Production of high value-added phenolic compounds through lignin catalytic pyrolysis over ion-exchanged hierarchical ZSM-5 and Beta zeolites.

Production of high value-added phenolic compounds through lignin catalytic pyrolysis over ion-exchanged hierarchical ZSM-5 and Beta zeolites M.I. ´ Avila a , M.M. Alonso-Doncel b , J. Cueto b , L. Briones a , G. G´ omez-Pozuelo a , J.M. Escola a,c , D.P. Serrano a,b , A. Peral a,* , J.A. Botas a,c,** a Chemical and Environmental Engineering Group, ESCET, Rey Juan Carlos University, c/ Tulip´ an s/n, 28933, M´ ostoles, Madrid, Spain b Thermochemical Processes Unit, IMDEA Energy, Avda. Ram´ on de la Sagra 3, 28935, M´ ostoles, Madrid, Spain c Instituto de Investigaci´ on de Tecnologías para la Sostenibilidad, Rey Juan Carlos University, c/ Tulip´ an s/n, 28933, M´ ostoles, Madrid, Spain ARTICLE INFO Keywords: Lignin pyrolysis Hierarchical zeolites ZSM-5 Beta Ion-exchanged zeolites Phenolic compounds ABSTRACT Synthesised H-ZSM-5 and H-Beta zeolites with hierarchical porosity (h-H-ZSM-5 and h-H-Beta) have been ionexchanged with alkali (Na + and K + ) and alkaline-earth (Mg 2+ ) metals and have been evaluated as catalysts to produce high value-added products through catalytic pyrolysis of lignin. In comparison with the thermal test, hierarchical zeolites in acid form are effective catalysts for lignin pyrolysis, favouring the production of valuable light compounds although reducing bio-oil* yield and increasing gas and coke formation. In this way, h-H-ZSM-5 zeolite promotes the formation of oxygenated aromatics, being guaiacols and syringols the major products. Alkali-exchanged variants of this zeolite enhance demethylation reactions improving the selectivity towards 2methoxy-phenol and syringol, by modifying acid site properties. On the other hand, h-H-Beta zeolite, with larger pore size and stronger acidity, leads to higher concentration of oxygenated aromatics for both parent and ion-exchanged catalysts. Specifically, h-KH-Beta promotes the production of 2-methoxy-phenol and syringol. Besides, h-MgH-Beta stands out for its greater selectivity towards phenol and alkylphenols, such as dimethylphenol. Overall, the combination of accessibility, provided by the hierarchical porosity, with the different nature and strength of the acid sites, induced by the ion-exchange with alkali and alkaline-earth metals, allows tailoring the lignin catalytic pyrolysis process to selectively produce high value-added compounds. 1. Introduction Lignocellulosic biomass is mainly composed of cellulose, hemicellulose, and lignin [1,2]. The former components have been widely used in bioethanol production, pulp and paper engineering, as well as in other industries [1–3]. However, the effective conversion of lignin into value-added products has always been a bottleneck due to their complex structural characteristics [4,5]. For pulp and paper engineering, up to 50 million tons of lignin waste residue can be produced every year [5], which is mainly used as a low-grade fuel for power generation. Likewise, lignin is considered as a waste in biorefineries [5], despite containing abundant aromatic groups that can act as a precursor for the synthesis of fuels, chemicals and advanced materials through a large variety of processes. Therefore, the valorisation of lignin residues is of great significance to realize full biomass utilization and the development of the biorefinery industry into circular economy. Lignin is a biopolymer formed by building blocks named monolignols, p-coumaryl alcohol (H), coniferyl alcohol (G), and sinapyl alcohol (S), linking through C-O and C-C bonds, resulting in large aromatic polymers [6,7] (Fig. 1). These monolignols present p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, with each containing 0, 1, and 2 o-methoxy groups, respectively [8]. The distribution of these units varies depending on the source of lignocellulose. Hardwood lignin primarily consists of S and G units, with S units being more abundant. Softwood lignin is predominantly made up of G units, with only minor amounts of H units, whereas in the lignin present in herbaceous plants G units are the primary components, followed by H and S units [9,10]. Several lignin chemical valorisation routes have been explored. Thus, catalytic oxidation [11,12], hydrogenolysis or depolymerisation [13–15], and catalytic pyrolysis [16–20] have been evaluated to produce both fuels and chemicals, such as aldehydes, phenols, and aromatic hydrocarbons [21,22]. Among them, catalytic pyrolysis is a very * Corresponding author. ** Corresponding author at: Chemical and Environmental Engineering Group, ESCET, Rey Juan Carlos University, c/ Tulip´ an s/n, 28933, M´ ostoles, Madrid, Spain. E-mail addresses: [email protected] (A. Peral), [email protected] (J.A. Botas). Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod https://doi.org/10.1016/j.cattod.2025.115343 Received 16 December 2024; Received in revised form 19 March 2025; Accepted 15 April 2025 Catalysis Today 456 (2025) 115343 Available online 23 April 2025 0920-5861/© 2025 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/bync-nd/4.0/ ). promising route to transform lignin into high value-added products. In these processes, lignin thermal pyrolysis vapours pass through the catalytic bed, promoting cracking reactions such as C-C bond cleavage, hydrogen transfer, isomerization, aromatic side chain scission, decarbonylation, decarboxylation, and dehydration. A large variety of catalyst types have been tested in the catalytic pyrolysis of lignin, such as metal salts, metal oxides, zeolite catalysts, and low-cost waste catalysts [23–27]. The liquid product, named bio-oil, is mainly a mixture of phenols, which is difficult to separate and purify. This aspect is mainly related to the complex structure of lignin, being a challenge for the scientific community