Influence of Secondary Porosity Introduction via Top-Down Methods on MOR, ZSM-5, and Y Zeolites on Their Cumene Cracking Performance

Abstract

The influence of secondary porosity and the dimensionality of zeolitic structures with 1D and 3D pore systems on the accessibility of cumene to Brønsted acid sites was evaluated in this study. Zeolites Y, ZSM-5, and MOR, obtained through NH4F leaching and basic and acid treatments, were studied. Zeolites Y and ZSM-5 showed a significant increase in specific surface area while maintaining the micropore volume as well as an increase in the concentration of Brønsted acid sites following treatment. Zeolite MOR exhibited an increase in mesopore volume and retained Brønsted acidity. The impact of the treatments on catalytic properties was evaluated through cumene cracking, which yielded high catalytic conversion for the materials. This result is consistent with the goal of the model reaction to characterize Brønsted acid sites, enhance accessibility, and reduce diffusion paths.

1. Introduction

Zeolites are crystalline aluminosilicates typically containing alkali and alkaline earth metals as counterions and are extensively used in catalytic cracking processes in the petrochemical industry. They are predominantly microporous and, despite possessing truly unique properties such as high crystallinity, thermal stability, and acidity, they exhibit inferior performance compared to some hierarchical zeolitic materials due to access and diffusion constraints in catalyzing reactions and processes involving molecules with kinetic sizes larger than their pore size [1,2,3]. The zeolite Y with FAU topology presents a cubic system and is commonly synthesized in the Si/Al ratios in the range of 2.5 to 2.9. Its large pore opening and three-dimensional (3D) channel system make it an excellent catalyst compared to USY used in fluid catalytic cracking and hydrocracking processes [4,5,6,7]. The zeolite ZSM-5 with MFI topology features accessible medium pores with a three-dimensional (3D) channel system, has a unique characteristic on the Si/Al ratios that can synthesized, at a range between 15 and ∞, therefore, high silica content. It is widely used in various catalytic processes on an industrial scale [8,9,10,11,12,13,14,15,16,17]. Continuously, the zeolite mordenite with MOR topology can be synthesized with a Si/Al ratio ≥ 5 and has only one network of accessible pores, thus featuring a pseudo-one-dimensional (1D) channel system and offers advantages in applications primarily in petroleum refining, such as catalytic cracking and hydrocracking, isomerization, and carbonylation [18,19,20,21,22].

Introducing mesoporosity into zeolite crystals enhances molecular accessibility and transport as the length of the intracrystalline diffusion path is reduced and the deactivation range is briefly diminished [2,7,23,24,25,26,27]. There are several methodologies to introducing secondary porosity into zeolitic materials. The methods which allow for hierarchization during synthesis are termed “bottom-up”, and they require the use of soft or hard templates such as surfactants, sacrificial inorganic materials, etc. These methods might generate controllable mesoporosity, but they are associated with high production cost and environmental risk due to the molds used [7,24]. The “top-down” methods are based on post-synthetic treatments, where one or more components of the zeolite structure are extracted. Among these methods, desilication involves the preferential removal of silicon, dealumination involves the preferential removal of aluminum, and unbiased demetallation involves the removal of both silicon and aluminum from the zeolite structure [26,28,29]. Demetallation by fluoride action is a treatment that yields materials with compositions similar to the original and controlled porosity. The combination of HF-NH₄F is effective, but it involves HF, which is highly toxic, and thus requires expensive equipment in subsequent effluent processes [30]. The use of NH₄F alone removes aluminum and silicon equally and is independent of the composition or structure of the zeolite, yielding a zeolite similar to the starting material without issues of HF poisoning [14].

There are numerous studies related to creating mesoporosity in various types of zeolitic structures. For example, obtaining mesoporous ZSM-5 through treatments with hydrofluoric acid has shown improved performance in n-hexane cracking to light alkenes. However, it is essential to understand the Si/Al ratio well for proper treatment and avoid collapsing the zeolite structure [31,32]. The ideal Si/Al ratio range for the desilication of ZSM-5 zeolite with secondary porosity development is 25–50. Below this range, there is no generation of mesopores, and significantly above it, silicon dissolution occurs, leading to the formation of only micropores [33]. However, within this range, increased activity and selectivity are achieved due to enhanced diffusivity observed in various processes such as biomass conversion, catalytic cracking, and n-octane hydroisomerization [14,18,34,35,36,37]. Improvements in accessibility and molecular transport with a reduction in diffusion path length are evident for ZSM-5 zeolite with secondary porosity, as reported in various reactions: methanol dehydration to DME, catalytic cracking of n-hexane, cumene, and triisopropylbenzene (TIPB), among others [27,31,38,39,40,41,42]. Zeolite Y zeolite exhibited controllable mesopore formation through acid and/or basic treatments, where it has shown favorability in cracking processes, toluene adsorption, benzyl alcohol alkytation, CO₂ adsorption, increased selectivity towards ethylene and propane in hydrocracking, hydrogenation and isomerization biomass processes [7,14,15,27,34,35,38,43,44,45,46]. The introduction of secondary porosity into zeolite mordenite has led to improvements in catalytic performance in DME carbonylation, nC9 adsorption, and benzene alkylation with propylene to cumene, without loss of the starting material’s acidic properties [21,22,27,29].

