m-Xylene Isomerization over IWW Zeolite with a Three-Modal Pore Structure: The Effect of Crystal Morphology

Journal Pre-proof m-Xylene Isomerization over IWW Zeolite with a Three-Modal Pore Structure: The Effect of Crystal Morphology Emad Shamma, Alica Seidlová, Subhajyoti Samanta, Michal Mazur, Maksym Opanasenko, Mariya Shamzhy PII: S0920-5861(25)00381-5 DOI: https://doi.org/10.1016/j.cattod.2025.115563 Reference: CATTOD115563 To appear in: Catalysis Today Received date: 27 June 2025 Revised date: 20 August 2025 Accepted date: 8 September 2025 Please cite this article as: Emad Shamma, Alica Seidlová, Subhajyoti Samanta, Michal Mazur, Maksym Opanasenko and Mariya Shamzhy, m-Xylene Isomerization over IWW Zeolite with a Three-Modal Pore Structure: The Effect of Crystal Morphology, Catalysis Today, (2025) doi:https://doi.org/10.1016/j.cattod.2025.115563 This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. m-Xylene Isomerization over IWW Zeolite with a Three-Modal Pore Structure: The Effect of Crystal Morphology Emad Shamma, Alica Seidlová, Subhajyoti Samanta, Michal Mazur, Maksym Opanasenko and Mariya Shamzhy* Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University, Hlavova 2030/8, 12843 Prague, Czech Republic Corresponding author: [email protected] Abstract: Xylene isomerization is a key zeolite-catalyzed petrochemical process for the production of pxylene, a highly demanded intermediate in the polymer industry. While MFI-type zeolites are widely used in industry as shapeselective catalysts, xylene isomerization also serves as a benchmark reaction for evaluating the shapeselectivity of new zeolite catalysts in relation to their porosity. In this study, IWW zeolite with a three-modal pore network of isolated 8and 12-ring pores intersected by 10-ring channels was investigated for m-xylene isomerization, with a focus on how both the multidimensional pore system and crystal morphology affect catalytic performance. IWW zeolites were synthesized as germanosilicates with platelet-like and needle-like crystals, functionalized by post-synthetic Ge-to-Al substitution, and tested in gas-phase m-xylene isomerization in comparison with reference zeolite catalysts containing unimodal 8-, 10-, or 12ring channels. Compared to MFI with a similar concentration of acid sites, Al-IWW catalysts exhibited higher m-xylene conversion across a wide range of WHSV values (4.4 – 40 h-1), regardless of crystal morphology. Needle-like Al-IWW crystals achieved para-selectivity comparable to that of MFI, while platelet-like Al-IWW outperformed MFI in p-xylene yield at short contact times (WHSV = 40 h-1). STEM analysis confirmed that the 10-ring channels are aligned along the length of the needle-like crystals, promoting shapeselective p-xylene formation. In contrast, the 12-ring channels running along the extended dimension of the platelet-like crystals facilitate the diffusion of reactant and product molecules to and from the active sites. All in all, the integration of multi-sized pores and tunable crystal morphology in IWW zeolites may offer a promising strategy for balancing selectivity and activity in p-xylene synthesis. Keywords: Zeolite, xylene isomerization, IWW, germanosilicate, crystal morphology, active site, shapeselectivity Journal Pre-proof 1. Introduction Xylene isomerization is an essential petrochemical process for the selective production of p-xylene, a key building block in the polymers industry. The highly demanded para-isomer typically constitutes only 25% of the xylene mixture obtained after hydrotreating C9 aromatics in refineries. To increase the p-xylene content, the mixture undergoes selective isomerization over solid acid catalysts, typically aluminosilicate zeolites, with appropriately sized channel entrances and voids which restrict product distribution. Zeolite catalysts are widely used in industry for xylene isomerization due to molecular diffusion control inside their micropores, abundance of acid sites, and long-term stability. Industrially used shapeselective zeolite catalysts (e.g., MFI, MOR, EUO) favor the preferential formation of p-xylene and play a key role in m-xylene transformation, which involves a complex reaction network, including (Fig. 1): (A) monomolecular or bimolecular isomerization, that enables the interconversion of xylene isomers; (B) monomolecular dealkylation, that yields toluene and light hydrocarbons through the removal of a methyl group from m-xylene; and (C) bimolecular disproportionation, that produces a mixture of toluene and trimethylbenzenes (TMBs) through methyl group transfer between two aromatic molecules [1-3]. Fig. 1. Scheme for the mechanisms of m-xylene transformation: (A) monomolecular isomerization, (B) monomolecular dealkylation, and (C) bimolecular disproportionation MFI zeolite with a unimodal 3D system of 10-ring pores is the most widely used catalyst for mxylene isomerization due to its high para selectivity. It contains two types of intersecting channels: straight channels (0.53 × 0.56 nm) and sinusoidal channels (0.51 × 0.55 nm), with a channel intersection of 0.86 nm. These structural features are key to both product and transition-state Journal Pre-proof shapeselectivity. The kinetic diameter of p-xylene (~0.58 nm) is smaller than those of mand oxylene (~0.68 nm), facilitating its preferential diffusion through the MFI pores [4]. Adsorption studies with pure xylene isomers show a diffusivity trend of para > ortho > meta in MFI, with pxylene exhibiting the highest diffusion coefficient. Moreover, the transition state complex for the meta-to-ortho isomerization is bulkier (0.67 nm) than that for meta-to-para isomerization (0.62 nm), further favoring p-xylene formation due to lower steric constraints. As a result, MFI zeolite typically delivers a para/ortho xylene ratio exceeding 2, and a low disproportionation-toisomerization (Dis/Iso) ratio of ~0.01, with unimolecular pathways dominating the reaction mechanism [3-6]. As such, xylene isomerization also serves as a benchmark reaction for assessing the shapeselectivity of new zeolite catalysts in relation to their porosity [1]. In this way, p-xylene selectivity over MFI zeolite has been further enhanced by tailoring crystal morphology. For instance, chain-like crystals with an extended b-axis showed a 10% increase in p-xylene selectivity compared to conventional coffin-shaped crystals. However, the narrow pores of MFI limit molecular diffusion and accessibility to active sites, which can promote undesired dealkylation reactions and reduce overall p-xylene yield [4, 7, 8]. These limitations have prompted the development of alternative zeolite catalysts. One such example is EUO zeolite, designed for industrial m-xylene isomerization. It features 1D 10-ring channels and special 12-ring side pockets, achieving higher para/ortho ratio than MFI (2 vs. 1.35) under similar conditions [9]. In combination with other components, EUO forms the basis of the commercial Oparis® catalyst family, which is widely used for the isomerization of ethylbenzene and m-xylene [8]. Hong and Jones [10, 11] compared the performance of zeolites with unimodal 1D 10-ring and 12ring pore systems to that of MFI. The 1D 10-ring MTT zeolite exhibited a higher para/ortho ratio (3.4) than the 3D 10-ring MFI (2.5) at 20% conversion. In contrast, increasing pore size, as in 1D 12-ring AFI and 3D 12-ring IFR, resulted in a para/ortho ratio nearly two times lower than that of MFI (approaching ~1). Moreover, the Dis/Iso ratio was lower for MTT compared to AFI and IFR, due to the stronger restriction of bimolecular reactions in the narrower 10-ring pores. MOR zeolite with bimodal 2D system of 8and 12-ring channels shows higher m-xylene conversion than 3D 10-ring MFI. Its larger 12-ring pores enable easier diffusion but also promote bimolecular disproportionation and transalkylation reactions, increasing the formation of toluene and trimethylbenzenes (TMBs), and thereby reducing p-xylene selectivity. In contrast, MFS zeolite, with a 2D network of 8and 10-ring pores, effectively suppresses bulky byproduct formation and shows a Dis/Iso ratio roughly half that of MFI (~0.005 vs. 0.01) [10, 12]. More recently, m-xylene isomerization has been employed to evaluate the shapeselectivity of new UTLderived isoreticular zeolite catalysts with tailored multimodal porosity, in comparison to MFI [13]. Zeolite PCR with 8and 10-ring pores achieved higher para/ortho ratios (4.5) than MFI (1.4). Journal Pre-proof Overall, the literature survey suggests that designing zeolite catalysts with a combination of different pore sizes represents a promising strategy for optimizing both shapeselectivity and diffusion control in xylene isomerization. Beyond pore size, crystal morphology-induced shapeselectivity, arising from variations in crystal size and shape, has been demonstrated to impact reaction pathways in both xylene isomerization and related acid-catalyzed transformations. The IWW zeolite, a three-modal germanosilicate with a system of 8-, 10-, and 12-ring pores and tunable crystal morphologies, offers a unique opportunity to integrate pore architecture and crystal morphology effects [14, 15]. Its 10-ring channels are associated with promoting monomolecular reactions and high p-xylene selectivity, while the 12-ring pores can facilitate the diffusion of reacting molecules. Our study investigates the under-explored zeolite IWW as a promising catalyst for m-xylene isomerization, while proposing strategies to balance product selectivity and catalytic activity. The performance of IWW was systematically compared with zeolites containing unimodal 8-, 10-, or 12-ring pore systems, allowing us to isolate the effects of pore connectivity and size, with MFI serving as the industrial benchmark. This comparison underscores the role of multi-sized micropore network in directing activity and selectivity of IWW catalysts. Furthermore, we demonstrate that tuning the crystal morphology of IWW offers an effective approach to optimize the trade-off between p-xylene selectivity and yield. Collectively, our findings suggest that integrating multisized pores with tunable morphology in IWW zeolites represents a promising strategy to balance selectivity and activity in p-xylene synthesis. 2. Experimental part The zeolites investigated in this study are listed in Table 1. They include IWW zeolites featuring a three-modal pore system with 8-, 10-, and 12-ring channels and either needle- (Al-IWW-needles) or platelet-like (Al-IWW-platelets) crystal morphologies, as well as reference zeolites with unimodal pore systems (i.e. having uniform sizes of the pores), either non-intersecting (1D) or intersecting (3D). *BEA (CP811E) and MFI (CBV8014) zeolites were provided by Zeolyst in ammonium form. Al-IWW-needles and Al-IWW-platelets catalysts were synthesized by the postsynthesis Al-for-Ge substitution in hydrothermally synthesized germanosilicate zeolites, IWWneedles and IWW-platelets, respectively. Other zeolites under study were prepared in aluminosilicate form by hydrothermal crystallization. Journal Pre-proof Table 1. Zeolites studied in this work Journal Pre-proof 2.1. Synthesis of zeolite catalysts CHA3D 8-ring zeolite was synthesized following the procedure reported in the previously published work [16], using N,N,N-trimethyl-1-adamantammonium hydroxide (TCI, 25% in H2O) as SDA and reaction mixture with the molar composition of 1 SiO2 : 0.35 Na2O : 0.05 Al2O3 : 0.35 SDA : 18 H2O. 1.6 g of sodium hydroxide (VWR, 99.2%) was first dissolved in the SDA hydroxide solution (32.96 g of SDA in 0.77 g of water). After the complete dissolution, 0.22 g of aluminium hydroxide (Acros Organics, extra pure) was added, and the mixture was left to stir for 30 minutes. Lastly, 16.67 g of colloidal silica HS-40 (Sigma Aldrich, 40% in H2O) was added dropwise to the mixture, and the reaction mixture was left to stir for 90 minutes at room temperature. 