to find a catalytic system to ensure the efficient valorisation of lignin [5]. Among all the catalytic systems evaluated for lignin valorisation through pyrolysis, the use of zeolites stands out, as both their acidity and porosity can be tuned during their synthesis. These two factors play a crucial role in the reaction performance, influencing both the extent of lignin depolymerisation and the distribution of resulting products. However, most commercial zeolites have pore sizes below than 1 nm, which is an important drawback for the decomposition of heavy lignin oligomers into lighter compounds of industrial interest. This limitation has motivated the use of hierarchical zeolites with enhanced accessibility for this application [18]. Hierarchically ordered zeolites are inorganic materials having multiple levels of porosity, involving micro-, mesoand/or macropores in the same crystalline structure [28–30]. Focusing on the catalytic pyrolysis of lignin over zeolitic catalysts, acid sites facilitate the breakdown of lignin by catalysing C-C and C-O bond cleavages, while the pore structure affects the selective diffusion of reactants and products. For instance, microporous ZSM-5 favours monocyclic aromatic hydrocarbons, while mesoporous ZSM-5 produces more alkylphenols [27,31,32]. However, the small micropore size of ZSM-5 zeolite limits access to larger phenols, potentially causing catalyst deactivation [27]. Some studies indicate that the optimal pore size for zeolites in lignin pyrolysis falls within the range of 6.5–8.4 Å, suggesting that hierarchical Beta zeolite, with larger 12-membered ring pores (7.6 ×6.4 Å) and an improved accessibility, may be one of the most suitable zeolitic materials in this application [18,33]. In previous works of our group, we reported an enhanced selectivity towards oxygenated aromatics (phenols and alkylphenols) employing as catalysts nanocrystalline K-ZSM-5 and K-USY zeolites in the catalytic pyrolysis of wheat straw due to its right combination of acid properties and good accessibility [19,20]. Other recent work has been focused on catalytic pyrolysis of lignin employing nanocrystalline zeolites ion-exchanged with metals, confirming the differences in products distribution depending on the Lewis and Brønsted acid sites concentration [17]. Since the cation exchange predominantly occurs over the Brønsted acid sites of both H-ZSM-5 and H-Beta parent zeolites, the resulting catalysts present an enhanced concentration of Lewis acid sites. Thus, guaiacols production was improved using NaH-ZSM-5 and KH-ZSM-5 as catalysts, whereas alkylphenols formation was increased over MgH-Beta and KH-Beta zeolites. Guaiacols are valuable compounds in a variety of industries and interesting chemicals in the synthesis of fragrances, cosmetics, pesticides and pharmaceuticals. Flavouring agents like vanillin and roasted coffee are produced employing guaiacol as a precursor, and it is also used as an analgesic, antiseptic and sedative drug. On the other hand, alkylphenols are used as valuable intermediates (antioxidants, pharmaceuticals and polymers) in different industrial processes [34]. Therefore, it has been confirmed that high value-added compounds can be obtained from the catalytic pyrolysis of residual lignin, contributing to the valorisation of this type of waste. In this context, the present study focuses on the production of high value-added phenolic compounds through lignin catalytic pyrolysis using ion-exchanged hierarchical H-ZSM-5 and H-Beta zeolites. This approach aims to prove that hierarchical porosity improves the accessibility of lignin pyrolysis products to the active centres of catalysts. In addition, the ion exchange of these hierarchical zeolites with alkali and alkaline-earth metals (Na + , K + and Mg 2+ ) allows the fine-tuning of their acid properties since the cation exchange occurs over Brønsted acid sites, resulting in an increase of the Lewis acidity concentration. This study also assesses the occurrence of a synergic effect, owing to the combination of good accessibility and balanced acidity, to find an optimal catalytic system to produce oxygenated aromatic compounds from lignin pyrolysis. Fig. 1. Schematic illustration showing representative structure of lignin with structures of the monomers coumaryl alcohol (H), coniferyl alcohol (G) and sinapyl alcohol (S). M.I. ´ Avila et al. Catalysis Today 456 (2025) 115343 2 2. Experimental procedures 2.1. Catalysts 2.1.1. Synthesis of hierarchical zeolites The H-ZSM-5 zeolite with hierarchical porosity (h-H-ZSM-5) was synthesised using the silanised nuclei crystallization method outlined in prior studies [28]. The materials used included aluminium isopropoxide (IPA, Aldrich, 98 wt.%) as the aluminium source, tetraethyl orthosilicate (TEOS, Aldrich, 98 wt.%) as the silica source, tetrapropylammonium hydroxide (TPAOH, Alfa Aesar, 40 wt.%) as the structure-directing agent, and Milli-Q water. The molar composition of the initial synthesis gel was 1 Al 2 O 3 : 90 SiO 2 : 16.5 TPAOH: 2250 H 2 O. Silanisation step was performed at 90 ◦C under reflux and atmospheric pressure for 6 h, with N-(2-N-benzylaminoethyl)-3-aminopropyltrimethoxysilane (Ph-2A, Gelest, 92 wt.%) used as the silanising agent (8 mol.% in respect to the silicon content in the initial gel). The resulting gel was placed into PTFE-lined steel autoclaves and subjected to crystallization under autogenous pressure at 170 ◦C for 7 days. After crystallization, the solid was recovered by centrifugation and dried at 110 ◦C. To remove both the structure-directing and silanising agents and activate the catalyst, the material was calcined in static air at 550 ◦C for 5 h with a heating rate of 1.8 ◦C/min. The H-Beta zeolite with hierarchical porosity (h-H-Beta) was also synthesised using the silanised nuclei crystallization method according to prior studies [29,30]. A solution comprising aluminium flakes (Aldrich, 99.9 wt.