It is possible to use certain reactions as a type of “model reaction” to better characterize catalysts, such as the cracking of n-hexane used to study the location of acid sites in a ZSM-5, or the isomerization and alkylation of benzene in the study of pore size and interconnectivity with medium-sized pores [11,13]. Catalytic cracking of cumene is used to study acidic properties, primarily Brønsted acid sites, where cumene is adsorbed and protonated initially, and then mainly converted into benzene and propylene [42,47]. However, it is possible to use this same reaction to study the influences of secondary porosity, morphology, and the network of 1D and 3D pores on accessibility and diffusivity in zeolitic catalysts, as the cumene molecule still experiences diffusion limitations in the pores and in the dimensionality of typical zeolitic materials channels [15,27,39,42,44,47,48,49,50,51,52].

This study demonstrates the influence that secondary porosity introduced via top-down methods has on the structural properties of zeolites with 1D and 3D pore systems: MOR, ZSM-5, and Y, and how it affects their performance as catalysts for cumene catalytic cracking. The results confirm a significant impact on the conversion of hydrocarbons, which is consistent with improvements in acidity, accessibility, and diffusivity of the materials.

2. Results and Discussion

2.1. Synthesis and Characterization of Mordenite Zeolite with Secondary Porosity

Figure 1 shows the XRD patterns of the mordenite zeolite before and after the introduction of secondary porosity. It is noticeable that the material after the treatments (HMOR-T) does not exhibit significant apparent changes in the characteristic peaks of the MOR phase compared to the commercial zeolite (NH₄MOR), except for the increase in the first peak, which may be related to the alkaline treatment, where the conditions used are those that promote less loss of crystallinity [29]. Therefore, even after desilication and dealumination treatments, the structure of the mordenite zeolite persists [18].

Table 1. Si/Al ratio, relative crystallinity and effective yield of Mordenite zeolites.

Samples	FRX (%)			DRX	NMR
	SiO2	Al2O3	Na2O	C (%)	Si/Al	SAR
NH4MOR	87.1	8.7	1.3	100	8.2	16.5
HMOR	87.3	8.6	1.3	108	9.4	18.8
HMOR-T	89.1	7.1	1.3	82.2	11.4	22.8

C: Relative crystallinity [(∑characteristic peaks of the starting zeolite/∑characteristic peaks of treated zeolite) × 100]. SAR: Structure Si/Al ratios.

presents the relative crystallinity, where the typical morphology of mordenite zeolite “worm-like particles” was preserved, with an increase in the organization of clustered crystal aggregates observed in its micrographs (Figure 2a,b) [53].

The increase in the Si/Al ratios observed in the SAR_NMR for the treated sample (HMOR-T) mentioned in Table 1, compared to the starting material (NH₄MOR), confirms that the acid-alkaline treatment led to a preferential dealumination of the structure, reflected in the partial decrease in the material’s crystallinity. However, the amount of Si removed may not have been significant, probably due to the low Si/Al ratio of the starting material [10,18,29,54]. It is observed that the HMOR sample showed a slight increase in its Si/Al ratio and the SAR_NMR, which may have been caused during the calcination process. The ²⁹Si NMR spectra (Figure S3) of the mordenite zeolites exhibit two characteristic peaks arising from the Q⁴Si(0Al) and Q³Si(1Al) species, respectively, with the latter possibly related to the acidic sites. The absence of other peaks may be associated with the desilication process, but the decrease in intensity corresponding to Q³Si(1Al) suggests limited silicon removal from the structure and a possible decrease in material acidity [6,29]. For the ²⁷Al NMR spectra (Figure S3) of the reference zeolite, the characteristic peak of tetrahedral aluminum species of the zeolite structure is exhibited. However, the sample subjected to cation exchange and subsequent calcination, as well as the one with introduced secondary porosity (HMOR and HMOR-T), showed the peak of tetrahedral aluminum, but also displayed that of octahedral aluminum, which is outside the zeolite structure [6,29].

The nitrogen adsorption isotherms for the mordenite zeolites (Figure 3) exhibited type I (a) profiles, with more pronounced uptake at low p/p⁰, which is associated with micropore filling. However, the hysteresis of the treated zeolite (HMOR-T) was type H4, which may be related to the formation of mesopores [55]. The results of the textural properties obtained from the applied methods are described in

Table 2. Results of textural parameters obtained through N₂ adsorption and desorption for Mordenite zeolites.

Samples	BET	t-Plot Harkins-Jura-de-Boer			Gurvich VTP
	SBET (m2/g)	Vo (cm3/g)	Vm (cm3/g)	St (m2/g)	Vt (cm3/g)
NH4MOR	529	0.169	0.081	81	0.250
HMOR	536	0.164	0.090	103	0.254
HMOR-T	507	0.159	0.104	89	0.263

S_BET: Specific surface area; V₀: Micropore volume; V_m: Mesopore Volume; S_t: Specific surface area of micropores; V_t: Total pore volume.

.

The textural properties of the mordenite zeolites are summarized in Table 2. The acidic zeolite (HMOR) exhibited properties similar to the starting zeolite (NH₄MOR), with a high micropore volume and a lower mesopore volume. However, the zeolite after the treatment for the introduction of secondary porosity maintained the specific surface area (S_BET) and showed an increase in the mesopore volume at the cost of a slight decrease in the micropore volume, corroborating the improvement in the material’s crystallinity (Table 1), a similar case reported by Stefanidis et al., 2013. Considering the increase in the Si/Al ratio and SAR_NMR (Table 1), it can be inferred that the introduction of mesoporosity is adjusted during the basic treatment step, and the subsequent acid treatment led to dealumination of the material. However, this increase in SAR_NMR and the presence of octahedral aluminum species in the ²⁷Al NMR spectrum (Figure S5) may indicate that the aluminum extracted from the structure remained in the liquid phase during treatment and was reintroduced into the treated zeolite, a process known as realumination [18,56].