0.10 g of CHA previously synthesized according to Zones [17] was added to the mixture as seeds. The reaction mixture was transferred into a 90 mL Teflon-lined autoclave and left to crystallize for 12 days in an oven under rotation (60 rpm) at 160 °C. The solid product was isolated by filtration, washed with distilled water and dried for overnight at 60 °C. The synthesis yielded 3.0 g of CHA zeolite. The zeolite was calcined at 550 °C for 5 hours with a heating rate of 1 °C/min under air flow. ESV1D 8-ring zeolite was synthesised following the procedure reported by B. J. Campbell et al. [18], using N,N-dimethylpiperidinium hydroxide as SDA, which was prepared by adding dimethylamine to 1,5-dibromopentane (1:1) in ethanol under reflux and then the bromide salt obtained was filtered, washed with ethanol, and dried. The bromide aqueous solution was ion exchanged with Ambersep® 900(OH) anion exchange resin (Sigma Aldrich, 0.8 mmol of solid per 1 g of resin) to the hydroxide form of the SDA. The molar composition of the reaction mixture was 1.0 SiO2 : 0.3 Na2O : 0.04 Al2O3 : 0.2 SDA : 40 H2O. In a Teflon beaker, 22.48 g of the hydroxide SDA was stirred with 7.04 g of water, and then 1.03 g of aluminium sulfate hexadecahydrate (Lechner, 99%) was added to the solution. The mixture was left to stir for 30 minutes to dissolve the aluminium salt. Then, sodium silicate solution (37% in H2O, 18.80 g) was added dropwise, and the reaction mixture was left to stir for 1 hour at room temperature. The mixture was then transferred into a Teflon-lined 90 mL autoclave. The mixture was crystallized for 14 days in an oven under rotation (60 rpm) at 170 °C. The solid product was isolated by filtration, washed with distilled water and dried at 60 °C. The synthesis yielded 2.7 g of ESV zeolite. The zeolite was calcined at 680 °C for 5 hours with a heating rate of 1 °C/min under air flow. TON1D 10-ring zeolite was synthesised following a modified synthesis process reported by K. Haysaka et al. [19], using a reaction mixture with the molar composition of 112 SiO2 : 10 K2O : 1 Al2O3 : 24.5 SDA : 3265 H2O, where 1,6-diaminohexane (Alfa Aeser, 98%) was used as the SDA. In a Teflon beaker under stirring, 0.69 g of the SDA was dissolved in 11.50g of distilled water. After a complete dissolution of the SDA, 0.33 g of potassium hydroxide (Lechner, PA) and 0.15 g of aluminium sulfate hexadecahydrate were added, and the mixture was left to stir to dissolve all components. Subsequently, 4.0 g of colloidal silica AS-40 (Sigma Aldrich, 40% in H2O) was added dropwise. The reaction mixture was left to stir for 90 minutes at room temperature. Afterwards, the mixture was transferred to a Teflon-lined 25 mL autoclave. The mixture was crystallized in 5 days Journal Pre-proof in an oven under rotation (60 rpm) at 160 °C. The solid product was isolated by centrifugation, washed with distilled water and dried at 60 °C. The synthesis yielded 3.1 g of TON zeolite. The zeolite was calcined at 550 °C for 5 hours with a heating rate of 1 °C/min under air flow. AFI1D 12-ring zeolite was synthesized according to Ref. [20] using N,N,N-trimethyl-1adamantammonium hydroxide as SDA and reaction mixture with the molar composition of 30 SiO2 : 3 NaOH : 1 Al(OH)3 : 3 AdaOH : 954 H2O. In a Teflon beaker under stirring, 7.78 g of the SDA was added to 47.00g of distilled water, and then sodium hydroxide (0.38 g) and subsequently aluminium hydroxide (0.24 g) were added to the solution. The mixture was left to stir for 30 minutes. Lastly, 5.50 g of CAB-O-SIL M5 fumed silica (Acros Organics) was added under vigorous stirring, and the reaction mixture was left to stir for 90 minutes at room temperature until it became homogenous. The reaction mixture was then transferred into a 90 mL Teflon-lined autoclave and left to crystallize for 8 days in an oven under rotation (60 rpm) at 160 °C. The solid product was isolated by centrifugation, washed with distilled water and dried at 60 °C. The synthesis yielded 2.2 g of AFI zeolite. The zeolite was calcined at 550 °C with a heating rate of 1 °C/min under air flow for 5 hours. The catalytically active H-form of CHA3D 8-ring, ESV1D 8-ring, TON1D 10-ring and AFI1D 12-ring aluminosilicates were obtained by ion-exchange with ammonium nitrate (Sigma Aldrich, ≥99%) and then calcination. The zeolite powders were mixed with 1 M ammonium nitrate solution (1 g/100 mL). This mixture was left to stir for 8 hours at room temperature before filtration. The ion-exchange process was repeated five times. Afterwards, the samples were collected, dried at 60 °C, and calcined at 350 °C for 3 hours with a heating rate of 1 °C/min under air flow. IWW-needles and IWW-platelets zeolites were synthesized as germanosilicates using 1,5-bismethylpyrrolidinium pentane as SDA. The preparation of the SDA is described elsewhere [14]. The molar composition of the reaction mixture was varied as