%), fumed silica (Aldrich), tetraethylammonium hydroxide (TEAOH, Alfa Aesar, 35 wt.% aqueous solution), and Milli-Q water was aged at room temperature for 20 h. Afterwards, it underwent precrystallisation in steel autoclave reactors lined with PTFE at 135 ◦C for 3 days to encourage the development of Beta protozeolitic nanounits. The molar composition of the initial synthesis gel was 1.5 Al 2 O 3 : 60 SiO 2 : 15.5 TPAOH: 2400 H 2 O. The resulting gel was subjected to the crystallization process (135 ◦C for 7 days) using the same silanising agent (Ph-2A) previously described for the h-H-ZSM-5 sample. Likewise, the solid product obtained from the crystallization treatment was recovered and processed following the same procedure described before for h-H-ZSM-5 zeolite. 2.1.2. Preparation of ion-exchanged zeolites The two previously synthesised hierarchical zeolites were subjected to ion exchange with alkaline earth (Mg 2+ ) and alkali (Na + and K + ) cations according to the procedure earlier described [17]. Typically, 5 g of zeolite powder were immersed in 50 mL of a 0.4 M aqueous solution of the corresponding chloride salt for 24 h at 25 ◦C. The salts employed for the ion exchange included NaCl (Sigma Aldrich, 99.0 wt.%), KCl (Scharlab, 99.0 wt.%) and MgCl 2 (Fisher Scientific, 99.0 wt.%). After the ion exchange, the zeolites were separated by centrifugation, rinsed with Milli-Q water, dried at 110 ◦C overnight, and subsequently calcined in static air at 550 ◦C for 5 h with a heating rate of 1.8 ◦C/min. Prior to their utilisation as catalysts, both the hierarchical and ionexchanged zeolites were pelletised by compression under 8 tons of pressure force for 30 s, then crushed and sieved through ASTM standard sieves to obtain particles in the 180–250 μ m size range. 2.1.3. Catalysts characterisation The contents of aluminium and Na, K and Mg metals in the catalysts were analysed using inductively coupled plasma - optical emission spectroscopy (ICP-OES) with a Varian Vista AX spectrometer. Previously, the samples were dissolved in a H 2 SO 4 /HF acid solution. X-ray diffraction (XRD) data were collected using a Philips X ′ pert diffractometer with CuK α radiation. Argon adsorption-desorption isotherms were measured with a Micromeritics 3Flex Adsorption Analyzer, the samples being prior outgassed at 300 ◦C under vacuum for 5 h. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method, and the Non-Local Density Functional Theory (NL-DFT) method was employed to differentiate the contributions of micropores, mesopores and external surface area to the total surface area [35]. Total pore volume was estimated from the adsorbed amount at a relative pressure of 0.95. Transmission electron microscopy (TEM) images and energy-dispersive X-ray (EDX) analysis were performed using a JEOL JEM1400-Flash microscope operating at 120 kV. The acidity and strength of the acid sites in the zeolites were examined through pyridine adsorption and desorption followed by Fourier transform infrared spectroscopy (FTIR/PYR). Self-supporting wafer samples (8–15 mg/cm²) were placed into a high-temperature cell with a CaF₂ window. Activation was carried out under vacuum at 525 ◦C for 1 h. Pyridine was then adsorbed at 150 ◦C under 4 mbar pressure, and FTIR spectra were recorded between 4000 and 400 cm⁻¹ (4 cm⁻¹ resolution with 128 scans). A Jasco FT/IR-4600 spectrometer with a triglycine sulphate (TGS) detector was used for this analysis. To assess the strength of the acid sites, FTIR spectra were also recorded after desorption of pyridine at high vacuum and different temperatures (150, 250, 350, and 450 ◦C), using an equilibration time of 25 min for each temperature. Quantification of the acid sites was performed by analysing specific pyridine vibration modes and corresponding absorption coefficients available in the literature [36]. These included the pyridinium PyH⁺ band at 1545 cm⁻¹ ( ε h-H-ZSM-5 = 1.09 cm⋅µmol −1 ; ε h-H-Beta =1.12 cm⋅µmol −1 ) and the pyridine PyL band at ~1455 cm⁻¹ ( ε h-H-ZSM-5 and ε h-H-Beta =1.71 cm⋅µmol −1 ; ε Meh-H-ZSM-5 and ε Meh-H-Beta =1.72 cm⋅µmol −1 ). 2.2. Lignin used as feedstock The lignin selected as raw material to be processed in the pyrolysis tests was a Tanovis Protobind 1000 sample, which is obtained through Soda process extraction from non-woody biomass and provided in powder form by PLT Innovations [37]. The lignin was characterised according to European Standards to determine the proximate (moisture, ash, and volatile matter contents) and ultimate analyses, which are summarised in Table 1. The full characterization details of this material can be found in a previous publication by our research group [17]. Before conducting the pyrolysis tests, the lignin was compressed into uniform tablets using a hydraulic press with an 8 tons pressure force applied for 30 s. The tablets were then crushed and passed through ASTM standard sieves to achieve the required particle size for feeding into the pyrolysis reactor (0.5 – 1 mm). 2.3. Lignin pyrolysis tests A detailed description of the experimental setup used for the catalytic pyrolysis tests is provided in a previous work [38]. It consists of a laboratory-scale down-flow fixed-bed stainless steel reactor (16 mm inner diameter, 400 mm length) with an ex-situ configuration, featuring separate thermal and catalytic zones, each heated by independent electric furnaces. The temperatures are monitored by K-type thermocouples positioned in both zones. In each experiment, lignin (4 g, previously dried to eliminate moisture) was used as feedstock. For the catalytic tests, catalyst-to-lignin mass ratios (C/L) of 0.2 and 0.4 were evaluated. The lignin was fed with a particle size in the 0.5–1 mm interval, while the catalyst particles ranged from 180 to 250 μ m. The experiments were carried out at atmospheric pressure, with temperatures Table 1 Proximate and ultimate analyses of lignin. Proximate analysis a (wt.