The FT-IR spectra of the mordenite zeolites (Figure 4) display characteristic bands of the zeolite structure in the range of 1200–1065 cm⁻¹ corresponding to internal and external asymmetric stretching vibrations of TO₄ units, in the 900–500 cm⁻¹ range originating from symmetric stretching vibrations between TO₄ units and deformations of the S4R rings, and exhibit the structural degree of the zeolite or bending vibrations of the D5R rings [57]. The decrease in the band in the 3500–1600 cm⁻¹ range arises from water removal during the calcination process or possible dissolution or amorphization of Si-OH-Al groups during alkaline treatment, which also accounts for the increase in the band of isolated Si-OH groups located between 1500 and 1000 cm⁻¹. However, the treated samples maintained the zeolitic character of the material, as evidenced by the preservation of characteristic bands of the structure in the 1000–500 cm⁻¹ range [19,57].

The mordenite zeolite after secondary porosity introduction treatments (HMOR-T) exhibited a high concentration of Brønsted acid sites, as shown in

Table 3. Concentration of Brønsted and Lewis acid sites of HMOR and HMOR-T zeolites.

Samples	Pyridine Acidity in IR (µmol·g−1) 150 °C		Pyridine Acidity in IR (µmol·g−1) 450 °C
	Sites of Brønsted [HPy]	Lewis Sites [LPy]	Sites of Brønsted [HPy]	Lewis Sites [LPy]
HMOR	833	143.3	130	158.5
HMOR-T	708	135.3	201	78.5

, although lower than the zeolite with only cation exchange and calcination (HMOR), this decrease in acidity was influenced during the dealumination step, which occurred preferentially, as evidenced by the Si/Al ratio and SAR_NMR data [18]. However, the stability of pyridine at higher temperatures highlights the acidic strength of the sites present in the treated zeolite.

2.2. Synthesis and Characterization of ZSM-5 Zeolite with Secondary Porosity

The diffractograms of ZSM-5 zeolites in Figure 5 exhibit characteristic peaks corresponding to the Miller indices for the typical crystalline structure [58]. After alkaline treatment, the samples maintain the MFI-type crystalline structure similar to the starting zeolite, without the appearance of any other undesirable crystalline phase, amorphous materials, or crystal rearrangement, presenting a single crystalline phase [28,54,59,60]. The well-defined morphology, where the square crystals with the flattest surface extend throughout the crystal, is preserved after the introduction of mesoporosity treatments in Figure 6a,b, with any observations of voids on the crystal surface being caused by the alkaline treatment [8,12,61].

For zeolite ZSM-5, the structural Si/Al ratios and the SAR_NMR (

Table 4. Si/Al ratio, relative crystallinity and effective yield of ZSM-5 zeolites.

Samples	FRX (%)			DRX	NMR
	SiO2	Al2O3	Na2O	C (%)	Si/Al	SAR
NH4ZSM-5	93.1	6.9	1.10	100	14.4	28.8
HZSM-5	91.7	8.3	1.40	105.9	21.1	42.1
HZSM-5-T	92.9	7.1	1.20	103	24.2	48.4

C: Relative crystallinity [∑(characteristic peaks of the starting zeolite/∑characteristic peaks of treated zeolite) × 100]. SAR: Structure Si/Al ratios.

) of the treated sample (HZSM-5-T) increase compared to the starting sample, indicating controlled removal of possible amorphous phases after treatment with NaOH. Silicon removal from the structure is expected due to the hydroxide ion, owing to the facile base-catalyzed hydrolysis of silanols; however, aluminum tetrahedra are inert to hydroxide attack due to the associated negative charges [62]. The amounts of removed Al were small and could have been caused by the dissolution of zeolite grain fragments, or by the extracted aluminum being reinserted into the external or internal surface of the structure [63]. The crystallinity did not undergo any significantly apparent reductions; therefore, a highly crystalline material was obtained [28,32,56,60].

The ²⁹Si NMR spectra (Figure S3) of the starting ZSM-5 zeolite and HZSM-5 exhibit two characteristic peaks of the respective aluminosilicates species Q³Si(1Al) and Q⁴Si(0Al) [6]. The material treated with NaOH showed a decrease in the peak of the Q³Si(1Al) species. This may be related to the removal of amorphous phases since the presence of AlO₄⁻ stabilizes neighboring silicon atoms by repelling hydroxyl groups, as Si-OH-Al is less prone to OH⁻ attack than Si-O-Si, showing agreement with slightly higher SAR_NMR values [57].

The ²⁷Al NMR spectra (Figure S3) of the starting zeolite exhibit only the characteristic peak of tetrahedral aluminum present in the zeolite framework at a chemical shift around 57 ppm. The zeolite in its acidic form (HZSM-5) presented, in addition to tetrahedral aluminum, octahedral aluminum species at a chemical shift near 0 ppm. Furthermore, the sample that underwent the secondary porosity introduction process (HZSM-5-T) also contains octahedral aluminum species, suggesting that aluminum atoms may not be in a structural position, given the decrease in material crystallinity [57,64,65].

All ZSM-5 zeolites exhibit type I (a) N₂ adsorption isotherms (Figure 7) with pronounced adsorption at lower p/p⁰ typical of materials with filled micropores. However, the acidic HZSM-5 zeolite and the treated HZSM-5-T showed H4 type hysteresis, enabling the presence of mesopores [55]. The results of the textural properties obtained from the applied methods are described in

Table 5. Results of textural parameters obtained through N₂ adsorption and desorption for ZSM-5 zeolites.