follows: (y) SiO2 : (1-y) GeO2 : 0.25 SDA : 15 H2O, where y = 0.90 resulted in crystallization of IWW-needles and y = 0.66 resulted in the formation of IWW-platelets. For the IWW-platelets, 0.78 g of the germanium source (GeO2, Sigma Aldrich, 99.99%) was first dissolved in 4.40 g of the SDA solution plus 3.37 g of water, and then 3.18 g of TEOS (Thermo Scientific, 98%) was added to the mixture as the silicon source. To synthesize the IWW-needles, 0.21 g of GeO2 was dissolved in the aqueous solution of 4.40 g of the SDA, containing 3.37 g of water, before adding 4.10 g of TEOS (Thermo Scientific, 98%) to the mixture. To grow the needles and accelerate the synthesis, 70 mg of a previously prepared IWW was added as seeds to the synthesis mixture as a 5% mole fraction of the silica source. The mixture was left to stir for enough time to evaporate the alcohol before charging the final gel into a 25 mL autoclave. The synthesis continued for ten days at 175 °C under rotation with a speed of 20 rpm. Later, the synthesis was quenched, and the solids were collected by centrifugation, washed with water and dried at 60 °C. The described synthesis procedure yielded and 1.4 g of IWW-platelets and 1.6 g of IWW-needles. The as-synthesized IWW samples were calcined at 550 °C for 5 hours with a heating rate of 1 °C/min under air flow. Journal Pre-proof Al-IWW-needles and Al-IWW-platelets were prepared by post-synthesis Al-for-Ge substitution in IWW-needles and IWW-platelets zeolites according to Ref. [21]. For that, the parent germanosilicate was treated with 1 M aluminium nitrate (Thermo Scientific, 98%) solution (1 g/100 mL) at 96 ℃ for four days. The solid product was collected by filtration, washed with excess water and 0.1 M HCl, and then dried at 60 °C for one day. The dry solid was calcined at 450 °C for 4 hours. 2.2. Characterization Powder XRD patterns were collected on a Bruker D8 Advance diffractometer equipped with a LYNXEYE XE-T detector and a Cu-Kα radiation generator (λ=1.54 Ǻ). Zeolite samples were homogenized using a mortar and pestle and loaded into plastic holders for measurements. Diffractograms were obtained in the 2θ range between 3° and 40°. Physisorption measurements were performed on a Micrometrics 3Flex volumetric Surface Area Analyzer using 0.1 g of the catalyst. Each sample was outgassed at 110 °C for 1 hour, with a heating rate of 1 °C/min, using the Micrometrics Smart Vac Prep system. Subsequently, the sample was activated at 250 °C for 8 hours with the same heating rate. A total of 40 data points with an equilibration interval of 180 s were collected for each nitrogen adsorption isotherm at –196 °C (77 K) in the p/p° range between 0 and 1. The micropore volume and external surface area were obtained using the t-plot method [22, 23]. The total pore volume was calculated at p/p° = 0.95. SEM images were obtained using JEOL JSM-IT800 and a Thermo Fisher Scientific Scios 2 DualBeam FIB-SEM microscopes using secondary and backscattered electron detectors. The sample was ground and spread over carbon tape. Images were acquired at an accelerating voltage of 1–3 kV, a beam current of 10–50 pA, and a working distance up to 10 mm. The size of crystals was estimated by measuring and averaging the dimensions of at least seven crystals using the ImageJ software. Elemental composition analysis was conducted by EDS using the FEI Quanta 200F X-ray detector integrated into the SEM system. Data collection was performed at an accelerating voltage of 15 kV, a beam current of 10 nA, and a working distance of 10 mm. In situ FTIR spectroscopic measurements were performed using a Nicolet iS50 spectrometer equipped with a DTGS detector in Transmittance mode. Spectra were collected in the range 4000– 400 cm-1 with a resolution of 4 cm-1 with 64 scans. The sample was homogenized by grinding and formed into a self-supported wafer with a density between 8 and 12 mg/cm2. The wafers were activated under high vacuum (≈6×10-4 Torr) at 450 °C for 4 hours with a heating rate of 5 °C/min. Trideuteroacetonitrile (AN), pyridine (Py) and 2,6-di-tert-butylpyridine (DTBPy) were used as probe molecules and were degassed by freezing-pump-thaw cycles prior to adsorption over activated sample wafers. AN was adsorbed at room temperature at a partial pressure of 3.5 Torr for 20 minutes. Subsequently, it was desorbed at room temperature for 20 minutes. Py was adsorbed at 150 °C at a partial pressure of 3.5 Torr for 20 minutes. Afterwards, it was desorbed at 150 °C, 250 °C, 350 °C and 450 °C for 20 minutes. DTBPy was adsorbed at 150 °C at the equilibrium Journal Pre-proof smaller probe molecule can access the acid centres in all 8-, 10and 12-ring pores. Py has a size comparable to that of the targeted product in the xylene isomerization reaction, p-xylene, and was used to probe acid sites relevant to catalysis. DTBPy was employed to assess acid sites located on the external surface of zeolites with unimodal 8and 10-ring pore systems, as its bigger size prevents diffusion into <12-ring micropores. The concentration of acid sites accessible for AN ranges from 0.06 to 0.70 mmol/g across the samples, with BAS (active sites in xylene isomerization [1, 31, 32]) fractions between 39% and 88%, indicating evident Lewis acidity. In contrast, the number of acid sites accessible for AN was lower than the total Al content, as reflected by the ΣAN/Al (Table 3), which ranges from 0.04 to 0.80. This discrepancy is commonly observed in literature; for instance, the results of Zholobenko et al. suggest (BAS + LAS)/Al ratios between 0.47 and 0.82 for various zeolites [25]. In our study, this