ZSM-5, a typical zeolite with the topological structure of MFI, plays an important role in catalysis ^[1]. Owing to its high catalytic activity and unique shape-selectivity, it has been widely studied and used in many industrial processes such as alkylation, disproportionation, isomerization, and cracking ^[2-5]. However, the relatively small pore size (＜ 2 nm) of conventional ZSM-5 zeolites causes mass-transfer issues, thereby limiting the broad applications of ZSM-5 ^[6-9]. To overcome this drawback, hierarchically porous ZSM-5, possessing at least two levels of porosity (typically micropores and mesopores), has been extensively investigated. The combined microporosity and mesoporosity can alleviate diffusion limitation issues, thereby improving accessibility to acidic sites of large molecules. These advantages can lead to reduced catalyst deactivation and enhanced activity during specific catalysis processes ^[6-9]. Various strategies for preparing hierarchically porous ZSM-5 have been described in the literature over the past few decades ^[10-14]. Among them, desilication is a promising approach owing to its low cost, high efficiency ^{[15, 16]}, and convenient operation ^{[17, 18]}.

Additionally, the stability of the mesoporous network plays an important role in the activity and lifetime of a catalyst in practical applications. When the catalyst is exposed to a thermal and/or hydrothermal environment, dealumination typically occurs, leading to collapse of the mesoporous structure and depletion of acidic sites. Thus, it is necessary to stabilize the zeolite structure and acid sites in hierarchically porous zeolites. Phosphorus modification has been proven to be an efficient method for structure stabilization ^[19-21]. The framework aluminum pairs can be stabilized by extra- framework cationic species formed by protonation of orthophosphoric acid ^[22]. As a result, dealumination is inhibited and lattice aluminum is less susceptible to leaving the zeolite framework upon interaction between the framework aluminum and phosphorus ^{[23, 24]}. These modifications and resulting stabilization are typically conducted on the microporous zeolites. However, mesopores formed by silicon extraction are always disordered and less stable than the microporous structure ^[25]. Thus, it is important to stabilize the mesoporous network in hierarchically structured zeolites. Although stabilization by phosphorus is demonstrated to be a promising technique to enhance the hydrothermal stability of zeolites, reduction in acid strength occurs inevitably ^[26-28]. Upon interaction with the zeolitic framework, phosphorus species can substitute the protons on the oxygen of the bridging hydroxyl groups in the zeolitic framework to form P-OH by replacing Al-OH ^[29]; such a mechanism maintains the level of acidity of the resulting framework. Accordingly, achieving a balance between stabilization and weakening of acid sites upon phosphorus modification is important to design highly efficient catalysts.

In our previous work ^[30], hierarchically porous high-silica ZSM-5 with high hydrothermal stability was prepared by desilication and subsequent inorganic phosphorus modification. The obtained hierarchically porous ZSM-5 displayed excellent performance in various acid-catalyzed reactions. However, the synthesis procedure was complex and tedious as it involves multiple steps. Therefore, synthesizing hierarchically porous ZSM-5 with high hydrothermal stability using a more direct and convenient approach is of great interest. Moreover, the stabilization modification was limited to the following phosphorus species: H₃PO₄, sodium phosphates, and ammonium phosphates. These phosphorus species would non-selectively distribute in the zeolite channels ^[31-33]. To more effectively use the phosphorus species, understanding the insertion of phosphorus into the zeolitic framework is important.

Herein, organic quaternary phosphonium hydroxide (TEPOH) was chosen as a novel phosphorus source to enhance the accessibility and interaction with aluminum in the zeolite framework and to avoid leaching of phosphorus from the zeolite channels. Hierarchically porous ZSM-5 zeolites containing phosphorus were prepared using a newly developed method. Subsequently, the catalytic performance of the prepared hierarchically porous ZSM-5 toward dealkylation of 1, 3, 5- triisopropylbenzene (TIPB) and cracking of 1-octene was examined.