Samples	BET	t-Plot Harkins-Jura-de-Boer			Gurvich VTP
	SBET (m2/g)	Vo (cm3/g)	Vm (cm3/g)	St (m2/g)	Vt (cm3/g)
NH4ZSM-5	406	0.096	0.070	165	0.166
HZSM-5	446	0.113	0.106	165	0.219
HZSM-5-T	858	0.271	0.119	144	0.390

S_BET: Specific surface area; V₀: Micropore volume; V_m: Mesopore Volume; S_t: Specific surface area of micropores; V_t: Total pore volume.

.

The textural properties of the starting ZSM-5 zeolite (Table 5) indicate the high BET specific surface area and predominant total micropore volume in the material. The zeolite in its acidic form (HZSM-5) showed a slight increase in specific surface area and total micropore volume, possibly due to the calcination process [66]. The zeolite treated with NaOH (HZSM-5-T) exhibits a significant increase in specific surface area, indicating the presence of both micro- and mesopores. Therefore, it is suggested that the alkaline treatment at low concentrations mainly removes amorphous components or defects from the material while preserving the micropores [56,59,66].

The FTIR spectra (Figure 8) for the starting ZSM-5 zeolite exhibit bands corresponding to water molecules and silanol groups (Si-O-Si) in the range of 3700–3400 cm⁻¹, bands representing external linkages between TO₄ groups present at 1200 cm⁻¹ and characteristic bands of the zeolitic structure, namely vibrations of pentasil rings (S5R), displayed at approximately 550 cm⁻¹ [67]. The decreases in bands are minimal for the HZSM-5 sample, which can be explained by the elimination of water molecules and silanol groups after the calcination process. In the sample with secondary porosity, the bands practically disappear in the range of 3700–3400 cm⁻¹ after alkaline treatment with NaOH, consistent with the desilication process removing Si-OH groups and water molecules during calcination for the formation of mesoporosity initiated at sites and defects in the zeolite crystals [56,67].

The results of IR-Pyridine in

Table 6. Concentration of Brønsted and Lewis acid sites of HZSM-5 and HZSM-5-T zeolites.

Samples	Pyridine Acidity in IR (µmol·g−1) 150 °C		Pyridine Acidity in IR (µmol·g−1) 450 °C
	Sites of Brønsted [HPy]	Lewis Sites [LPy]	Sites of Brønsted [HPy]	Lewis Sites [LPy]
HZSM-5	813.0	156.9	348.1	79.1
HZSM-5-T	892.7	168.7	411.7	104.9

confirm that the cation exchanges and successive calcination in the HZSM-5 sample resulted in a material with a high concentration of Brønsted acid sites, originating from tetrahedrally coordinated aluminum and possible removal of debris that hinder access through the channels in the porous network of the zeolite [68]. An increase in the concentration of Brønsted acid sites is identified in the sample treated with NaOH (the best scenario of the syntheses performed). This occurred not only due to the aforementioned reasons, but also due to the presence of mesoporosity in the crystals, further facilitating molecule diffusion and increasing accessibility to the acid sites [37]. The still high concentration of Brønsted acid sites at temperatures above 450 °C is noteworthy, indicating predominant strong sites.

2.3. Synthesis and Characterization of Zeolite Y with Secondary Porosity

In Figure 9, it is noticeable that the zeolite maintains its crystalline order after the secondary porosity introduction treatment, as no significant changes in the peak positions are observed. On the other hand, the demetallation process promotes an increase in crystallinity when comparing the NaY and HY-T samples, reflected in the increase in relative crystallinity (

Table 7. Si/Al ratio, relative crystallinity and effective yield of Y zeolites.

Samples	FRX (%)			DRX	NMR
	SiO2	Al2O3	Na2O	C (%)	Si/Al	SAR
NaY	67.7	21.1	8.8	100	2.7	5.3
HY	71.9	22.9	2.9	90.2	3.7	7.5
HY-T	73.7	20.8	2.9	108	3.4	6.8

C: Relative crystallinity [∑(characteristic peaks of the starting zeolite/∑characteristic peaks of treated zeolite) × 100]. SAR: Structure Si/Al ratios.

). The changes in crystallinity can be attributed to recrystallization, as the X-ray diffraction pattern of the demetallized zeolite does not exhibit any broadening of diffraction lines or anomalous intensities of lines [69]. The cuboid morphology with fine edges expected for a Y zeolite is preserved after the treatments according to the scanning micrographs in Figure 10a,b [6].

The commercial NaY zeolite used as a starting material for the post-synthesis treatment has a Si/Al ratio of 2.7 (Table 7). The sample subjected only to cation exchange and subsequent calcination (HY) for use in catalytic reactions showed a slight increase in the Si/Al ratio and SAR_NMR, which may have been caused by the calcination process. For comparison, the material subjected to the creation of mesoporosity (HY-T) had a ratio of 3.4. Simultaneous removal of silicon and aluminum may have occurred more effectively, with preferential removal of aluminum considering these increases in SAR_NMR values from the starting (NaY) to the treated (HY-T) zeolite. This increase in the Si/Al ratio in the zeolite framework, besides favoring the hydrothermal stability of the material, may also lead to the generation of secondary porosity [30].

The ²⁹Si NMR spectra (Figure S3) of the reference zeolite and after cation exchange followed by calcination (NaY and HY) show four of the five characteristic peaks in the range of aluminosilicates, with the following chemical shifts: −88.0, −93.7, −99.3, and −104.8 ppm, which originate from the following species Q⁰Si(4Al), Q¹Si(3Al), Q²Si(2Al), Q³Si(1Al) respectively, are responsible for describing most of the -Si-O-Al-O- groups present in the zeolite structure [6,64,65]. The HY-T treated sample exhibits three of the five characteristic peaks with chemical shifts in the following regions: −94.3, −100.5, and −105.3 ppm. The shift originating from Q⁰Si(4Al) disappeared, while the intensities of Q¹Si(3Al), Q²Si(2Al), Q³Si(1Al) species showed a slight increase. These results are consistent with the increase in SAR_NMR.

