditions.^269,270 Particular emphasis should be placed on distinguishing classical solution-mediated pathways from nonclassical mechanisms, including particle attachment. These studies may also reveal inorganic guidance mechanisms relevant to natural zeolite formation, potentially resolving long-standing questions surrounding systems such as EAB.

6.3. Performance enhancement through post-synthetic engineering

Post-synthetic modifications offer a powerful route to expand the functional scope of OSDA-free zeolites. Strategies such as controlled dealumination, heteroatom incorporation, and surface functionalization can tune acidity, hydrophobicity, and active-site distribution. For instance, mild dealumination of low-silica CHA can optimize acid-site distribution and enhance catalytic performance in CO₂ hydrogenation, methanol-to-olefins, and NH₃-SCR reactions—sometimes matching or surpassing OSDA-derived materials.^271,272

In parallel, circular-economy approaches—such as using waste catalysts as feedstocks combined with low-temperature activation—should be further developed to enable scalable and sustainable production.

6.4. Systematic screening of CBU-based targets for green synthesis

A coordinated effort is needed to identify OSDA-dependent zeolites that contain favorable CBUs and to explore their OSDA-free synthesis via seed-assisted or cation-pairing strategies. Such systematic screening will expand the library of sustainable zeolites and deepen understanding of how CBU stability correlates with nucleation energy barriers. Ultimately, this approach may contribute to universal design principles for green zeolite synthesis.

6.5. Towards a life-cycle perspective: the true footprint of OSDA-free synthesis

A comprehensive life-cycle assessment of OSDA-free zeolite synthesis must account for all post-synthesis processing steps. As-synthesized materials are typically obtained in their cationic form (e.g., as Na⁺- or K⁺-exchanged) and require ion exchange and calcination to produce the H+ catalysts essential for most acid-driven applications. These steps incur material and energy costs. However, a balanced comparison highlights a key distinction: while calcination to convert NH₄⁺-exchanged zeolites to H⁺-zeolites is common to all synthesis routes, the OSDA-free pathway avoids the initial high-temperature combustion required to remove complex OSDA—a step that is typically the most energy-intensive and high-emission in OSDA-assisted synthesis. For applications that do not require Brønsted acidity, such as adsorption, ion exchange, or preparation of advanced composites, the as-synthesized cationic zeolites are often functionally optimal, fully realizing the environmental advantages of OSDA-free synthesis without further processing.

Author contributions

All authors contributed to the preparation of the manuscript. Haonan Sun: investigation, writing – original draft, writing – review and editing. Sihui Li and Yushan Xie: investigation, visualization. Wenfu Yan and Jihong Yu: conceptualization, funding acquisition, project administration, supervision, writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.