A stable zeolite with atomically ordered and interconnected mesopore channel

Main

The quest for synthesizing mesoporous zeolites with pores larger than 20 Å (refs. ^10,11) has persisted for decades, as the large-pore industrially relevant zeolites exhibit pore openings of less than 7.5 Å (refs. ^12–14). Several aluminosilicate molecular sieves with intrinsic mesopores of size in the range 20–300 Å, such as M41S family materials^15,16, were reported, but they demonstrated relatively low thermal/hydrothermal stability and weak acidity owing to their amorphous nature¹⁷. So far, there are only two reported meso-microporous germanosilicate zeolites with pore openings that reached the mesopore range, that is, ITQ-37 (-ITV) (30-ring, 19.3 × 4.3 Å)¹⁸ and ITQ-43 (-IRT) (28-ring, 21.2 × 4.1 Å)¹⁹ (Supplementary Table 1). Unfortunately, those zeolites are not stable after the removal of OSDAs, thus hindering their practical application. Recently, stable aluminosilicate and pure-silicate zeolites with extra-large pores (pores delimited by rings greater than 12 tetrahedral atoms) of 16MR (10.6 Å, JZO)²⁰, 16MR (10.36 Å, JZT)²¹ and 20MR (14.3 Å, ZEO-5)²² were realized by using a phosphorus-containing bulky OSDA, pushing the channel size towards the mesopore region. However, the synthesis of stable aluminosilicate zeolites with intrinsic mesopores has not yet been achieved. Here we demonstrate that ZMQ-1 is the first attainment, to our knowledge, of such a goal reaching a 28-ring pore with a free diameter of 22.76 Å.

Synthesis of ZMQ-1 zeolite

ZMQ-1 was synthesized using a bolaform phosphonium-based OSDA (denoted as Tri-Cy-dC8; see Extended Data Fig. 1a and Supplementary Figs. 1 and 2) through conventional hydrothermal crystallization (see Methods for detailed synthesis procedures). Comparing ammonium-based OSDAs massively used in zeolite synthesis and phosphonium-based ones, the latter offer a stronger positive charge and flexibility of cyclohexyl groups, an advantage in the quest for extra-large-pore zeolites. Furthermore, the phosphonium-based OSDAs exhibit higher hydrothermal stability and can be used under harsher hydrothermal conditions. Our focus on a bolaform phosphonium-based OSDA with a relatively long methylene chain between the positively charged bulky and rigid heads is justified by the fact that the latter feature could generate a large pore space, whereas the long distance between the charged heads leaves enough space for the formation of zeolite framework.

ZMQ-1 was first discovered in a hydroxide medium at a synthesis temperature of 180 °C and the crystallization was accelerated at 190 °C (Extended Data Fig. 2a). Despite the high synthesis temperature, the OSDA was found to be intact in the as-synthesized zeolites (Extended Data Fig. 1b,c). The Si/Al molar ratio of ZMQ-1 zeolite could be tuned in a wide range from about 15 to 70 (Extended Data Fig. 2b), which endows it with versatilities for applications that require different acidic properties. Besides, we successfully obtained ZMQ-1 zeolites using different Si and Al sources and various compositions (Extended Data Fig. 2c), proving the robustness of the synthesis. It took longer crystallization time to obtain ZMQ-1 zeolite in a fluoride medium (Extended Data Fig. 2d). The morphology of ZMQ-1 obtained in hydroxide and fluoride media is individual and aggregated rectangular prisms, respectively, with the maximum size of about 2 μm (Extended Data Fig. 3). The synthesis compositions, crystallization conditions and phases obtained are summarized in Supplementary Table 2.