The properties of chemicals used in this work are as follows: Al₂(SO₄)₃·18H₂O (AR, SCRC), H₂SO₄ (98%, SCRC), water glass (SiO₂, 27.1 wt%), TEABr (99%, Sigma-Aldrich), TPAOH (1 mol/L in water, Sigma-Aldrich), NaOH (AR, SCRC), TEPBr (99%, Alfa Aesar), Ag₂O (AR, SCRC), TEAOH (25 wt% in water, SCRC), and (NH₄)₂HPO₄ (AR, SCRC).

Parent ZSM-5 (denoted as Z5) was synthesized according to a literature procedure ^[30]. Briefly, Al₂(SO₄)₃·18H₂O was used as the aluminum source and dissolved in sulfuric acid solution (0.65 mol/L). Then, the solution was mixed with water glass, tetraethylammonium bromide (TEABr), and seeding gel to obtain a mixture with a molar composition of 230SiO₂:1Al₂O₃: 11.5Na₂O:11.5TEABr:6900H₂O. The seeding get, containing 0.5 wt% SiO₂, was prepared with a molar composition of SiO₂:0.35 tetraethylammonium hydroxide (TPAOH):19.6H₂O at 80 ℃ for 3 d. The mixture was hydrothermally treated at 175 ℃ for 3 d to induce crystallization. Then, the as-prepared powder was recovered by filtration, washed with deionized water several times until the pH of the liquor was neutral, and dried at 100 ℃ overnight. The resulting zeolite Z5 was obtained after calcination at 550 ℃ in atmospheric pressure and ion-exchanged to obtain the H-form of the zeolite.

To prepare the hierarchically porous ZSM-5 containing phosphorus, Z5 was subjected to a one-step treatment at 70 ℃ in an aqueous alkaline solution containing 0.2 mol/L of OH−-based solution. Typically, 1 g of Z5 was uniformly dispersed into a pre-heated alkaline solution containing NaOH, TEPOH (prepared by TEPBr and Ag₂O), and TEAOH with a concentration molar ratio of (TEP⁺ + TEA⁺)/OH− of 0.2 ^[34]. The amount of TEP⁺ was pre-calculated based on the expected P₂O₅ loading of 1.5 wt% on the zeolite. After stirring for 30 min, the mixture was quickly cooled to room temperature and washed until the pH of the supernatant was neutral. After drying overnight at 100 ℃, the hierarchically porous ZSM-5 containing phosphorus was obtained and denoted as Z5-AT@TEP⁺-a, where AT@TEP⁺ refers to the concurrent alkaline treatment (AT) and phosphorus (TEP⁺) modification applied.

For comparison, three reference samples, i.e., Z5-AT (obtained upon alkaline treatment only, i.e., without phosphorus modification), Z5-AT@PO₄³⁻-a (obtained upon alkaline treatment in an inorganic phosphorus-containing solution with concurrent inorganic (NH₄)₂HPO₄ modification), and Z5-ATPO₄³⁻- a (obtained upon alkaline treatment, followed by inorganic (NH₄)₂HPO₄ impregnation), were prepared as follows.

Alkali treatment of Z5 was performed in an aqueous solution of 0.2 mol/L OH−-based solution at 70 ℃. Typically, 1 g of Z5 was uniformly dispersed into a pre-heated alkaline solution containing both NaOH and TEAOH at a TEAOH-to-OH− concentration molar ratio of 0.2. After stirring for 30 min, the mixture was cooled rapidly in an ice-water bath. After thorough washing, drying overnight at 100 ℃, calcination at 550 ℃ for 6 h in atmospheric pressure, and ion-exchange, Z5-AT in H-form was obtained ^[30].

Z5-AT@PO₄³⁻-a was prepared by desilication in a pre-heated alkaline solution of 0.2 mol/L OH−-based solution, which contained NaOH, TEAOH ([TEAOH]/[NaOH] = 0.25) and a known amount of (NH₄)₂HPO₄ (based on the expected P₂O₅ loading of 1.5 wt% on the zeolite ). After 30 min of stirring, the mixture was quickly cooled to room temperature and washed until the pH of the supernatant was neutral. Z5-AT@PO₄³⁻-a was obtained after drying at 120 ℃ for 8 h.