ratio was significantly lower for zeolites with 1D unimodal pores (0.04 for ESV, 0.28 for TON, 0.16 for AFI) than that for zeolites with 3D unimodal channel systems (0.58 for CHA, 0.80 for MFI, 0.72 for *BEA) and multimodal-pore IWW catalysts (0.51 for both Al-IWW-needles and AlIWW-platelets). As we can exclude the inaccessibility of BAS for AN (the disappearance of 3610 cm-1 band of bridging ≡Si-(OH)-Al≡ groups after -AN adsorption is observed for all the samples), the results are most probably related to variations in the spatial distribution and coordination environment of aluminium, which affect the strength and accessibility of LASs, rendering some inactive or undetectable by AN under the applied conditions. Py accessed the same number of acid sites as AN in zeolites AFI and *BEA with the unimodal 12ring pores, and in the multimodal Al-IWW samples of both morphologies, while no acid sites were accessible to Py in zeolites ESV and CHA with unimodal 8-ring channels (Table 3). In turn, based on DTBPy adsorption, all 8and 10-ring zeolites with unimodal pore systems showed negligible concentration of external BASs (0.01 – 0.03 mmol/g), representing < 12% of the total Brønsted acidity determined with AN. On the other hand, all BAS detected by smaller probe molecules were also accessible to DTBPy in the 3D 12-ring *BEA zeolite and the multimodal AlIWW zeolites. A clear difference in acid site accessibility to DTBPy was observed between 12-ring zeolites with 1D and 3D pore systems: only 67 % of BAS were accessible to DTBPy in 1D AFI compared to 100% in 3D *BEA. Journal Pre-proof Table 3. Concentration of BASs and LASs in Al-IWW catalysts and reference zeolites with unimodal pore systems, as determined by FTIR spectroscopy of adsorbed probe molecules of variable sizes. Catalyst c(Al)a (mmol/g) c(AN)b (mmol/g) c(Py)b (mmol/g) c(DTBPy)b (mmol/g) BAS LAS Σ ΣAN/Al BAS LAS Σ ΣPy/Al BAS BASDTBPy/ ΣBAS ESV 1.54 0.04 0.02 0.06 0.04 *n.d. *n.d. *n.d. *n.d. *n.d. *n.d. CHA 1.20 0.47 0.23 0.70 0.58 n.d. n.d. n.d. n.d. 0.03 0.06 TON 0.60 0.15 0.02 0.17 0.28 0.29 0.02 0.31 0.52 0.01 0.02 MFI 0.46 0.25 0.12 0.37 0.80 0.26 0.11 0.36 0.78 0.03 0.12 AFI 0.64 0.05 0.06 0.11 0.17 0.08 0.07 0.15 0.23 0.04 0.67 *BEA 0.43 0.14 0.17 0.31 0.72 0.20 0.15 0.35 0.81 0.19 1 AlIWWplatelets 1.20 0.30 0.31 0.61 0.51 0.29 0.23 0.53 0.44 0.30 1 AlIWWneedles 1.12 0.22 0.36 0.57 0.51 0.32 0.27 0.59 0.53 0.23 1 a – based on EDS results b – based on in situ FTIR spectroscopic results * n.d.- not detected The comprehensive acidity characterization of the studied zeolites reveals two important trends: 1) The total BAS concentrations vary widely across the zeolites (0.04 - 0.47 mmol/g). Multimodal-pore Al-IWW catalysts exhibit BAS concentrations comparable to the benchmark MFI zeolite (0.22 - 0.30 mmol/g), while 1D zeolites such as ESV and AFI showed much lower values (0.04 - 0.05 mmol/g). This variation in active site numbers, along with the differences in pore size and dimensionality, is expected to strongly influence m-xylene conversion in the isomerization reaction. 2) The concentration of external BAS is negligible in 8and 10-ring zeolites (<0.03 mmol/g; <12% of total BAS), suggesting a minimal role in catalysis. Journal Pre-proof Overall, according to their basic characteristics, the prepared series of zeolites (Table 1) constitutes a representative and well-defined set of catalysts suitable for addressing the role of the multi-size pore architecture and crystal morphology of IWW zeolite catalysts in the acid-catalyzed transformation of m-xylene. 3.2. Catalytic performance in m-xylene isomerization The catalytic performance of the Al-IWW and reference zeolites was analyzed based on the mxylene conversion, p-xylene selectivity and yield, Dis/Iso, and p-xylene/o-xylene ratios. MFI with unimodal 3D system of 10-ring pores was used as the core reference for comparison because of its industrial significance. A direct comparison between zeolites listed in Table 1 at similar WHSV does not clearly reveal the effect of pore size or channel architecture on key catalytic performance characteristics in m-xylene isomerization. The key parameters of the catalytic performance (i.e., p-xylene selectivity, pxylene/o-xylene and Dis/Iso ratios) are strongly dependent on the conversion and vary widely among the studied catalysts due to significant differences in BAS concentrations (0.03–0.47 mmol/g, Table 3). Specifically, at comparable WHSV (4.4 – 8.8 h-1), m-xylene conversion ranged from 0 (for 1D 8-ring ESV) to 48% (for 3D 10-ring MFI, Fig. S6-A). Therefore, to concentrate on the effect of pore structure, the values of WHSV were tuned, yielding a narrower range of conversion between 20% and 35%, for better comparison (Fig. S6-B). Based on the analysis of the catalytic performance of zeolites with unimodal pore systems under optimized conditions and taking into account pore size-catalytic performance correlations known from the literature, the influence of pore size of zeolite catalysts on the selectivity in m-xylene isomerization can be summarized as follows: • Although the 8-ring pores of 1D ESV and 3D CHA are generally considered too small for m-xylene (0.73 nm) to enter [5, 33, 34], ESV shows minimal activity (1– 2% conversion), whereas CHA achieves 58% p-xylene selectivity at 20% conversion. Given limited external BAS (Table 3), the performance of CHA can be attributed to its dynamic framework flexibility, enabling molecular diffusion and promoting monomolecular isomerization, as supported by a low Dis/Iso (0.04) and high p/o-xylene ratio (1.5), unlike ESV, whose rigid pores and scarce accessible BAS result in poor activity (Fig. S6-A; Fig. S7). 