Structure determination of ZMQ-1

The size of ZMQ-1 crystals was too small for single-crystal X-ray diffraction. Its structure was determined by 3D ED, more specifically, continuous rotation electron diffraction (cRED)²³ (Fig. 1 and Extended Data Fig. 4). cRED data show that the calcined ZMQ-1 is orthorhombic and the unit-cell parameters were further refined against the powder X-ray diffraction (PXRD) data by Pawley fitting to be a = 18.83(10) Å, b = 54.9(2) Å and c = 20.36(5) Å (Supplementary Table 3 and Extended Data Fig. 5a). The space group Cmmm (no. 65) was used for the structure determination, based on the observed reflection conditions (Extended Data Fig. 4a–c). The structure of the calcined ZMQ-1 has a 3D 28 × 10 × 10-ring channel system with the extra-large pore delimited by 28 T-atoms (Fig. 1 and Supplementary Video 1). The 28-ring channels are interconnected by 10-rings (Fig. 1b). There are three symmetry-independent 10-ring windows; each contains two rings and connects two adjacent 28-ring channels. Two windows are perpendicular to the a axis: one built by double 10-rings (#1, in cyan) and one by 10-rings and 12-rings (#2, in green). The third one is perpendicular to the [±510] directions, built from two 10-rings (#3, in blue). Four 10/12-ring windows (#2) are located at z = 0 and six 10/10-ring windows at z = ½ in sequence #133133. To our knowledge, ZMQ-1 is the first stable aluminosilicate zeolite containing 28-ring channels. It has a fully tetrahedrally connected zeolite structure instead of the ‘interrupted framework’ featured by other extra-large-pore zeolites (EMM-23 (-EWT), NUD-6 and SSZ-70)^24–26. The free pore diameters are 22.76 Å × 11.83 Å for the 28-ring channel and 5.91 Å × 5.32 Å (#1), 5.56 Å × 5.58 Å (#2) and 5.11 Å × 5.90 Å (#3) for the 10-ring windows (Fig. 2a). Thus, ZMQ-1 exhibits the largest pore size ever reported among all existing stable zeolites (Supplementary Table 1). It has the second lowest framework density (11.4 Si atoms nm⁻³) among all fully connected silicate zeolites and a very low density (1.14 g cm⁻³). This is comparable with the 20-ring ZEO-5 zeolite (11.1 Si atoms nm⁻³) obtained by a post-synthetic expansion of  a complex chain silica zeolite precursor²². The presence of the extra-large 28-ring channels is also confirmed by annular dark field (ADF)-scanning transmission electron microscopy (STEM) images (Fig. 1c). The simulated PXRD pattern from the structure of calcined ZMQ-1 agrees well with the experimental PXRD pattern, further confirming the structure obtained from the cRED data (Extended Data Fig. 5b).

The dimensions of the extra-large pore in ZMQ-1 are extraordinary, yet its structural features bear similarities to common zeolite structures. The structure is built from wavy layers perpendicular to the b axis (in green), which are connected by flat strips extending along the c axis (in purple), as shown in Extended Data Fig. 6a–c. The strip contains 10-rings built of butterfly units commonly found in zeolites²⁷ (Extended Data Fig. 6c). A unique feature in ZMQ-1 is that all TO₄ tetrahedra in the butterfly net point in the same direction and connect to another butterfly net to form a double strip (Extended Data Fig. 6a,b). This differs from other zeolites built from butterfly units, in which different TO₄ tetrahedra in a butterfly net point in different directions²². The structure of ZMQ-1 contains typical composite building units of zeolites, d5r, d6r and non.

To understand the role of the OSDAs in the formation of the extraordinary channels in ZMQ-1, it is crucial to determine the position of the OSDA molecules in the structure. Therefore, we also investigated the as-synthesized ZMQ-1 by cRED (Extended Data Fig. 4d–f). cRED data of the as-synthesized ZMQ-1 collected at room temperature show that the crystals are monoclinic with the space group P2/m and unit-cell parameters of a = 18.94(3) Å, b = 20.21(4) Å, c = 27.51(7) Å and β = 103.56(19)°. The framework structure was determined on the basis of cRED data merged from three different crystals to achieve high data completeness. Similar to the calcined material, the as-synthesized ZMQ-1 also has extra-large 28-ring channels that are interconnected by 10-ring windows (Fig. 1e and Extended Data Fig. 6d). The main difference is that the 28-ring channels are tilted and bilobal shaped. This is confirmed by integrated differential phase contrast (iDPC)-STEM images (Fig. 1f). Local positional disorder was found for two symmetry-independent SiO₄ tetrahedra during the structure refinement, which were modelled using two different parts (Si29,30, O62–66 for PART 1 and Si31,32 and O67–70 for PART 2) with 56% and 44% occupancy, respectively. The tetrahedra associated to Si30 and Si32 have terminal groups at O66 and O70, respectively. Another terminal group is at O58, bonded to Si27. ADF-STEM images of as-synthesized and calcined ZMQ-1 show that the crystals are well ordered and free of defects (Extended Data Fig. 7a,b). The locations of symmetry-independent OSDAs were determined and refined using the low-dose cRED data (see Methods for detailed technical procedures). The final refined unit-cell compositions of calcined ZMQ-1 and as-synthesized ZMQ-1 are [Si₂₄₀O₄₈₀] and |C₄₄H₈₂P₂|_2.65[Si₁₂₀O₂₄₀(OH)₄], respectively. The number of OSDAs per unit cell is close to that obtained by elemental analysis (Supplementary Table 4). The PXRD pattern simulated on the basis of the structure of as-synthesized ZMQ-1 closely matches the experimental PXRD pattern, confirming the structure with OSDAs obtained from the cRED data (Extended Data Fig. 5c).