The ²⁷Al NMR spectra (Figure S3) of the NaY reference sample exhibit the characteristic peak of tetrahedral aluminum species in the zeolite structure at a chemical shift of approximately 60 ppm. However, for the sample subjected to cation exchange and calcination (HY), in addition to the characteristic peak of tetrahedral aluminum, evidence of octahedral or extrastructural aluminum is observed at a chemical shift of 0.8 ppm, which may be an effect of the calcination process. For the HY-T treated sample, in addition to showing the peak of tetrahedral aluminum at a chemical shift near 60 ppm, there is also a shift at 1.2 ppm, which can be attributed to the extrastructural aluminum present on the surface of the zeolite after treatment [6]. These results are consistent with the preferential removal of aluminum from the framework, as also evidenced by the values obtained for the SAR_NMR in Table 7 of the reference and treated samples.

The nitrogen adsorption isotherms of Y zeolites (Figure 11) display type I (a) isotherms with pronounced adsorption at lower p/p⁰ typical of materials with narrow micropores, but the HY-T zeolite exhibited H4 type hysteresis elucidating the formation of mesopores [55]. The results of the textural properties obtained from the applied methods are described in

Table 8. Results of textural parameters obtained through N₂ adsorption and desorption for Y zeolites.

Samples	BET	t-Plot Harkins-Jura-de-Boer			Gurvich VTP
	SBET (m2/g)	Vo (cm3/g)	Vm (cm3/g)	St (m2/g)	Vt (cm3/g)
NaY	796	0.259	0.053	100	0.312
HY	524	0.163	0.055	94	0.218
HY-T	858	0.266	0.124	158	0.390

S_BET: Specific surface area; V₀: Micropore volume; V_m: Mesopore Volume; S_t: Specific surface area of micropores; V_t: Total pore volume.

.

The textural properties of Y zeolites (Table 8) show that the reference material NaY exhibits high specific surface area and micropore volume. However, the external surface area and low mesopore volume hinder accessibility, as expected for predominantly microporous zeolite. The sample subjected to cation exchange and subsequent calcination (HY) demonstrates a decrease in specific surface area and micropore volume, which occurs due to the calcination process [23]. The treated zeolite (HY-T) shows significant increases in specific surface area and mesopore volume to S_BET = 858 m²/g and V_m = 0.124 cm³/g, respectively, as well as preservation of the micropore volume with a slightly larger external surface area (V₀ = 0.266 cm³/g and S_t = 158 m²/g). This is in accordance with the proposed methodology [30] for the introduction of mesopores and increasing the surface area of Y zeolite without major alterations to its intrinsic properties such as Si/Al ratio, micropore volume, and crystallinity.

The FTIR spectra in Figure 12 display characteristic peaks of the zeolitic structure found in the range of 1100 to 450 cm⁻¹, originating from vibrations due to symmetric stretching of internal tetrahedra of the zeolite and asymmetric stretching of Si-O or Al-O bonds. The bands between 3450 and 1640 cm⁻¹ correspond to water molecules present in the cavities [24,70]. The HY and HY-T samples showed decreases in bands corresponding to the removal of water molecules during the calcination process, and of Si-OH-Al groups that were dissolved or amorphized during demetallation [57].

The starting acidic zeolite (HY) exhibited notable acidic properties (

Table 9. Concentration of Brönsted and Lewis acid sites of HY and HY-T zeolites.

Samples	Pyridine Acidity in IR (µmol·g−1) 150 °C		Pyridine Acidity in IR (µmol·g−1) 450 °C
	Sites of Brønsted [HPy]	Lewis Sites [LPy]	Sites of Brønsted [HPy]	Lewis Sites [LPy]
HY	614.6	269.3	7.9	3.3
HY-T	625.4	501.1	no acidity activity considerated	5.4

). The concentration of Brønsted acid sites is high, mainly influenced by tetrahedrally coordinated aluminum species and the removal of debris located in the channel accesses. The concentration of Lewis acid sites present is derived from extrastructural or octahedral aluminum species, as evidenced by the ²⁷Al NMR data (Figure S7) [68]. The sample with secondary porosity showed improved accessibility of pyridine molecules and had preferential aluminum removal, thus presenting a significant increase in the concentration of Lewis acid sites. Simultaneously, there was an increase in the concentration of Brønsted sites caused by improvements in material accessibility [68]. Additionally, the acidity quantified at 450 °C showed that for the HY-T zeolite, the acid sites are slightly stronger than for the HY sample, a behavior that may also be associated with increased accessibility to the zeolite’s acid sites conferred by the creation of additional porosity.

2.4. Catalytic Evaluation

The series of obtained zeolites were applied in the cumene cracking reaction to investigate the catalysts’ performance based on the previously studied properties. The starting Zeolite Y in its acidic form showed high cumene conversion initially, reaching a maximum of approximately 90% (Figure 13), an expected outcome since the material has high acidity, as cumene cracking predominantly occurs over Brønsted acid sites [49]. Notably, the Zeolite Y with secondary mesoporosity (HY-T) exhibited high catalytic performance, reaching conversions close to 100% within 30 min of reaction. This initial similar behavior for both is expected, given their acidic properties, as there is a considerable concentration of Brønsted acid sites for both Zeolite HY and HY-T (Table 9). However, the treated zeolite proved to be more efficient during the reaction due to improvements in its textural properties. The increase in specific surface area and mesopore volume (Table 8) provided greater accessibility to the micropores and active sites of the zeolite.