8-ring zeolites (e.g., CHA) were unexpectedly active and selective, achieving up to 58% p-xylene selectivity with negligible side reactions. • Both 1D and 3D 10-ring zeolites, TON and MFI, showed the highest p-xylene selectivity of 63% (in the range of 20-35% conversion) and the lowest Dis/Iso (0.01), Journal Pre-proof confirming monomolecular isomerization predominance, though only MFI maintains stable activity without deactivation. • Despite lower BAS (0.14 vs. 0.25 mmol/g) and shorter contact time, 3D 12-ring BEA shows similar m-xylene conversion to MFI but the lowest p-xylene selectivity (44%) due to its large pores promoting bimolecular pathways, as evidenced by the highest Dis/Iso (~0.30). In contrast, 1D 12-ring AFI, with comparable pore size, surpasses BEA in selectivity (51% vs. 44%) after 35 min, rising to 60% with time as coke formation likely narrows pores and shifts the reaction toward monomolecular isomerization (Dis/Iso drops from 0.18 to 0.11). 12-ring zeolites (e.g., *BEA) showed the lowest p-xylene selectivity due to the prevalence of bimolecular sidereactions, such as disproportionation. When comparing the catalysts of interest (IWW) with the well-established standard (MFI), both plateletand needle-like Al-IWW samples consistently showed greater m-xylene conversion than MFI across all the tested contact times (Fig. 5). At long contact time (WHSV 4.4 h-1), Al-IWW-platelets and Al-IWW-needles exhibited similar conversions (~65%), notably exceeding that of MFI zeolite (~47 %). Given their comparable BAS concentrations (0.26 - 0.32 mmol/g, Table 2), the higher conversion of Al-IWW catalysts can be attributed to the enhanced molecular transport through their 12-ring pore system. Decreasing the contact time (WHSV increase 4.4 →20 →40 h-1) resulted in a steady decline in m-xylene conversion in MFI (47 → 35 → 32%) and Al-IWW-needles (65 → 55 → 44%) (Fig. 5) in relevance with the common explanations: (i) shorter residence time limits the extent of the reaction, (ii) diffusion constraints hinder access to active sites, and (iii) insufficient time to reach equilibrium lowers observed conversion [10, 35, 36]. Journal Pre-proof Fig. 5. Conversion of m-xylene vs. T-O-S over AlIWW-platelets, Al-IWW-needles, and MFI, catalysts at 350 °C and at WHSV of A) 4.4 h-1; B) 20 h-1; C) 40 h-1. Notably, in comparison to Al-IWW-needles and MFI, Al-IWW-platelets maintained m-xylene conversion in the range 65-55 % across the same WHSV range, suggesting more stable performance under reduced contact time (Fig. 5). Further insights into the long-term stability of Al-IWW-platelets and Al-IWW-needles were obtained from catalytic tests extended to 24 h on stream (Fig. S8). The results suggest that Al-IWW-platelets deactivate more slowly than Al-IWWneedles, with deactivation constants of 0.020 and 0.025 h-1 (considering the 1st order deactivation model). This stability may theoretically originate from (i) higher intrinsic activity of BAS in AlIWW-platelets leading to faster reaction rates; (ii) higher accessibility of BASs leading to their better utilization at high throughput due to more favourable pore arrangement, minimizing internal diffusion limitations. Hypothesis (i) was ruled out by pyridine-FTIR thermodesorption results (Fig. S9), which showed similar acid strength distributions in IWW catalysts of both studied morphologies. In contrast, hypothesis (ii) was in line with FTIR-DTBPy results, showing higher concentrations of accessible BAS in Al-IWW-platelets (Table 3). Furthermore, STEM results reveal the pore arrangement in the Al-IWW samples, which may result in more efficient mass Journal Pre-proof transport in the Al-IWW-platelets. Indeed, the platelet-shaped crystals (0.43 × 0.21 × <0.10 µm) offer more balanced pore orientation and diffusion paths compared to the needle-shaped Al-IWW crystals, where 10-ring channels run along the long edge of a crystal (~0.48 µm), and 12-ring pores are confined to much shorter dimensions (<0.1 µm). This anisotropy in the needle morphology likely restricts diffusion and contributes to the observed decline in conversion at higher WHSV. While providing benefits for m-xylene conversion at short contact time, the platelet-like morphology of Al-IWW was less efficient for achieving high para selectivity. Morphology-related differences in p-xylene selectivity between the Al-IWW catalysts are apparent in Fig. 6-A. At similar conversion levels (44-55%), MFI outperforms both Al-IWW catalysts with 50% p-xylene selectivity, while the Al-IWW-needles (45%) is more selective than the Al-IWW-platelets (41%). High para selectivity of MFI is consistent with the literature, where the 10-ring channels and their intersections contribute minimally to the bimolecular mechanism. Accordingly, MFI exhibited a near-zero Dis/Iso ratio (0.03), characteristic of a monomolecular isomerization pathway [10, 3739]. In the case of Al-IWW catalysts, the contribution of the active sites within the 12-ring channels seems non-negligible and lowers the selectivity towards the targeted para-isomer. This interpretation is supported by in situ FTIR spectroscopic studies using DTBPy adsorption, which showed that ~100% of BASs in Al-IWW-platelets and ~72% in Al-IWW-needles are accessible for bulky reactants, such as TMBs formed due to the bimolecular side reactions, as verified by the higher Dis/Iso ratio compared to the Al-IWW-needles (Fig. 6-B). While MFI outperforms Al-IWW catalysts in both p-xylene selectivity and Dis/Iso, all samples show a similar p-xylene/o-xylene of ~1.05 (Fig. 6-C). This trend outlines that Al-IWW samples yield a larger variety of side-products, such as TMBs, while maintaining a consistent distribution of xylene isomers. Moreover, the yield of p-xylene is increased at shorter contact time over the AlIWW catalyst (Fig. 6-D), especially on the Al-IWW-platelets (23%), which exceeds the p-xylene yield on MFI (20%) due to the high accessibility to Brønsted acid sites in addition to improved mass transport, as mentioned previously. All in all, the combination of 8-, 10and 12-ring channels in a 3D interconnected pore system makes Al-IWW a more active (in terms of p-xylene yield) to the benchmark MFI zeolite for gasphase m-xylene isomerisation at short contact times. Clear crystal morphology-related differences in catalytic behaviour were observed for Al-IWW catalysts, notably in both p-xylene selectivity and yield. These differences can be attributed to variations in diffusion pathways, accessibility, and the distribution of BASs within pores of different sizes and orientation. Journal Pre-proof Fig. 6. T-O-S dependence of: A) p-xylene selectivity, B) Dis/Iso; C) p-xylene/o-xylene, and D) pxylene yield at m-xylene conversion of 44 - 55% over the catalysts MFI, Al-IWW-platelets and Al-IWW-needles. 4. Conclusions Compared to the industrially relevant MFI zeolite used in xylene isomerization, the IWW zeolite, with its multimodal system of 12-, 10-, and 8-ring pores, demonstrates higher m-xylene conversion. Notably, only the platelet-like Al-IWW exhibited higher overall catalytic activity than MFI in terms of p-xylene yield (an increase of ~10%), which is likely associated with enhanced diffusion. However, this advantage is accompanied by lower para-selectivity, owing to significant bimolecular side reactions occurring in the 12-ring channels. Comparative analysis of the catalytic performance of IWW catalysts with distinct crystal morphologies revealed pronounced morphology-induced effects in m-xylene isomerization: Journal Pre-proof • Material containing “extended”10-ring channels aligned along the direction of crystal growth, promoted p-xylene formation through monomolecular isomerization and achieved p-xylene selectivity, approaching that of MFI. • Material possessing “extended” 12-ring transport pores showed lower selectivity but outperformed both MFI and the alternative IWW in terms of p-xylene yields, especially at shorter contact times. This was attributed to more efficient molecular transport. In conclusion, the combination of pores of different sizes in the IWW zeolite catalyst, along with its tuneable crystal morphology, presents a promising approach to achieving an optimal balance between selectivity and activity in the industrially important p-xylene synthesis. However, further optimization, such as selective passivation of acid sites in the 12-ring channels or targeted placement of active sites within the 10-ring channels, may be required to suppress non-selective pathways and enhance overall p-xylene selectivity over IWW catalysts. Author contribution E.S.: Investigation (synthesis, SEM-EDS, catalytic tests); Formal analysis; Data curation; Writing – original draft. A.S.: Investigation (post-synthesis, PXRD, catalytic tests); Formal analysis; Visualization; Writing – original draft. S.S.: Investigation (FTIR); Formal analysis. M.M.: Investigation (STEM); Formal analysis. M.O.: Supervision. Writing – review & editing. M.S.: Conceptualization; Project management; Funding acquisition; Writing – review & editing. All authors contributed to the final version of the manuscript. Acknowledgement The authors gratefully acknowledge Dr. M. Kubů for performing the nitrogen physisorption measurements and Dr. J. Přech for supervising catalytic tests. This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic through the ERC_CZ project LL 2104. The authors acknowledge Charles University Centre of Advanced Materials (CUCAM – OP VVV Excellent Research Teams, no. CZ.02.1.01/0.0/0.0/15_003/0000417) and TECHSCALE project (No. CZ.02.01.01/00/22_008/0004587) for providing infrastructure enabling this research. References [1] M. Guisnet, N.S. Gnep, S. Morin, Mechanisms of xylene isomerization over acidic solid catalysts, Microporous Mesoporous Mater., 35-36 (2000) 47-59. [2] Y. Byun, D. Jo, D.N. Shin, S.B. Hong, Theoretical Investigation of the Isomerization and Disproportionation of m-Xylene over Medium-Pore Zeolites with Different Framework Topologies, ACS Catal., 4 (2014) 1764-1776. Journal Pre-proof [3] N.R. Demikhova, M.I. Rubtsova, V.A. Vinokurov, A.P. Glotov, Isomerization of Xylenes (a Review), Pet. Chem., 61 (2021) 1158-1177. [4] Y. Liu, X. Zhou, X. Pang, Y. Jin, X. Meng, X. Zheng, X. Gao, F.-S. Xiao, Improved para-Xylene Selectivity in meta-Xylene Isomerization Over ZSM-5 Crystals with Relatively Long b-Axis Length, ChemCatChem, 5 (2013) 1517-1523. [5] K. Gugeler, J. Kästner, M. Dyballa, Computational Approach for Determining Molecular Diameters and Access to Pores, ACS Catal., 15 (2025) 4798-4816. [6] G. Mirth, J. Čejka, J.A. Lercher, Transport and Isomerization of Xylenes over HZSM-5 Zeolites, J. Catal., 139 (1993) 24-33. [7] S. Liu, S. Yang, J. He, D. Mao, C. Yin, Efficient synthesis of chain-like ZSM-5 zeolite for the m-xylene isomerization reaction, Inorg. Chem. Commun., 128 (2021) 108564. [8] J. Shi, Y. Wang, W. Yang, Y. Tang, Z. Xie, Recent advances of pore system construction in zeolite-catalyzed chemical industry processes, Chem. Soc. Rev., 44 (2015) 8877-8903. [9] G.N. Rao, R. Kumar, P. Ratnasamy, Shape selectivity of zeolite EU-1 in reactions of aromatic hydrocarbons, Appl. Catal., 49 (1989) 307-318. [10] H.