Z5-AT-PO₄³⁻ -a was prepared by subsequent phosphorus impregnation of Z5-AT. Z5-AT was added to a (NH₄)₂HPO₄ solution (the mass percentage of zeolite was 1.5% calculated by P₂O₅). The mixture was stirred at room temperature for 4 h and subsequently heated at 80 ℃ under stirring until the mixture turned to a paste. The paste was dried at 120 ℃ for 8 h to remove residual moisture ^[30].

Activation of the phosphorus element in the prepared zeolites was performed on an FCC catalyst steam-aging equipment (Beijing Wekindu Technology Co. Ltd.). Specifically, Z5-AT@TEP⁺-a, Z5-AT@PO₄³⁻-a, and Z5-AT-PO₄³⁻ -a were activated at 550 ℃ for 3 h in 100% steam (4.4 g H₂O/min). Following calcination and ion-exchange, the phosphorus- containing samples were obtained in H-form and labeled as Z5-AT@TEP⁺, Z5-AT@PO₄³⁻, and Z5-AT-PO₄^3‒, respectively. The H-type zeolites were also steam-aged at 800 ℃ for 17 h in 100% steam (4.4 g H₂O/min), and the aged products were denoted with the suffix “-HT”.

Powder X-ray diffraction (XRD) patterns were collected on a Rigaku Ultima IV powder diffractometer using Cu K_α radiation (λ = 0.154184 nm) over a 2 θ range of 5°-50°; the accelerating voltage and applied current were 35 kV and 25 mA, respectively. The relative crystallinity of the products was determined from the peak area within a 2 θ range of 22.5°-25° using a commercial ZSM-5 sample, as a reference, supplied by Nankai University Catalyst Co., Ltd. Scanning electron microscopy (SEM) was performed on a HITACHI S-4800 scanning electron microscope, operating at an accelerating voltage of 3 kV. High-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM) images were collected on a JEM-2010, operating at 200 kV. The SiO₂/Al₂O₃ ratio and P content in the products were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) on a Thermo IRIS Intrepid II XSP atomic emission spectrometer. N₂ adsorption-desorption isotherms were recorded on a Quantachrome Autosorb-3B instrument at −196 ℃. The mesopore size distribution was determined from the adsorption branch of the isotherms using the Barrett-Joyner-Halenda (BJH) model. ²⁷Al solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded on a VARIAN VNMRS-400WB spectrometer. ³¹P NMR measurements were performed with a 4.0-mm MAS probe on a Bruker Avance III spectrometer. NH₃ temperature-programmed desorption (NH₃-TPD) was conducted on a Tianjin XQ TP5080 automatic adsorption apparatus fitted with a thermal conductivity detector. For the measurements, 100 mg of the H-type samples was pretreated at 550 ℃ for 2 h under helium flow, and NH₃ adsorption was conducted at 100 ℃. After removing the physically adsorbed NH₃, the samples were heated to 550 ℃ at a rate of 10 ℃/min.

The cracking of 1-octene was performed at atmospheric pressure in a microreactor fitted with a quartz tube with an inner diameter of 8 mm. Before the reaction, 50 mg of 20-60 mesh mixture pellets of zeolite and silica gel (Qingdao), prepared by mass-equal grinding, was packed in a fixed-bed reactor and heated at 650 ℃ for 1 h in a stream of N₂. Then, 1-octene was fed with N₂ at a constant temperature of 650 ℃. The weight hourly space velocity of 1-octene was 121 h⁻¹. The products were analyzed by two on-line gas chromatograph instruments, GC-9890A (Shanghai Linghua, HP-5 capillary column, 60 m × 0.32 mm × 0.25 μm) and GC-14B (Shimadzu, DM-plot alumina capillary column, 50 m × 0.25 mm × 10 μm), both equipped with a flame ionization detector. Thermogravimetric (TG) analysis of the spent catalysts was performed on NETZSCH/STA 449 F3; the measurements were conducted in flowing air at a heating rate of 10 ℃/min.