Figure 13 shows that the initial ZSM-5 zeolite in its acidic form (HZSM-5) proved to be quite effective in terms of cumene conversion, reaching conversion values of 95% around 120 min of reaction. However, for the sample with secondary porosity (HZSM-5-T), the cumene conversion stands out from the beginning of the reaction, which may be due to greater accessibility and diffusion of reactants and products, given the increase in its textural properties (Table 5) [39]. The abundance of strong Brønsted sites ensures that the cumene cracking conversion rate is higher than the hydrogen transfer from propylene conversion to form propane (parallel reaction). These values of high conversion and selectivity for the formation of benzene and propylene from the Y and ZSM-5 zeolite series are in line with their respective concentrations of Brønsted acid sites, although in some cases there are traces of other by-products such, as ethylbenzene and toluene (Figure S4).

The mordenite zeolite in its acidic form (HMOR) obtained lower conversion compared to other materials (Figure 13), reaching a maximum of around 84% and rapidly declining in efficiency. This behavior may be a consequence of its one-dimensional (1D) channel causing faster deactivation due to coke deposition [35]. On the other hand, the zeolite with secondary porosity (HMOR-T), which exhibits similar properties, also achieved similar cumene conversions, reaching a maximum of approximately 74%; however, the sample shows slower deactivation, which may be directly related to the meticulous improvement in mesopore volume [22]. However, the high selectivity of the catalysts in this series for benzene and propylene is notable, without the formation of any by-products (Figure S4). The analysis of post-reaction catalysts (Figures S5 and S6) corroborates these results, considering the persistence of the zeolitic structures even after deactivation in some cases.

Comparatively, the samples (HY-T, HZSM-5, and HZSM-5-T) exhibited similar behaviors in conversion. HY-T starts with a higher conversion compared to HZSM-5, which is consistent with its improvements in accessibility and micropore volume (active sites). However, due to its higher concentration of Brønsted acid sites, after longer reaction times, HZSM-5 shows a conversion similar to the HY-T sample. This fact becomes more evident when comparing the HZSM-5-T sample, which has both superior textural properties and a higher concentration of Brønsted acid sites than the two previously mentioned samples, thus maintaining a higher conversion than the others throughout the entire evaluated reaction period [15,39].

Figure 14 presents the correlation of the studied results of average arithmetic cumene conversion (%), mesoporosity (cm³·g⁻¹), and Brønsted acidity (μmol·g⁻¹) of the investigated catalyst series. The combination of improvement in the concentration of acid sites present in the zeolites (mainly Brønsted acid sites), along with textural properties and increases in mesoporosity, directly impacted the catalytic capacity improvement of the materials in the catalytic cumene cracking reaction. For zeolite Y, ZSM-5, and mordenite, which have 12 MR pores (approximately 0.78 nm), 10 MR pores (around 0.6 nm), and 12 MR pores (approximately 0.59–0.71 nm), respectively, allowing easy access of the cumene molecule with a molecular diameter of 0.68 nm, even with this favoritism, the materials with secondary porosity (HY-T, HZSM-5-T, and HMOR-T) showed better catalytic performance due to enhanced accessibility and/or diffusivity, thus making better use of the zeolite crystals compared to the starting materials after 60 min of reaction [15,27,35,39,42,51]. However, the influence of pore dimensionality for zeolite HY-T (3D) is evident when evaluating the catalytic performance (Figure 10) compared to zeolite HMOR-T (1D), resulting from the greater ease of diffusion in its pore network [15,27,35].

3. Materials and Methods

3.1. Materials

For this study, the following reagents were used: Sodium hydroxide (NaOH, 97%, Dinâmica—São Paulo, Brazil), nitric acid (HNO₃, 99% Vetec Química fina—Rio de Janeiro, Brazil), ammonium fluoride (NH₄F, 98%, Sigma-Aldrich—San Luis, MO, USA), ammonium chloride (NH₄Cl, 99.5%, NEON—São Paulo, Brazil), cumene (98%, Sigma-Aldrich—San Luis, MO, USA), ethylbenzene (99.8%, Sigma-Aldrich—San Luis, MO, USA), toluene (Sigma-Aldrich—San Luis, MO, USA), benzene (99.9%, Sigma Aldrich—San Luis, MO, USA). The commercial zeolites employed were CBV 21A, CBV 2314, and CBV 100 manufactured by Zeolyste International—Conshohocken, PA, USA.

3.2. Synthesis of Zeolites with Secondary Porosity

The introduction of secondary porosity in mordenite is in accordance with previously reported work [18] and involves two treatment steps. Firstly, desilication was carried out by dispersing 5 g of the starting commercial mordenite zeolite (CBV 21A) in 500 mL of 0.2 mol·L⁻¹ NaOH solution under magnetic stirring at 80 °C for 0.5 h, followed by washing with hot distilled water until a neutral pH was reached. The material was then dried overnight at 100 °C. The second step involves dealumination of the material resulting from the first step. For this, the mordenite after desilication is dispersed in 500 mL of 3 mol·L⁻¹ HNO₃ solution under continuous magnetic stirring at 25 °C for 1 h. Subsequently, the resulting material is filtered, washed, and dried overnight at 100 °C.

For the starting commercial ZSM-5 zeolite (CBV 2314), the studies conducted by [32] were adapted with a single desilication step due to the Si/Al ratio range. First, 5 g of the zeolite were dispersed in 500 mL of a 0.2 mol·L⁻¹ NaOH solution under magnetic stirring and heating at 65 °C for 0.5 h. Subsequently, the resulting material was cooled in an ice bath, filtered, washed until neutral pH was reached, and dried overnight at 100 °C.