Notably, the octamethylene chain in each OSDA is interlocked by 10-ring windows. The tricyclohexylphosphonium heads extend into two different 28-ring channels. Because of their flexibility, the tricyclohexyl groups could not be located from the Fourier maps and they were instead added to complete the OSDA using Materials Studio (Fig. 1e and Extended Data Fig. 6d). The tricyclohexylphosphonium heads fill up the space of the extra-large 28-ring channel, which we believe plays an important role in directing the formation of the 3D framework and the intrinsic microporous and mesoporous 28 × 10 × 10-ring channel system. There are two symmetry-independent terminal hydroxyl groups (associated to O58 and O66/O70) in the as-synthesized ZMQ-1. Following calcination, the terminal hydroxyl groups condense and a fully connected framework is formed without any observed disorder (Fig. 1d,e and Supplementary Fig. 3). This results in the transformation of the bilobal-shaped channels into elliptical channels (Fig. 1d).

Physicochemical properties of ZMQ-1

It is well known that residual phosphorus remains within the micropores following calcination of zeolites obtained using phosphonium-based OSDAs. However, this residual phosphorus can be completely removed by washing with an ammonium chloride solution, as demonstrated in our previous work on extra-large-pore zeolites²⁸. This removal is crucial as it allows for the evaluation of the true micropore volume without any blocking effect being caused by the residual phosphorus species. The Ar at 87 K physisorption of the ZMQ-1 zeolite after calcination and washing with NH₄Cl solution exhibits a type I(b) isotherm (Fig. 2b), consistent with the presence of extra-large micropores or narrow mesopores (<2.5 nm)¹⁰. The corresponding cumulative pore volume plot and pore size distribution were calculated using state-of-the-art nonlocal density functional theory (NLDFT) methods, considering a cylindrical pore model and a zeolitic surface (Fig. 2c). The data revealed two main volume uptakes associated with different pore sizes: one monomodal distribution centred around 0.5 nm and a more complex bimodal one with mode peaks centred at around 2.05 nm and 1.10 nm. The lower value (0.5 nm) can be naturally ascribed to a medium-pore-sized zeolite, consistent with the size of the 10MR channel (Fig. 2a). In the same way, the prominent peak centred at 2.05 nm fits well with the largest opening dimension of the 28MR and is close to the crystallographic mesopore size (2.28 nm). The other embedded peak located at 1.10 nm could originate from the smallest dimension of the 28MR (1.18 nm). The exact assignment of this intermediate volume to a specific network feature is not straightforward. Indeed, it should be stated that the NLDFT model used to calculate the pore size distribution only considers independent cylinders with molecularly smooth walls as a pore model. The apparent Brunauer–Emmett–Teller surface area of ZMQ-1 zeolites calculated from Ar adsorption data and using the Rouquerol criteria is about 1,447 m² g⁻¹, with a record-high zeolite pore volume of about 0.47 cm³ g⁻¹ (Supplementary Table 4). The term ‘zeolite pore volume’ is proposed here to supplement the classical term ‘micropore volume’, as the mode pore size distribution of the material falls within the arbitrary limit set by the International Union of Pure and Applied Chemistry (IUPAC) to define the micropore and mesopore regions. Therefore, discussing an intraframework zeolite volume rather than micropore and mesopore volumes seems more logical. The as-synthesized and phosphorus-containing ZMQ-1 zeolites show high thermal and hydrothermal stability up to 800 °C (Extended Data Fig. 8c,d). Notably, the phosphorus-free ZMQ-1, after ammonium exchange, exhibits thermal stability comparable with the directly calcined phosphorus-containing ZMQ-1, maintaining stability even up to 1,000 °C (Extended Data Fig. 8e). Furthermore, phosphorus-free ZMQ-1 shows excellent hydrothermal stability up to 800 °C (Extended Data Fig. 8f) in a 50% relative humidity medium, comparable with its phosphorus-containing counterpart.