%) Ultimate analysis b (wt.%) Volatile matter Ash Fixed carbon C H N S O c 61.7 1.3 37.0 60.0 5.2 0.7 0.4 33.7 a Dry basis; b Dry ash-free basis; c Determined by difference. M.I. ´ Avila et al. Catalysis Today 456 (2025) 115343 3 set to 550 ◦C in the thermal zone and 450 ◦C in the catalytic one. Prior to each test, the lignin chamber and reaction system were purged with 100 mL/min of nitrogen. Once the target temperatures in both zones were reached, the lignin was loaded into the reactor, and pyrolysis was performed for 10 min under a constant nitrogen flow of 100 mL/min. During pyrolysis, the lignin decomposes into char (accumulating in the thermal zone), non-condensable gases and pyrolytic vapours, which pass through the catalytic zone. The vapours are condensed using a system of two stainless steel vessels cooled by an icewater bath, while the non-condensable gases are collected in a totaliser, which also measures gas volume via water displacement. After each experiment, the composition of gases, char, coke (formed on the catalyst), and bio-oil are determined using various analytical techniques, as next described. 2.3.1. Analysis of the lignin pyrolysis products The condensed bio-oil from the catalytic pyrolysis tests was separated into two phases by decantation, which are named as aqueous-rich liquid phase and organic-rich liquid phase. The aqueous-rich liquid phase contains most of the polar compounds, including water, in comparison to the organic-rich liquid phase which contains the compounds with higher molecular weight. The water content in each phase was measured using Karl Fischer titration, following the ASTM E202–08 procedure after dilution to 50 % with ethanol. The carbon (C), hydrogen (H), nitrogen (N), and sulphur (S) content of both phases was determined with a ThermoFisher Flash Smart micro-elemental analyser, and the oxygen (O) content was calculated by difference. The organic compounds present in the two liquid phases (referred to as bio-oil*, on a water-free basis) were identified using gas chromatography-mass spectrometry (GC-MS) with a Bruker® SCION 436-GC system (electron energy: 70 eV, emission: 300 V, helium flow rate: 1 mL/min). The column used was a WCOT fused silica HP5-MS UI (30 m ×0.25 mm ×0.25 mm). Compounds were identified by comparing their spectra with the NIST EIMS library (v.2.0), with a minimum match score of 700. Once identified, compounds were classified into different families based on their functional groups: carboxylic acids (CA), light oxygenates (LO, including aldehydes, alcohols, ketones, and ethers), furans (FUR), oxygenated aromatics (O-AR), and aromatic hydrocarbons (AR). Quantitative analysis was achieved through external calibration, using 19 standard compounds commonly found in thermal and catalytic bio-oils: acetic acid, propanoic acid, butanone, diethoxypropane, cyclopentenone, furfural, phenol, guaiacol, cresol, dimethylphenol, creosol, syringol, trimethoxybenzene, toluene, xylene, styrene, trimethylbenzene, naphthalene, and n-heptane. For other detected compounds, response factors were averaged according to their group. The difference between the total bio-oil yield and GC-MS detected compounds was used to estimate the amount of heavy or oligomeric species not detected by GC-MS. Non-condensable gases were analysed using a Varian CP-4900 micro-gas-chromatograph equipped with Molsieve 5 Å and PPQ columns, and a thermal conductivity detector (TCD), with argon and helium as carrier gases, respectively. Quantification was performed using calibration standards for N 2 , O 2 , H 2 , CO, CO 2 , CH 4 , C 2 H 4 , C 2 H 6 , C 3 H 6 , C 3 H 8 , C 4 H 8 , and C 4 H 10 . The C, H, N, S, and O content in the char and coke was measured using the ThermoFisher Flash Smart micro-elemental analyser. The amount of coke deposited on the catalyst was determined by temperature-programmed oxidation (TPO) under an air atmosphere, using a TA Instruments SDT 650 thermobalance (up to 900 ◦C at a heating rate of 10 ◦C/min). Finally, the overall mass balance was closed with a recovery of over 95 wt.% from the combined weight of the gas, bio-oil, char, and coke fractions. 3. Results and discussion 3.1. Catalysts characterisation Two different hierarchical zeolites were synthesised and named as hH-ZSM-5 and h-H-Beta. According to the database of zeolite structures published by the IZA Structure Commission, both zeolites showed diffraction patterns characteristic of MFI and BEA structures, respectively (Fig. S1). h-H-ZSM-5 (Fig. S1a) possesses wide diffraction peaks, with low intensity, typical of hierarchical zeolites consisting of small crystalline domains, as confirmed by TEM (Fig. S2a and b). Small crystalline units with sizes lower than 20 nm are detected, while these entities are partially grouped in globular aggregates of larger dimensions (150–300 nm), which is typical of hierarchical zeolites with interparticle mesoporosity [28]. Wide peaks of low intensity are also detected in the h-H-Beta diffraction pattern (Fig. S1b), suggesting the presence of small crystals or a high proportion of structural defects in this material [39]. Thus, TEM micrographs (Fig. S3a and b) show that this material is formed by elliptical particles (300–600 nm in size) possessing a polycrystalline distorted structure. After magnification (Fig. S3b), it can be observed that these particles are formed by small and different nanocrystalline domains (<20 nm) and diffraction fringes are clearly visible, denoting the crystallinity of these zeolites [28,40]. The physicochemical properties of both synthesised zeolites are summarised in Table 2. Si/Al molar ratios of h-H-ZSM-5 and h-H-Beta (determined by ICP-OES) were 61 and 20, respectively. Furthermore, both hierarchical