For the creation of additional porosity in the starting commercial Y zeolite (CBV 100), the methodology from [43] was adopted, which determines its capability to remove both aluminum and silicon from the crystal lattice. For this, a solution containing 10 g of NH₄F and 30 g of distilled water was prepared, resulting in a concentration of 25%. To this solution, 7.5 g of zeolite was added and treated in an ultrasonic bath for 1 h at 25 °C. After completion, the material was filtered, washed, and dried overnight at 100 °C.

To obtain the zeolitic catalysts in their acidic form (H⁺-Zeolite), cationic exchanges were carried out with a 0.1 mol/L NH₄Cl solution under magnetic stirring for 3 h at 80 °C. The resulting material was then filtered, washed until no more chloride ions (Cl⁻) precipitate, and dried overnight at 100 °C. This procedure was repeated three times. Subsequently, the materials were calcined by increasing the ambient temperature to 500 °C at a heating rate of 5 °C/min and holding it at that temperature plateau for 3 h, except for the starting commercial samples of mordenite and ZSM-5, which were only calcined as they were already in their ammonium form. In

Table 10. Nomenclatures adopted for materials.

Materials	Meaning
NH4MOR	Commercial Mordenite Zeolite
HMOR	Calcined Commercial Mordenite Zeolite
HMOR-T	Commercial Mordenite Zeolite after treatment, cation exchange and calcination
NH4ZSM-5	Commercial ZSM-5 Zeolite
HZSM-5	Calcined Commercial ZSM-5 zeolite
HZSM-5-T	Commercial ZSM-5 zeolite after treatment, cation exchange and calcination
NaY	Commercial Zeolite Y
HY	Zeolite Y after cation exchange and calcination
HY-T	Commercial Zeolite Y after treatment, cation exchange and calcination

, the nomenclatures of all the samples used are described.

3.3. Catalyst Characterizations

The powder X-ray diffraction (XRD) patterns were obtained at room temperature using a Bruker D2Phaser instrument equipped with a LYNXEYE detector and copper radiation (CuKα, λ = 1.54 Å) with a Ni filter, operating at a current of 10 mA and voltage of 30 kV, with angles ranging from 5 to 50°. Semi-quantitative chemical analyses were acquired by X-ray fluorescence on a Bruker S2 Ranger apparatus using Pd or Ag anode with a maximum power of 50 W, maximum voltage of 50 kV, maximum current of 2 mA, and XFlash^® Silicon Drift Detector. The two equipment units were acquired by Bruker São Paulo/Brazil.

The morphological pattern of the samples was evaluated using the ZEISS Auriga Scanning Electron Microscope with a Field Emission Gun (FEG) emitter, operating at a voltage of 20 kW, with chemical analysis detector by Energy-Dispersive X-ray Spectroscopy (EDS) from Bruker, model XFlash Detector 410-M. The equipment was acquired by ZEISS São Paulo/Brazil.

The FT-IR spectra were obtained using the Shimadzu IRAffinity1 Fourier Transform Infrared Spectrophotometer in the range of 4000 to 400 cm⁻¹ with a resolution of 4 cm⁻¹. The equipment was acquired by Shimadzu Kyoto/Japan.

For the specific surface area analysis, the Brunauer–Emmet–Teller method (SBET, m²/g) was utilized. The determination of micropore volume (V₀), micropore specific surface area (S_t), and mesopore volume (V_m) was performed using the t-plot method. The Gurvich rule was applied to obtain the total pore volume (V_t). The analyses were conducted using a Micromeritics ASAP 2020 Plus instrument through nitrogen physisorption at a temperature of 77 K, with sample degassing for 10 h under vacuum at 300 °C. The thermogravimetric analyses were obtained on a NETZSCH TG 209F3 with a platinum crucible under a synthetic air atmosphere with a heating rate of 10 °C/min up to 1000 °C. The equipment was acquired by Micromeritics Georgia/USA.

The ²⁹Si and ²⁷Al NMR spectra were obtained using the 300 MHz nuclear magnetic resonance equipment, model AVANCE III HD NMR SPECT. 300 from Bruker, with a superconducting magnet system with a 5.4 cm bore (operating field at 7.0463 Tesla). Rotor size: 4 mm. Working temperature range: −10 to 80 °C. The conditions for ²⁹Si NMR analysis were rotation at 10 kHz, room temperature, 10 s pulse delay, 10 ms contact time, and 1536 scans. For ²⁷Al NMR, the conditions were: rotation at 10 kHz, room temperature, 1 s pulse delay, and 0.5 ms contact time. The equipment was acquired by Bruker Massachusetts/USA.

The pyridine adsorption followed by infrared spectroscopy was performed using a Nicolet 5700 apparatus with an optical resolution of 2 cm⁻¹. The sample is pressed (0.5 ton) into a 2 cm² pellet and pre-treated from 20 to 450 °C under a dry air flow (100 cm³ min⁻¹) overnight. Sample degassing occurred at 10⁻⁵ bar for 1 h at 200 °C. For pyridine adsorption, the sample was adsorbed at 150 °C, at around 1–2 mBar for 5 min. The removal of physiosorbed pyridine from the sample was carried out under vacuum for 1 h. After this time, the sample was heated gradually to 250, 350, and 450 °C. IR spectra were obtained at each of these temperatures. For acidity estimation, the Beer–Lambert–Bouguer rule was applied, where the areas under the peaks were used to quantify Brønsted acid sites [PyH⁺], wavenumber at 1545 cm⁻¹, and Lewis acid sites [PyL] at 1454 cm⁻¹, according to the equation:

$$\left[{\mathrm{P}\mathrm{y}\mathrm{H}}^{+}\right]\mathrm{o}\mathrm{r}\left[\mathrm{P}\mathrm{y}\mathrm{L}\right]=\left({\displaystyle \frac{S}{m}}\right){\displaystyle \frac{A}{Ɛ}}$$

(1)

where [PyH⁺] and [PyL] represent the concentration of Brønsted and Lewis acid sites (μmol·g⁻¹), respectively. A is the area of the peak corresponding to the IR bands (1545 and 1455 cm⁻¹), S is the area of the pellet, m is the weight of the sample, and ε is the integrated molar adsorption coefficient, where ε1545 cm⁻¹ = 1.13 and ε1454 cm⁻¹ = 1.28 cm mol⁻¹.