The ²⁹Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectrum of the as-synthesized ZMQ-1 zeolite exhibits a wide peak ranging from −120 to −90 ppm. This corresponds to Q⁴ and Q³ species (Extended Data Fig. 9a), evidencing the structural disorder revealed by the structure refined against cRED data (Fig. 1d–f). Those defective sites condensed to become fully connected on thermal treatment starting at 400 °C, as observed during the temperature-evolved calcination of an as-synthesized ZMQ-1 through in situ Fourier-transform infrared spectroscopy (FTIR). Figure 3a shows the spectra collected under dynamic vacuum (10⁻⁶ torr) at room temperature and after heating at 150 °C for 75 min. The two bands between 3,000 and 2,800 cm⁻¹ correspond to the C–H stretching vibrational band of the di-phosphonium OSDA. On heating at 150 °C, the broad band spanning between 3,600 and 3,000 cm⁻¹ (corresponding to hydrogen-bonded hydroxyl groups and H₂O) and a sharp band at around 1,610 cm⁻¹ (corresponding to the scissor bending vibrational band of H₂O) vanish, indicating the evacuation of residual water molecules. Furthermore, a band with a well-resolved maximum at 3,700 cm⁻¹ and an overlapping sideband at 3,735 cm⁻¹ are observed in the hydroxyl group stretching vibration region. On the basis of the literature findings^29–32, such bands are attributed to isolated and weakly hydrogen-bonded hydroxyl groups. This finding agrees with the figures describing the location of these species within the porous surface of ZMQ-1. The existence of two different vibrational bands for these silanol species is a result of their involvement in different hydrogen-bonding interactions within the ZMQ-1 framework. Figure 3b shows the spectra recorded under dynamic vacuum (10⁻⁶ torr) after heating the as-synthesized ZMQ-1 up to 450 °C. As the heating temperature reaches 400 °C for 1 min, both silanol bands are maintained and undergo a blueshift of 6 cm⁻¹, leaving the band at 3,741 cm⁻¹ with a well-resolved maximum, yet remaining overlapped. Notably, on heating for an extra 4 min at 400 °C (total of 5 min), both silanol bands merge into a single band centred at 3,734 cm⁻¹ with a broad half-bandwidth, accompanied by the appearance of a weak band at 3,620 cm⁻¹, characteristic of Brønsted acid sites. After heating ZMQ-1 for 4 h at 450 °C, a sharp, well-resolved band also appears at 3,745 cm⁻¹, attributed to isolated hydroxyl groups. This observation suggests that the silanol species within ZMQ-1 undergo condensation along with the calcination of the OSDA, which leads to the healing of the defects observed with cRED. Mainly isolated silanol species remain after calcination. This is further supported by the ²⁹Si NMR of ZMQ-1 calcined-washed with NH₄Cl, for which sharper peaks above −106 ppm may be identified (Extended Data Fig. 9a), indicating a transformation in the hydroxyl groups following calcination. The complex multicomponent band between 3,660 and 3,550 cm⁻¹ mainly corresponds to Brønsted acid sites formed after the removal of the OSDA, for which it is possible to distinguish a well-resolved maximum at 3,620 cm⁻¹ with a weak shoulder at 3,635 cm⁻¹ and a medium-intensity overlapping band at 3,595 cm⁻¹. This aspect of the Brønsted acid sites band might suggest that these sites are perturbed within ZMQ-1 owing to hydrogen bonding with neighbouring oxygen atoms.

In the as-synthesized zeolite, one main ²⁷Al environment is observed at 53.8 ppm, which corresponds to tetrahedrally coordinated sites in the zeolite framework (Extended Data Fig. 9b). After calcination, the zeolite shows two ²⁷Al broad environments at about 56.4 and −10 ppm. The former corresponds to Al in the tetrahedral sites. By contrast, the latter corresponds to different Al–O–P species³³, noting that a broad peak appears between 0 and −40 ppm in the ³¹P NMR spectrum after calcination (Extended Data Fig. 1c). When the calcined zeolite is washed with ammonium chloride and then calcined again, tetrahedral ²⁷Al environments persist and a further sharp peak appears at 0 ppm assigned to extra-framework aluminium species (Al^VI). Besides, the phosphorus was not detected in the ³¹P NMR spectrum (Extended Data Fig. 1c) and the ²⁷Al NMR peak at −10 ppm disappears as well (Extended Data Fig. 9b).