parent zeolites were ion-exchanged using alkali and alkaline-earth metal salts according to the procedure explained in the experimental section. Consequently, Na + , K + and Mg 2+ cations are introduced by partially substituting H + cations and modifying the chemical composition of the samples. All the ion-exchanged materials exhibit Si/Al molar ratios similar to those of their corresponding parent zeolites, with values close to 60 and 20 for the ZSM-5 and Beta-type zeolites, respectively. Moreover, no significant structural changes are appreciated over ion-exchanged zeolites (Fig. S1, S2 and S3) [17]. Both, XRD diffractograms and TEM micrographs indicate that ion-exchanged zeolites exhibit similar crystallinity and structural features than their parent ones. Comparing the results attained over both types of zeolites, a more successful ion exchange process is achieved over the h-H-ZSM-5 zeolite. In this case, the cation to aluminium molar ratio (Me/Al) is closer to the maximum theoretical value attainable if the negative charges related to Al atoms were completely counterbalanced by cations (Me/Al =1 for Na + and K + , and 0.5 for Mg 2+ ). Moreover, homogeneous Me dispersion is appreciated over the zeolite surface and no impurity phases related to metal oxides are detected (Fig. S1). Furthermore, no metal oxide particles were detected by TEM (Fig. S2 and S3), and TEM-EDX elemental mapping demonstrates the uniform distribution of Na, K and Mg over both h-H-ZSM-5 and h-H-Beta zeolites (Fig. S4 and S5). On the other hand, after ion exchange, despite the higher mass of cation incorporated (Table 2, m Me , wt.%), h-MeH-Beta zeolites reach only Me/Al molar ratio values close to 50 % of the maximum theoretical value achievable. In this case, the highest degree of ion exchange is obtained over the h-NaHBeta (Na/Al =0.61). This difference in the degree of ion exchange over both types of zeolites is the same as that observed and discussed in a previous work of our group where H-ZSM-5 (MFI; Si/Al =52) and HBeta (BEA; Si/Al =20) nanocrystalline commercial zeolites were ionexchanged following a similar procedure [17]. Significant differences in textural properties, the highly distorted BEA framework structure, and the very different Si/Al molar ratios and acid properties of both zeolites are some of the reasons that explain the existence of a higher proportion of effective ion exchange sites over h-H-ZSM-5 in comparison to h-H-Beta zeolite. M.I. ´ Avila et al. Catalysis Today 456 (2025) 115343 4 After ion exchange a small modification in the textural properties of synthesised zeolites is observed (Fig. S6 and S7). Both parent zeolites, hH-ZSM-5 (Fig. S6a) and h-H-Beta (Fig. S7a) display adsorption at very low relative pressures due to micropore filling, followed by an important Ar adsorption at intermediate relative pressures (0.1 <P/P 0 <0.8) related to the continuous adsorption in the mesopores of the hierarchical zeolites. Ion-exchanged zeolites show similar shape to their corresponding H-form zeolites but denote a slightly lower sorption capacity. This fact is corroborated in Table 2, where the textural properties of the different materials derived from the Ar sorption analyses are listed. Cation-exchanged materials manifest an appreciable reduction in their total surface area (BET). Moreover, metal cation exchange generates a clear micropore blockage in h-H-Beta zeolite, while this effect is lower, but not negligible, over h-H-ZSM-5 (V MIC and S MIC ). This fact is consistent with the exchange of protons by metal cations of larger ionic radius. The difference observed in both families of materials can be related to the higher cation mass amount incorporated in the h-H-Beta zeolite (m Me , wt.%, Table 2) due to its higher Al content. FTIR analyses of adsorbed pyridine at different temperatures were performed to identify and quantify the acid centres (type, nature and strength) present in the different materials. Table 3 lists the Brønsted and Lewis acid sites concentration detected by this technique at 150 ◦C. h-H-ZSM-5 contains a higher proportion of Brønsted acid sites, resulting in a molar ratio of Lewis to Brønsted acid centres (L/B) of 0.6. After ion exchange this ratio is modified, increasing the proportion of Lewis acid sites and giving rise to higher L/B ratios, from values close to 1 for hMgH-ZSM-5, to 8 for h-KH-ZSM-5. h-H-Beta zeolite presents a more balanced L/B ratio than h-H-ZSM-5, with a value of 0.9. After ionexchange, the L/B ratio almost doubles for h-MgH-Beta, increasing this ratio in higher extent for h-KH-Beta (L/B =3.2) and h-NaH-Beta (L/ B =6.6). Therefore, a large decrease in Brønsted acidity is observed for alkaline (Na + and K + ) ion-exchanged zeolites, due to the substitution of H + associated with this type of acid sites by alkaline cations. Bridged hydroxyl groups responsible of BAS are preferential ion-exchange sites for alkaline cations resulting in the large reduction of BAS. Consequently, the ion-exchange of H + by extraframework alkali cations generates LAS, increasing the population of this type of acid sites, as pointed out above. Additionally, defect sites such as SiO - and silanols (Si-OH) or extraframework Al, can also contribute as ion-exchange sites and to the formation of LAS of different nature. The amount of this type of ionexchange sites is expected to be significant in these types of hierarchical zeolites exhibiting a high proportion of external surface area. On the other hand, it would be reasonable to expect similar results when magnesium is used as cation in the ion-exchange process. However, in this case, despite the proper efficiency of the ion-exchange process, the reduction in the amount of BAS is less important. While for alkaline cations after the ion-exchange process, a BAS reduction between 75 % and 90 % for ZSM-5 type zeolites and 70–85 % for