3.4. Catalytic Reaction

Cumene (isopropylbenzene) cracking was conducted in a fixed-bed quartz reactor with an internal diameter of 300 mm. In each reaction, 100 mg of the catalyst supported on rock wool was fixed and subjected to a pretreatment under a flow of 15 mL·min⁻¹ of N₂ at 300 °C for 1 h. Subsequently, the cumene/N₂ flow was adjusted to 15 mL·min⁻¹ for the measurement of the reference blank, and the reaction was initiated at atmospheric pressure and a temperature of 300 °C for 3 h. The obtained products were analyzed and quantified by GC-FID, using a gastight syringe to collect a 200 µL aliquot and injected into the PerkinElmer Clarus 680 Gas Chromatograph with an Equality-5 capillary column (30 m, 0.25 mm, 0.25 µm) and flame ionization detector (FID) using the following method conditions: detector temperature 150 °C, injector 200 °C, column with a ramp from 30 °C to 85 °C, remaining 2 min at each plateau, and a heating rate of 5 °C. For qualitative analysis, the GC-MS technique using the Shimadzu GC-2010 Plus model with a polar column (30 m, 0.25 mm, 0.25 µm) was employed, with injection of 1 µL using a gastight syringe and the method conditions were as follows: detector temperature 150 °C, injector 200 °C, column with a ramp from 25 °C to 50 °C, remaining 4 min at each plateau, followed by 75 °C to 150 °C, remaining 2 min at each plateau, and a heating rate of 10 °C. The NIST (National Institute of Standards and Technology) library was used for subproduct identification. The conversion of cumene (C_IPB) was calculated according to Equation (2), and the product selectivity (S_prod) was calculated according to Equation (3) [71]:

$$\%{\mathrm{C}}_{\mathrm{I}\mathrm{P}\mathrm{B}}={\displaystyle \frac{\mathrm{A}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{e} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{d} \left(\mathrm{a}\mathrm{r}\mathrm{e}\mathrm{a}\right)}{\mathrm{A}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{e} \mathrm{i}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{e}\mathrm{d} \mathrm{i}\mathrm{n}\mathrm{t}\mathrm{o} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r} \left(\mathrm{a}\mathrm{r}\mathrm{e}\mathrm{a}\right)}}\times 100$$

(2)

$$\%{\mathrm{S}}_{\mathrm{P}\mathrm{r}\mathrm{o}\mathrm{d}}={\displaystyle \frac{\mathrm{Q}\mathrm{u}\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{t}\mathrm{y} \mathrm{o}\mathrm{f} \mathrm{d}\mathrm{e}\mathrm{s}\mathrm{i}\mathrm{r}\mathrm{e}\mathrm{d} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t} \left(\mathrm{a}\mathrm{r}\mathrm{e}\mathrm{a}\right)}{\mathrm{Q}\mathrm{u}\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{t}\mathrm{y} \mathrm{o}\mathrm{f} \mathrm{a}\mathrm{l}\mathrm{l} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s} \mathrm{o}\mathrm{b}\mathrm{t}\mathrm{a}\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{d} \left(\mathrm{a}\mathrm{r}\mathrm{e}\mathrm{a}\right)}}\times 100$$

(3)

4. Conclusions

The synthesis of materials with different dimensionalities and secondary porosity was successfully achieved for zeolite Y and ZSM-5. They showed a significant increase in surface area reaching 858 m²/g, without drastic changes in the main properties of the zeolites, such as crystallinity, Si/Al ratio, microporosity, while also obtaining an increase in acidity (625.37 and 892.67 µmol·g⁻¹, respectively). Mordenite zeolite did not show an increase in surface area; in contrast, it experienced an increase in mesopore volume (0.104 cm³·g⁻¹) and a decrease in the concentration of Brønsted acid sites (707.74 µmol·g⁻¹). Furthermore, the acidic properties of the materials remained high after the cation exchange process to which they were subjected. All synthesized materials showed high conversion values in the cumene cracking reaction (above 70%). However, the best scenarios are in zeolites with secondary porosity, which showed higher acid site concentration and improved accessibility (HMOR-T, ZSM-5-T, and HY-T). The combination of these properties, morphology, and pore network dimensionality of the zeolite defines the best performance related to catalytic activity in terms of conversion and selectivity. Moreover, especially for HY-T (3D), its conversion remained high even during the 3 h reaction, demonstrating that the secondary porosity creation treatment enhanced its performance as a catalyst for the cumene cracking reaction. The starting ZSM-5 zeolite indicated an excellent catalyst for cumene cracking, achieving high conversion values (approximately 90%) as the reaction progressed. This study is important for evaluating the influence of post-synthesis treatments on zeolites with different pore network dimensionalities, aiming to enhance their performance as catalysts. However, the results showed that greater catalytic efficiency regarding these treatments does not apply to all cases and all zeolites, as evidenced by the inferior behavior exhibited by mordenite (1D) compared to zeolite Y (3D).