The dealkylation of TIPB was evaluated in a microreactor fitted with a quartz tube of an inner diameter of 6 mm. Prior to the catalytic reaction, 50 mg of 20-60 mesh zeolite ZSM-5 pellets prepared without a binder was pre-treated at 380 ℃ for 60 min in a stream of N₂. Then, while maintaining a constant temperature of 380 ℃, 0.5 μL of substrate was injected into the catalyst bed with N₂ fed from the top of the reactor. The flow of the carrier gas was 80 mL/min. The TIPB cracking products were analyzed by an on-line gas chromatograph instrument (GC-9800A, Shanghai Kechuang; FFAP capillary column, 30 m × 0.25 mm × 0.25 μm), equipped with a flame ionization detector.

XRD analysis was performed to investigate possible structural changes in ZSM-5 that occurred upon alkaline treatment and phosphorus modification. The alkaline-treated samples and the organic and inorganic phosphorus-modified samples displayed diffraction patterns similar to that of the untreated zeolite (Z5) (Fig. 1). The intensity of the peaks of all post-synthesis-treated samples decreased slightly. This decrease could be attributed to the removal of Si from the framework ^[17]. Though the different post-synthesis treatments led to different degrees of structural collapse, the long-range ordering of the microporous network of all the samples were maintained. Furthermore, the relative crystallinity of all samples was higher than 80%. This result indicates that the framework silicon was removed in a controlled fashion with the aid of TEA⁺ cations ^[35].

Fig. 2 presents the N₂ adsorption-desorption isotherms and pore size distributions of the different ZSM-5 samples prepared in the present study. Parent Z5 displayed a typical Type Ⅰ isotherm, which is characteristic of a microporous system. In contrast, Z5-AT and Z5-AT@TEP⁺ exhibited Type Ⅳ isotherms, characterized by a high N₂ uptake at high p/p 0 values. This feature indicated that mesoporosity was efficiently introduced in the zeolite framework during desilication. Table 1 presents the physicochemical properties of all the samples prepared. The external surface area (S _ext) and mesopore volume (V _meso) of Z5-AT increased considerably from 35 cm²/g and 0.07 cm³/g (Z5) to 159 cm²/g and 0.37 cm³/g, respectively. When compared with the S _ext and V _meso of Z5-AT, those of Z5-AT@TEP⁺ increased to 166 cm²/g and 0.37 cm³/g, respectively. Z5-AT@PO₄³⁻ and Z5-AT-PO₄³⁻ displayed similar isotherms and textural properties.

Additionally, Z5-AT, Z5-AT@PO₄³⁻, and Z5-AT-PO₄³⁻ featured comparable pore size distribution profiles. This result indicates that the free inorganic phosphate or organic quaternary phosphonium species in solution had minimal influence on desilication. However, the physicochemical properties of Z5-AT following steam-aging changed considerably, indicating an obvious collapse in the framework, as listed in Table 1. Specifically, S _ext of Z5-AT-HT decreased to 74 cm²/g (from 159 cm²/g for Z5-AT); however, the Type Ⅳ isotherm features were preserved. In contrast, the phosphorus-modified hierarchically porous ZSM-5 could maintained their textural properties even under severe steam-aging. The level of preservation of the S _ext value (calculated by the ratio of S _ext before steam-aging to S _ext after steam-aging, presented in parentheses below) decreased in the order of Z5-AT@TEP⁺-HT (86%) > Z5-AT-PO₄³⁻ -HT (79%) > Z5-AT@PO₄³⁻-HT (51%) > Z5-AT-HT (46%), relative to the S _ext before steam-aging. The phosphorus-containing zeolite sample, Z5-AT@TEP⁺-HT, displayed a lower degree in re-duction in S _ext than the phosphorus-free zeolite Z5-AT-HT. Similarly, the level of retention of V _meso decreased in following order: Z5-AT-PO₄³⁻ -HT (126%) > Z5-AT@TEP⁺-HT (106%) > Z5-AT@PO₄³⁻-HT (79%) > Z5-AT-HT (65%). This trend demonstrates that the phosphorus-modified samples possess higher hydrothermal stability than the phosphorus-free sample. The textural properties of Z5-AT@TEP⁺-HT, especially the mesoporosity, were better preserved than those of Z5-AT-PO₄³⁻ -HT. Thus, higher hydrothermal stability of the mesoporous network was achieved in Z5-AT@TEP⁺-HT than in Z5-AT-PO₄³⁻ -HT. Furthermore, the degree of complexity of the one-step post-synthesis treatment employed is lower when compared with that of previous report ^[11]. Notably, similar textural properties of both Z5-AT@PO₄³⁻-HT and Z5-AT-HT were obtained following steam-aging. As observed in Table 1, no phosphorus species was detected in Z5-AT@PO₄³⁻. This result indicates that almost no inorganic phosphorus species remains after desilication and related subsequent treatments. Thus, the stabilizing effect provided by phosphorus modification was minimal in Z5-AT@PO₄³⁻-HT.