Beta type ones is obtained, when Mg 2+ is ion-exchanged this decrease is lower than 20 % or 25 % for ZSM-5 and Beta type zeolites, respectively. Differently to alkaline (univalent) cations, the distance between ion-exchange sites is an important factor when Mg 2+ is involved, because this cation interacts with two bridged hydroxyl ion-exchange sites only if they are located within a certain distance. The probability of this proximity between two bridged hydroxyls is not elevated in both h-H-ZSM-5 and h-H-Beta zeolites, because their high Si/Al molar ratios (61 and 20, respectively), and therefore the amount of BAS decreases to a lesser extent than when monovalent alkali cations are ion-exchanged. These results are in agreement with previous findings, indicating that exchangeable protons are less efficiently replaced by Mg 2+ via ion-exchange, than via other methods such as impregnation [41]. Thus, for h-MgH-ZSM-5 and h-MgH-Beta, only a limited number of bridged hydroxyls are transformed into Mg(OH) + sites (LAS) due to their interaction with solvated Mg 2+ [42,43]. Additionally, and similarly to alkaline cations, LAS of different nature could also be generated due to the interaction of Mg 2+ with distinct defect sites existing in the hierarchical zeolitic structures, contributing to the notable increase in the LAS population observed for magnesium ion-exchanged hierarchical zeolites [17,19,44]. Fig. 2 displays the concentration of Brønsted and Lewis acid centres measured after evacuation at different temperatures (from 150 ◦C to 450 ◦C). This allows the quantification of the concentration and strength of the different acid sites to be determined. h-H-ZSM-5 consists mainly of Brønsted acid sites (BAS) of medium-high acid strength. After the evacuation at 350 ◦C it maintains most of the 60 % of BAS, and at 450 ◦C this value is still higher than 37 % (Fig. 2a). Magnesium exchange reduces the concentration of BAS, increasing that of Lewis acid sites (LAS), maintaining an elevated number of acid centres (acidity), characterised by medium to high-strength BAS and medium-strength LAS. After the Table 2 Physicochemical and textural properties of hierarchical and ion-exchanged h-H-ZSM-5 and h-H-Beta zeolites. Sample m Mea (wt.%) Me/Al b Si/Al b S BETc (m 2 /g) S MICd (m 2 /g) S EXT-MESOe (m 2 /g) V MICd (cm 3 /g) V TOTf (cm 3 /g) h-H-ZSM¡5 - - 61 540 337 203 0.194 0.415 h-MgH-ZSM−5 0.38 0.60 62 448 295 153 0.170 0.356 h-KH-ZSM−5 1.00 0.96 61 443 298 145 0.172 0.377 h-NaH-ZSM−5 0.52 0.86 62 474 305 169 0.176 0.368 h-H-Beta - - 20 630 420 210 0.248 0.429 h-MgH-Beta 0.46 0.26 22 534 363 171 0.214 0.382 h-KH-Beta 1.25 0.43 21 527 348 179 0.205 0.399 h-NaH-Beta 1.13 0.61 20 508 339 169 0.200 0.391 a Metal weight percentage (ICP-OES); b Molar ratio (ICP-OES); c BET surface area; d Micropore surface area and micropore volume (NL-DFT method); e External-Mesopore surface area; f Total pore volume (P/P 0 ≈0.95). Table 3 Acid properties of h-H-ZSM-5 and h-H-Beta-based catalysts. Sample FTIR-PYR (150 ◦C) BAS (mmol/g) a LAS (mmol/g) b L/B c h-H-ZSM¡5 0.165 0.093 0.6 h-MgH-ZSM−5 0.136 0.147 1.1 h-KH-ZSM−5 0.017 0.136 8.0 h-NaH-ZSM−5 0.039 0.156 4.0 h-H-Beta 0.256 0.229 0.9 h-MgH-Beta 0.197 0.332 1.7 h-KH-Beta 0.072 0.232 3.2 h-NaH-Beta 0.040 0.264 6.6 a Concentration of Brønsted acid sites; b Concentration of Lewis acid sites; c LAS/BAS molar ratio. M.I. ´ Avila et al. Catalysis Today 456 (2025) 115343 5 evacuation at 350 ◦C, more than 30 % and 50 % of BAS and LAS, respectively, are retained (Fig. 2a and b). On the other hand, both, h-KHZSM-5 and h-NaH-ZSM-5 are composed mainly of weak LAS, possessing the latter a slightly higher proportion of BAS of low acid strength. As it was expected, considering its higher Al content and its characteristic distorted structure, h-H-Beta possesses an elevated acidity (Fig. 2c and d) and both BAS and LAS can be categorised as acid centres of medium to strong acid strength. After ion-exchange, and similarly to h-H-ZSM-5based zeolites, the proportion of LAS increases, exhibiting h-MgH-Beta both BAS and LAS of medium-high acid strength, while medium acid strength BAS and weak LAS are detected over both h-KH-Beta and hNaH-Beta catalysts. These results are in agreement with previous works from our group and other authors [17,45] confirming that the ion exchange with alkali and alkaline-earth cations produces mainly weak LAS. Furthermore, as the degree of ion exchange increases, the acid nature of the catalysts turns into Lewis-type, resulting in a reduction in their acid strength. 3.2. Thermal pyrolysis of lignin Fig. 3a shows the products yield distribution obtained from the thermal pyrolysis of lignin. It can be observed that the main product is the solid by-product char, followed by bio-oil* (water-free basis), noncondensable gases and water. Permanent gases (Fig. 3b) primarily include CH 4 , CO 2 , CO, and light hydrocarbons (ethane, ethylene, propene, propylene, butane and butene), which result from the breakdown of lignin’s methoxy, carbonyl, and ether linkages. CO 2 and CO are produced through decarboxylation and decarbonylation reactions, respectively, while methane and other hydrocarbons are formed from the cleavage of methoxy groups and the cracking of C-C bonds within the lignin polymer. On the other hand, the components present in the bio-oil* fraction have been determined employing GC-MS analyses, using calibration methods to obtain quantitative data on both mass yield related to the starting raw lignin and their concentration in the bio-oil* fraction. Although results are presented on a water-free basis, the aqueous-rich and organic-rich liquid phases of the bio-oil were analysed separately to assess the distribution of different compound families. The concentrations of each family in the bio-oil* have been summarised in Fig. 3c and d. Minority families (CA, LO, FUR and AR) with proportions lower than 1.5 wt.