-K. Min, S.H. Cha, S.B. Hong, Mechanistic Insights into the Zeolite-Catalyzed Isomerization and Disproportionation of m-Xylene, ACS Catal., 2 (2012) 971-981. [11] C.W. Jones, S.I. Zones, M. E. Davis, m-Xylene reactions over zeolites with unidimensional pore systems, Appl. Catal. A, 181 (1999) 289-303. [12] N.M. Tukur, S. Al-Khattaf, Comparison studies of xylene isomerization and disproportionation reactions between SSZ-33, TNU-9, mordenite and ZSM-5 zeolite catalysts, Chem. Eng. J. (Lausanne), 166 (2011) 348-357. [13] N. Remperová, J. Přech, M. Kubů, K. Gołąbek, J.F. Miñambres, M.-F. Hsieh, A. Turrina, M. Mazur, Gas-phase isomerisation of m-xylene on isoreticular zeolites with tuneable porosity, Catal. Today, 390-391 (2022) 78-91. [14] A. Corma, F. Rey, S. Valencia, J.L. Jordá, J. Rius, A zeolite with interconnected 8-, 10and 12-ring pores and its unique catalytic selectivity, Nat. Mater., 2 (2003) 493-497. [15] O. Veselý, M. Shamzhy, W.J. Roth, R.E. Morris, J. Čejka, Controlling Crystal Morphology of Anisotropic Zeolites with Elemental Composition, Cryst. Growth Des., 24 (2024) 2406-2414. [16] J. Devos, M.L. Bols, D. Plessers, C.V. Goethem, J.W. Seo, S.-J. Hwang, B.F. Sels, M. Dusselier, Synthesis–Structure–Activity Relations in Fe-CHA for C–H Activation: Control of Al Distribution by Interzeolite Conversion, Chem. Mater., 32 (2020) 273-285. [17] S.I. Zones, Zeolite SSZ-13 and its method of preparation, 1985, US4544538A. [18] B. J. Campbell, A. K. Cheetham, G. Bellussi, L. Carluccio, G. Perego, R. Millini, D. E. Cox, The synthesis of the new zeolite, ERS-7, and the determination of its structure by simulated annealing and synchrotron X-ray powder diffraction, Chem. Commun. (Cambridge, U. K.), (1998) 1725-1726. [19] K. Hayasaka, D. Liang, W. Huybrechts, B.R. De Waele, K.J. Houthoofd, P. Eloy, E.M. Gaigneaux, G. Van Tendeloo, J.W. Thybaut, G.B. Marin, Formation of ZSM‐22 Zeolite Catalytic Particles by Fusion of Elementary Nanorods, Chem. Eur. J., 13 (2007) 10070-10077. [20] Verified Syntheses of Zeolitic Materials, International Zeolite Association Structure Commission, Weinheim, Germany, 2016. [21] J. Zhang, Q.D. Yue, M. Mazur, M. Opanasenko, M.V. Shamzhy, J. Čejka, Selective Recovery and Recycling of Germanium for the Design of Sustainable Zeolite Catalysts, ACS Sust. Chem. Eng., 8 (2020) 8235-8246. [22] J. Rouquerol, F. Rouquerol, P. Llewellyn, G. Maurin, K. Sing, Adsorption by Powders and Porous Solids: Principles, Methodology and Applications, Academic Press2013. Journal Pre-proof [23] B.C. Lippens, J.H. de Boer, Studies on pore systems in catalysts: V. The t method, J. Catal., 4 (1965) 319-323. [24] B. Wichterlová, Z. Tvarůžková, Z. Sobalık, P. Sarv, Determination and properties of acid sites in H-ferrierite: A comparison of ferrierite and MFI structures, Microporous Mesoporous Mater., 24 (1998) 223-233. [25] V. Zholobenko, C. Freitas, M. Jendrlin, P. Bazin, A. Travert, F. Thibault-Starzyk, Probing the acid sites of zeolites with pyridine: Quantitative AGIR measurements of the molar absorption coefficients, J. Catal., 385 (2020) 52-60. [26] K. Góra-Marek, K. Tarach, M. Choi, 2,6-Di-tert-butylpyridine Sorption Approach to Quantify the External Acidity in Hierarchical Zeolites, J. Phys. Chem. C, 118 (2014) 12266-12274. [27] https://www.iza-structure.org/databases/. [28] M. Shamzhy, F.S.d.O. Ramos, Tuning of acidic and catalytic properties of IWR zeolite by post-synthesis incorporation of three-valent elements, Catal. Today, 243 (2015) 76-84. [29] M.V. Shamzhy, P. Eliašová, D. Vitvarová, M.V. Opanasenko, D.S. Firth, R.E. Morris, PostSynthesis Stabilization of Germanosilicate Zeolites ITH, IWW, and UTL by Substitution of Ge for Al, Chem. Eur. J., 22 (2016) 17377-17386. [30] V. Kasneryk, M. Shamzhy, J. Zhou, Q. Yue, M. Mazur, A. Mayoral, Z. Luo, R.E. Morris, J. Čejka, M. Opanasenko, Vapour-phase-transport rearrangement technique for the synthesis of new zeolites, Nat. Commun., 10 (2019) 5129. [31] G.P. Babu, S.G. Hegde, S.B. Kulkarni, P. Ratnasamy, Active centres over HZSM5 zeolites: I. Xylene isomerization, J. Catal., 81 (1983) 471-477. [32] É. Miklósy, J. Papp, D. Kalló, Xylene isomerization on H-mordenites and H-clinoptilolites, Zeolites, 3 (1983) 139-148. [33] G. Jajko, K. Tarach, K. Góra-Marek, P. Kozyra, W. Piskorz, Toward the Mechanism of oXylene Isomerization in Selected Zeolites of Different Si/Al Ratios and Channel Sizes— Experiment Corroborated by Periodic DFT + D Simulations, J. Phys. Chem. C, 125 (2021) 1033410348. [34] J. Toda, A. Corma, G. Sastre, Diffusion of Trimethylbenzenes and Xylenes in Zeolites with 12and 10-Ring Channels as Catalyst for Toluene-Trimethylbenzene Transalkylation, J. Phys. Chem. C, 120 (2016) 16668-16680. [35] X. Chang, Y. Li, Z. Zeng, Kinetics study of the isomerization of xylene on HZSM-5 zeolite. 1. Kinetics model and reaction mechanism, Ind. Eng. Chem. Res., 31 (1992) 187-192. [36] S. Laforge, D. Martin, M. Guisnet, Xylene transformation over H-MCM-22 zeolites: 3. Role of the three pore systems in o-, mand p-xylene transformations, Appl. Catal. A, 268 (2004) 3341. [37] F.J. Llopis, G. Sastre, A. Corma, Xylene isomerization and aromatic alkylation in zeolites NU87, SSZ-33, β, and ZSM-5: molecular dynamics and catalytic studies, J. Catal., 227 (2004) 227241. [38] S. Al-Khattaf, Xylenes Reactions and Diffusions in ZSM-5 Zeolite-Based Catalyst, Ind. Eng. Chem. Res., 46 (2007) 59-69. [39] S. Zheng, A. Jentys, J.A. Lercher, Xylene isomerization with surface-modified HZSM-5 zeolite catalysts: An in situ IR study, J. Catal., 241 (2006) 304-311. Journal Pre-proof