SEM images of the samples subjected to different treatment are presented in Fig. 3. The parent Z5 consisted of uniform crystals of ~2 μm, possessing a typical coffin-like morphology (Fig. 3(a)). No obvious changes in the morphology were observed after the various post-synthesis treatments, indicating that desilication occurred in a controlled fashion in the presence of organic cations ^[34]. Fig. 3(f) shows a HAADF-STEM image of Z5; the bright area highlights a crystalline particle, featuring microporosity only. The presence of extensive intracrystalline mesoporosity formed in Z5-AT@TEP⁺ zeolite could readily be distinguished by the dark spots against the original bright area, outlining the crystalline particle, in Fig. 3(g). The presence of the newly created mesopores in Z5-AT@TEP⁺ observed by HAADF-STEM was consistent with the N₂ adsorption-desorption results. The results of the mesopore size and V _meso (developed upon treatment in a mixed solution of NaOH, TEAOH, and TEPOH) of Z5-AT@TEP⁺ were considerably different from those of Z5 (Fig. 2 and Table 1).

NH₃-TPD measurements were conducted to determine the strength and amount of acid sites on the zeolite samples. As observed from Table 1 and Fig. 4, the NH₃ capacity on Z5-AT and Z5-AT@PO₄³⁻ was slightly larger than that of the parent Z5 because of the lower SiO₂/Al₂O₃ ratio after desilication ^[36]. The NH₃-TPD profile of Z5-AT@TEP⁺ indicated that the introduction of phosphorus species into the zeolite, especially those adsorbed on strong acid sites, reduced the NH₃ capacity. This result confirmed the acid-weakening effect of phosphorus on the zeolite ^[24]. A similar phenomenon was observed for Z5-AT-PO₄³⁻ , as consistent with the results in the literature ^[30]. The total adsorption capacity of NH₃ for all samples decreased significantly after steam-aging at 800 ℃ for 17 h. This phenomenon was attributed to the occurrence of dealumination and related reduction in acid site density on these samples. The level of retention of acid site density of the steamed samples decreased in the order of Z5-AT@TEP⁺-HT (50%) > Z5-AT-PO₄³⁻ -HT (35%) > Z5-AT@PO₄³⁻-HT (18%) > Z5-AT-HT (16%) > Z5-HT (12%). The acid site density in the phosphorus- containing samples, especially Z5-AT@TEP⁺-HT, after steam-aging was better preserved than in the phosphorus-free samples ^[22]. It could be concluded that the acid sites were stabilized because dealumination of the zeolitic framework was inhibited by phosphorus modification ^[24]. The higher level of preservation of acid site density in Z5-AT@TEP⁺-HT when compared with that of Z5-AT-PO₄³⁻ -HT suggests the efficacy of this simple post-synthesis treatment.