% in the bio-oil* are depicted in Fig. 3c, while the concentration of the main bio-oil* family, i.e. oxygenated aromatics, is close to 32 wt.% (Fig. 3d). The detail of O-AR family according to its major components, grouped in alkylphenols (ALK), guaiacols (GUA), syringols (SYR) and catechols (CAT), is also shown in Fig. 3d. The bio-oil* derived from thermal pyrolysis is rich in oxygenated aromatics, specifically alkylphenols (4.1 wt.%), syringols (9.3 wt.%), and guaiacols (18 wt.%). The presence of these compounds agrees well with the lignin composition, which includes in their structure p-hydroxyphenyl, syringyl, and Fig. 2. Concentration of a,c) Brønsted (C B ), and b,d) Lewis (C L ) acid sites of the zeolite samples, probed by FTIR/pyridine, as a function of the desorption temperature. M.I. ´ Avila et al. Catalysis Today 456 (2025) 115343 6 guaiacyl units [46]. However, in the absence of catalysts (pure thermal process), the bio-oil* presents a high oxygen content (23.7 wt.%), which may negatively affect its stability and quality as a fuel. Moreover, a relatively low concentration of light components, detected by the GC-MS analyses, was identified in the bio-oil* (38.3 wt.%), indicating a limited conversion of lignin into high value-added compounds [17]. As it has been represented in the scheme of Fig. 4, during thermal decomposition, lignin undergoes numerous reactions, the C–C and C–O–C side chains break apart, producing phenols or hydroxybenzenes, which contain functional groups such as carboxyl, methoxy, or carbonyl [47]. These groups can further decompose, generating light gases. Depolymerisation reaction takes place, leading to an increase in heavy methoxyphenols in the bio-oil. Additionally, repolymerization reactions contribute to the formation of char [48]. In parallel, hydrogenation, dehydrogenation, demethylation, and demethoxylation reactions take place, resulting in the formation of major compounds such as guaiacols, catechols, and alkyl phenols [49]. Additionally, the presence of a small amount of furan derivatives in the bio-oil composition obtained through lignin pyrolysis Fig. 3. Thermal pyrolysis of lignin a) Fractions yields, b) Gaseous components yield (GO: gaseous olefins, GP: gaseous paraffins), c) Overall concentration of compound families in the bio-oil* (excluding O-AR), and d) Concentration of total oxygenated aromatic compounds (O-AR) and their sub-families. Fig. 4. Main type of products resulting from the non-catalytic pyrolysis of lignin. M.I. ´ Avila et al. Catalysis Today 456 (2025) 115343 7 arises from the degradation of residual holocellulose remaining after the Protobind 1000 lignin extraction process, accounting for less than 4 wt. % [50]. 3.3. Catalytic pyrolysis of lignin employing h-MeH-ZSM-5 and h-MeHBeta zeolites This section shows the results obtained in the catalytic pyrolysis of lignin employing hierarchical H-ZSM-5 and H-Beta zeolites, including both parent and cation-exchanged materials. Based on studies performed by our research team on lignocellulosic biomass and lignin pyrolysis [17,19,20], the selected temperatures were 550 ◦C and 450 ◦C for the thermal and catalytic zones, respectively, and two different catalyst-to-lignin (C/L) ratios were tested (0.2 and 0.4) to evaluate the catalyst impact on the products distribution. The mass yields of lignin pyrolysis products referred to the fed lignin are summarised in Figs. 5 and 6, for parent and cation-exchanged ZSM-5 and Beta-type zeolites, respectively. For comparative purposes, the non-catalytic test results have also been included. It is important to note that, given that the thermal pyrolysis stage is the same for all the tests, the amount of char produced was relatively constant (~ 40 wt.%). In terms of pyrolysis fraction distribution, the use of h-H-ZSM-5 and h-H-Beta zeolites, combined with an increased C/L ratio, results in lower bio-oil* yields. This is related to the higher formation of water, gases, and coke, which likely occurs as the vapours from the thermal zone are converted over the catalyst bed. It can be observed (Fig. 6) that the bio-oil yields for h-H-Beta zeolite are lower compared to those produced with h-H-ZSM-5 (Fig. 5). Regarding water production, the increase compared to the non-catalytic process is very similar for both zeolites, with slightly higher values observed for h-HZSM-5. This effect becomes more pronounced with an increase in the C/ L ratio. The acidic properties of zeolites, which are primarily determined by the Si/Al ratio, are crucial in influencing the mechanisms and efficiency in conversion of lignin decomposition vapours. Zeolites with higher Si/ Al ratios, such as h-H-ZSM-5 (Si/Al =61), have lower acidity compared to those with lower Si/Al ratios, like h-H-Beta (Si/Al =20) [51]. This difference in acidity can lead to distinct lignin vapours conversion pathways, including variations in deoxygenation reactions [52]. h-H-Beta, with its lower Si/Al ratio and higher acidity, promotes deoxygenation processes increasing gas production and higher coke yield, which is formed by the carbon deposits generated from successive oligomerization and polymerization reactions [53]. In contrast, h-H-ZSM-5, with higher Si/Al ratio and lower acidity, exhibits lower gas and coke production [54]. Larger pore volumes favour cracking reactions, which increases gas production, as well as aromatization and polymerization reactions, leading to a higher yield of coke [55,56]. This behaviour aligns with the larger micropore size and volume of H-Beta zeolite, which contributes to its higher coke production compared to h-H-ZSM-5. Specifically, produced coke yields of 3.4 wt.% at a C/L ratio of 0.2 and 8.2 wt.% at 0.4, are detected for h-H-ZSM-5 zeolite. In contrast, h-H-Beta zeolite showed higher coke yields of 6.0 wt.% at C/L =0.2 and 11.0 wt.