Catalytic properties of ZMQ-1

Since the adoption of zeolites by the petrochemical industry as an alternative to amorphous aluminosilicate catalysts, the quest to develop extra-large-pore zeolites has been primarily driven by the need to treat the heavy oil fraction of petroleum for economic value. This makes it of great interest to evaluate the catalytic performance of ZMQ-1 in vacuum gasoil (VGO) cracking as a benchmark test for its ability to valorize bulky molecules into value-added products. Here we tested ZMQ-1 zeolites after calcination (ZMQ-1(C)), which contains residual phosphorus species, and after subsequent washing with ammonium chloride (ZMQ-1(CW)) in the catalytic cracking of VGO and compared the results with commercial USY and Beta zeolites, as well as aluminium containing MCM-41 molecular sieve (see Methods for detailed experimental conditions and optimizations). Figure 4 shows that ZMQ-1 exhibits a comparable conversion rate of VGO to USY and Beta, and a much higher rate than MCM-41. In terms of product selectivity, phosphorus-free ZMQ-1 shows nearly the same selectivity as USY, with both materials favouring the formation of higher fractions of gasoline and diesel compared with Beta zeolite. Notably, phosphorus-containing ZMQ-1 shows a twofold higher selectivity towards diesel, accompanied by a twofold lower percentage of coke formation, which aligns with previous reports on phosphorus-containing extra-large-pore aluminosilicate ZEO-1. This brings the fuel selectivity (gasoline + diesel) for phosphorus-containing ZMQ-1 up to 80%, which is higher than its counterparts (see Extended Data Table 1). These preliminary catalytic test results highlight the strong potential of ZMQ-1 for acid-based heterogeneous catalysis, driven by its intrinsic mesoporosity and inherent phosphorus content, which does not require any post-synthetic treatment and competes well with USY zeolite, the horsepower of fluid catalytic cracking catalysis.

The characteristics of ZMQ-1 provide a new platform for fundamental research and diverse applications, including adsorption, separation, storage and ion exchange. Its intrinsic mesopores, robust framework and inherent acidity present new opportunities for advancements in process industries.

Methods

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Any methods, additional references, Nature Portfolio reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at 10.1038/s41586-024-08206-1.

Supplementary information

Author contributions

V.V. and P.L. conceived and supervised the project. X.Z. led the structure determination part of the project, including structure refinement, location of OSDAs and topology analysis of the zeolites. Y.S. performed the syntheses and characterizations of OSDAs and zeolites. J.X. conducted all 3D ED experimental work, data collection, analysis and structure determination. R.G.-N. performed the physisorption analysis. M.F. performed the in situ FTIR analysis. E.D. carried out the solid-state NMR analysis. Z.L., Z.Q. and B.W. performed the reaction of catalytic cracking of VGO. T.W. conducted STEM imaging, model building, topology analysis and prepared structure models in the figure. H.X. co-supervised J.X. with data processing. J.C. collected and analysed the low-dose cRED data of as-synthesized sample at SciLifeLab. X.Y., Q.L. and H.Y. optimized the syntheses and characterizations of selected samples. S.M. contributed to the discussion and analysis of data. V.V., X.Z., P.L., J.X. and M.F. wrote the manuscript. All authors contributed to the discussion and revisions of the paper.

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Data availability

The crystallographic information files (CIFs) for the structures of as-synthesized and calcined ZMQ-1 zeolites are archived at the Cambridge Crystallographic Data Center (https://www.ccdc.cam.ac.uk/) under reference numbers 2348667 (as-synthesized ZMQ-1) and 2348668 (calcined ZMQ-1).

Competing interests

P.L., V.V., Y.S., X.Y. and Q.L. have filed a patent on ZMQ-1 zeolite affiliated with Qingdao Institute of Bioenergy and Bioprocess Technology (China patent application no. 2023106333337; PCT application no. PCT/CN2023/106253).

Extended data

is available for this paper at 10.1038/s41586-024-08206-1.

Supplementary information

The online version contains supplementary material available at 10.1038/s41586-024-08206-1.