The ²⁷Al and ³¹P MAS NMR analysis results presented in Fig. 5 provide information on the coordination state of aluminum and phosphorus in the samples prepared using different treatments. In the ²⁷Al MAS NMR spectra of ZSM-5, several peaks, corresponding to different species, were observed at the following positions: ~55 ppm (tetrahedral aluminum in the zeolite framework), ~0 ppm (typical octahedral aluminum in extra- framework positions), ~36 ppm (intermediate aluminum species, tetrahedral aluminum in a distorted environment probably in the zeolite framework or (SiO) _x Al(OP)_{4− x} sites formed by substitution of some silicon with phosphorus atoms at high temperatures), ~10 ppm (octahedral aluminum in a distorted environment attached to phosphate) and ~ −10 ppm (octahedral aluminum attached to phosphorous) ^{[20, 31, 37-41]}. Only one intense and sharp signal at 57 ppm was observed in the spectrum of Z5-AT, indicating that all the aluminum atoms were tetrahedrally coordinated. Z5-AT-HT contains approximately half extra-framework aluminum (resonance signal observed at 0 ppm) owing to the occurrence of dealumination during steam-aging. Similar spectra were obtained for Z5-AT@PO₄³⁻ before and after steam-aging. In the phosphorus-containing samples, most aluminum maintained their tetrahedral coordination in a phosphorus-interacted environment (resonances at 52 ppm). In the spectra of Z5-AT-PO₄³⁻ and Z5-AT@TEP⁺, minor signals were observed at −10 ppm, indicating the interaction between newly formed octahedral coordinated aluminum and phosphate during hydrothermal activation. After severe steam-aging, an intense signal at 36 ppm was observed in the spectrum of Z5-AT@TEP⁺-HT. In contrast, the spectrum of Z5-AT-PO₄³⁻ -HT displayed a weak signal at 30-58 ppm and an intense peak at 10 ppm. These results reveal that more framework aluminum have been preserved and stabilized during organic phosphorus modification than during inorganic phosphorus modification. This phenomenon can be ascribed to the fact that TEP⁺ can appropriately ion-exchange with Na⁺ ions, which are mainly located near the aluminum atoms to balance the framework charge. Thus, the propensity of interaction between phosphorus and framework aluminum is enhanced.

³¹P MAS NMR spectroscopy was used to further investigate the state of phosphorus in the phosphorus-containing samples. As shown in Fig. 5(b), both the spectra of Z5-AT@PO₄³⁻ and Z5-AT@PO₄³⁻-HT did not feature any signals, attributed to the loss of phosphorus species during preparation of the zeolites. The spectra of the phosphorus-containing samples subjected to hydrothermal activation displayed broad peaks from −7 to −50 ppm and centered at −14 ppm. The signals could be ascribed to either phosphorus in pyrophosphoric acid or terminal groups of polyphosphates not attached to Al (−6 to −8 ppm), middle and terminal groups of polyphosphates either attached or not attached to Al (−12 to −30 ppm), and middle groups of highly condensed polyphosphates attached to Al (−32 to −40 ppm) ^{[22, 31, 42-44]}. Upon subsequent aging, the coordination of phosphorus changed further. For instance, the spectrum of Z5-AT-PO₄³⁻ -HT displayed a narrow and sharp signal centered at −31 ppm, which is related to amorphous aluminum phosphate ^[45]. In contrast, the spectrum of Z5-AT@TEP⁺-HT displayed a broad and intense resonance from 0 to −17 ppm. This result indicates that the phosphorus species is in a lower state of condensation than that in Z5-AT-PO₄³⁻ -HT. Condensation is closely related to the dispersion of phosphorus species on zeolites. The lower degree of phosphorus condensation indicates the presence of enhanced interaction between phosphorus and aluminum on Z5-AT@TEP⁺-HT rather than the occurrence of self-condensation.

Scheme 1 shows a possible stabilization mechanism for hydrothermal activation according to the information gathered on the coordination of Al and P species from the ²⁷Al/³¹P MAS NMR spectra in Fig. 5 and a previously reported mechanism ^[30]. During activation, the phosphorus species in TEP⁺ transform to (poly)phosphates ^[46]. Most of the framework aluminum retain their tetrahedral coordination state (52 ppm), whereas a few tetrahedral aluminum in Z5-AT@TEP⁺ are removed from the framework as octahedral aluminum (0 ppm in the ²⁷Al NMR spectrum of Z5-AT@TEP⁺), and the octahedral aluminum interact with phosphorus (−10 ppm). Consequently, in this process, two possible phosphorus-aluminum structures form as presented in Scheme 1 (structures 1 and 2). These structures include both the framework aluminum and extra- framework aluminum attached to the phosphorus. In the subsequent steam-aging process, most of the framework aluminum attach to the phosphorus in a newly distorted environment (36 ppm in the ²⁷Al NMR spectrum of Z5-AT@TEP⁺-HT). Additionally, a few extra-framework aluminum attach to the phosphorus. Based on this information, we propose that the stabilized structures (structures 3 and 4; broad signal detected ranging from −7 to −50 ppm in the 31NMR spectrum of Z5-AT@TEP⁺-HT) are derived upon changes in the structures 1 and 2 during steam-aging.