% at C/L =0.4. Additionally, h-H-Beta exhibited increased gas production concerning the thermal test, with values of 0.4 wt.% at C/L =0.2 and 1.6 wt.% at C/L =0.4, whereas h-H-ZSM-5 induces a lower gas production, with a reduction of up to 0.2 wt.% at C/L =0.4. For h-H-Beta, the higher BAS concentration enhances cracking reactions, resulting in greater olefin formation and overall gas yield. Conversely, h-H-ZSM-5, exhibiting a moderately reduced BAS content, intensifies deoxygenation reactions, particularly through water formation, further amplifying this effect [19]. On the other hand, significant differences in the products distribution are observed in the pyrolysis experiments employing ion exchanged h-H-ZSM-5 and h-H-Beta zeolites with Na + , K + , and Mg 2+ . The exchange of H + ions in the zeolite framework with Na + , K + , and Mg 2+ cations lead to important changes in the acidity and significant variations in the textural properties [57], provoking a reduction in the coke formation over the ZSM-5 ion-exchanged zeolites, particularly at a C/L ratio of 0.4. In the case of h-MgH-ZSM-5 zeolite, the concentration of LAS/BAS sites is more balanced, giving rise to a higher conversion of pyrolysis vapours over this catalyst and an increase in water and gas yields. In Figs. 7 and 8 the concentration of GC-MS detected compounds in the bio-oil* (dry-basis) are represented. The non-detected matter (difference until 100 %) is related with the presence of heavy compounds that cannot be analysed by GC-MS. It can be observed that the use of catalysts increases the concentration of GC-MS detected compounds in the bio-oil* . This can be attributed to the fact that the catalysts generate lighter compounds by conversion of oligomeric species and heavy alkoxy phenols typically found in pure lignin. In comparison to noncatalytic experiments, when using h-H-ZSM-5 and h-H-Beta zeolites, although bio-oil* mass yields diminish, the proportion of detected species increase up to 36 wt.% for h-H-ZSM-5 and 51 wt.% for h-H-Beta, respectively. These findings demonstrate that raw zeolites effectively convert oligomers into smaller molecules. On the other hand, employing h-MeH-ZSM-5 and h-MeH-Beta zeolites as catalysts results in significantly higher concentrations of GC-MS Fig. 5. Product mass yields from catalytic pyrolysis of lignin over hierarchical and ion-exchanged h-H-ZSM-5 zeolites. Fig. 6. Product mass yields from catalytic pyrolysis of lignin over hierarchical and ion-exchanged h-H-Beta zeolites. M.I. ´ Avila et al. Catalysis Today 456 (2025) 115343 8 detected components compared to those obtained using the parent zeolites, which indicates that the ion-exchanged zeolites are more efficient in transforming oligomers into bio-oil* detectable compounds compared to pure acidic zeolite. However, the exception is the h-NaH-Beta zeolite, particularly at a C/L ratio of 0.4, being the detected fraction in this case similar to that obtained using the parent zeolite. This phenomenon may be attributed to the markedly elevated amount of metal ion-exchanged reached. It is possible that this loading is promoting condensation and polymerisation reactions, which simultaneously inhibits cracking reactions that would increase lighter species. This finding is consistent with previous results obtained by our group working similarly with a Na + ion-exchanged commercial nanocrystalline H-Beta zeolite [17]. It is important to note that, in comparison to our previous work [17], where nanocrystalline zeolites were tested in the same reaction, the concentration of bio-oil* GC-MS detected components increases when hierarchical catalysts are employed (5 – 15 wt.%). Thus, the higher accessibility of hierarchical ZSM-5 and Beta zeolites favours the production of a bio-oil* with a higher concentration of detected compounds, not only in comparison to the non-catalytic test, but also in comparison to analogous zeolites with lower proportion of external surface area. Figs. 9 and 10 summarise the bio-oil* composition obtained in the catalytic pyrolysis of lignin employing hierarchical and ion-exchanged H-ZSM-5 and H-Beta zeolites, respectively, as catalysts, including the thermal results for comparison purposes. It can be observed that all the GC-MS detected compounds are also produced from the thermal decomposition of lignin, but their quantities increase with the use of catalysts. This fact can be attributed to the heavy/oligomeric species decomposition/conversion reactions that are promoted when the pyrolytic vapours pass through the catalytic bed. The use of appropriate catalytic systems for lignin pyrolysis serves to enhance cracking reactions, deoxygenation, dehydration, and demethylation. Fig. 11 displays a proposed processes scheme for the thermal and catalytic lignin decomposition, which will be described in detail in the subsequent sections. Significant increments, regarding the non-catalytic test, are detected Fig. 7. Overall concentration of GC-MS detected and non-detected components in the bio-oil* obtained in the lignin catalytic pyrolysis over hierarchical and ion-exchanged h-H-ZSM-5 zeolites. Fig. 8. Overall concentration of GC-MS detected and non-detected components in the bio-oil* obtained in the lignin catalytic pyrolysis over hierarchical and ion-exchanged h-H-Beta zeolites. Fig. 9. Bio-oil* GC-MS molecular analysis obtained in the lignin pyrolysis over hierarchical and ion-exchanged h-H-ZSM-5 zeolites (CA: carboxylic acids, LO: light oxygenates, FUR: furans, AR: aromatic hydrocarbons and O-AR: oxygenated aromatics). Fig. 10. Bio-oil* GC-MS molecular analysis obtained in the lignin pyrolysis over hierarchical and ion-exchanged h-H-Beta zeolites (CA: carboxylic acids, LO: light oxygenates, FUR: furans, AR: aromatic hydrocarbons and O-AR: oxygenated aromatics). M.I. ´ Avila et al. Catalysis Today 456 (2025) 115343 9