The catalytic activity of all steam-aged samples was evaluated toward cracking of 1-octene. The evolution of the catalytic activity of the five steam-aged samples as a function of time on stream is shown in Fig. 6(a). High initial conversions (> 95%) were obtained over all the steam-aged samples. However, the 1-octene conversion over Z5-HT rapidly decreased within 3-15 h of reaction from ~100% to ~75%. In contrast, the rate of conversion over Z5-AT-HT reduced at a slower pace; the reduced conversion rate was attributed to enhanced coke tolerance. Z5-AT@PO₄³⁻-HT displayed a trend similar to that of Z5-AT-HT; the lack of catalytic performance over that sample was attributed to the absence of effective phosphorus modification. The conversion of 1-octene remained mostly steady (i.e., ~98%) within 10 h of reaction over Z5-AT@TEP⁺-HT. A similar trend was observed for Z5-AT-PO₄³⁻ -HT, which displayed a conversion of ~97%. When the reaction period was extended further to 20 h, a slight decrease in conversion occurred over Z5-AT-PO₄³⁻ -HT (87%) and Z5-AT@TEP⁺-HT (91%). Based on these results, it can be deduced that the presence of hierarchical porosity and stable acid sites in the zeolite catalyst can realize sustainably high conversions of reduced spatial- constrained molecules in an acid-catalyzed reaction.

Coke formation within micropores causes more significant deactivation than that on the external surface owing to pore channel blockage and active site coverage ^[13]. To further investigate the catalytic performance of the prepared catalyst samples, the coke content on the used samples was analyzed by TG (Fig. 6(b)). The highest coke content was detected on the used Z5-AT@TEP⁺-HT. This result indicated that the relatively higher mesoporosity and acid density of Z5-AT@TEP⁺-HT led to higher coke tolerance and longer catalyst lifetime when compared with Z5-HT and Z5-AT-HT ^{[47, 48]}.

The catalytic performance of TIPB cracking is illustrated in Fig. 6(c). TIPB cracking conversion over the differently treated ZSM-5 samples decreased in the order of Z5-AT@TEP⁺-HT > Z5-AT-PO₄³⁻ -HT > Z5-AT-HT > Z5-AT@PO₄³⁻-HT > Z5-HT. This trend demonstrated that diffusion of TIPB was accelerated by mesopore introduction. Furthermore, higher product selectivity of terminal cracking products benzene (BZ) was observed over Z5-AT@TEP⁺-HT, indicating that the stabilized acid sites favored the typical acid-catalyzed reactions ^[49]. Furthermore, the catalytic testing results reveal that hierarchical ZSM-5 containing phosphorus performs better than the parent ZSM-5 and hierarchically porous phosphorus-free ZSM-5. Additionally, comparison of the zeolites stabilized by phosphorus reveals that the zeolite prepared via a one-step method shows superior catalytic performance than phosphorus-modified zeolites prepared by conventional methods.

A convenient, one-step post-synthesis treatment to introduce mesopores and phosphorus moieties in high-silica ZSM-5 was successfully developed by controlled desilication under the protection of TEA⁺. Phosphorus was integrated near framework aluminum in the form of quaternary phosphonium. The characterization studies showed that porosity optimization and structure stabilization were efficiently realized by the one-step method. Consequently, Z5-AT@TEP⁺ displayed superior catalytic performance than traditionally post-synthesis-treated hierarchically porous ZSM-5 modified with phosphorus. Higher conversions in TIPB dealkylation and remarkably prolonged catalytic performance of Z5-AT@TEP⁺-HT in 1-octene cracking were realized. Such excellent performance was ascribed to both the optimized porosity and the reduced propensity of structure collapse and acid sites poisoning.