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Review

Morphology and Microstructural Optimization of Zeolite Crystals Utilizing Polymer Growth Modifiers for Enhanced Catalytic Application

by
Junling Zhan
1,
Chongyao Bi
1,
Xiaohui Du
2,
Tao Liu
2,* and
Mingjun Jia
1,*
1
Key Laboratory of Surface and Interface Chemistry of Jilin Province, College of Chemistry, Jilin University, Changchun 130012, China
2
Lanzhou Petrochemical Research Center, Petrochemical Research Institute, PetroChina, Lanzhou 730060, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(6), 375; https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060375
Submission received: 30 April 2024 / Revised: 7 June 2024 / Accepted: 7 June 2024 / Published: 12 June 2024
(This article belongs to the Special Issue Surface Microstructure Design for Advanced Catalysts)
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Abstract

:
Rationally controlling the morphology and microstructure of the zeolite crystals could play a significant role in optimizing their physicochemical properties and catalytic performances for application in various zeolite-based heterogeneous catalysis processes. Among different controlling strategies, the utilization of zeolite growth modifiers (ZGMs), which are molecules capable of altering the anisotropic rates of crystal growth, is becoming a promising approach to modulate the morphology and microstructural characteristics of zeolite crystals. In this mini-review, we attempt to provide an organized overview of the recent progress in the usage of several easily available polymer-based growth modifiers in the synthesis of some commonly used microporous zeolites and to reveal their roles in controlling the morphology and various physicochemical properties of zeolite crystals during hydrothermal synthesis processes. This review is expected to provide some guidance for deeply understanding the modulation mechanisms of polymer-based zeolite growth modifiers and for appropriately utilizing such a modulation strategy to achieve precise control of the morphology and microstructure of zeolite crystals that display optimal performance in the target catalytic reactions.

Graphical Abstract

1. Introduction

Zeolites have received considerable attention for decades, since they are capable of acting as high-performance adsorbents and catalysts in the petrochemical, oil refining, coal chemical, and fine chemical industries [1,2,3]. The uniform micropores, large specific surface area, and adjustable acidity and redox properties of zeolite-based heterogeneous catalysts endow them with superb activity and shape selectivity for a variety of catalytic reactions [4,5]. It is well known that the catalytic properties of zeolite catalysts depend on not only their pore openings and cage/intersection cavities but also their various physicochemical properties, including morphology, crystal size, phase purity, composition, and acidity/redox properties [6,7,8]. Through the optimization of synthesis methods and parameters, these physicochemical properties could be modulated to a certain extent by adjusting the morphology and microstructural characteristics of the zeolite crystals, making them well fit for the requirement of a specific catalytic reaction [9,10,11,12,13,14]. Among the various modulation strategies, the introduction of so-called “zeolite growth modifiers” (ZGMs) has shown great advantages in precisely modulating of the morphology and microstructure of various zeolite crystals, with remarkable effects in terms of enhancing their catalytic activity, selectivity, and/or stability for a broad range of industrially important catalytic processes [10,15,16,17,18,19,20].
In contrast to the widely used mesoporous templates or organic structure-directing agents, ZGMs commonly refer to molecules (or macromolecules) that can interact either with the specific facets of zeolite crystals or with amorphous precursors to adjust the kinetics of nucleation and growth (inhibiting or promoting), thereby changing the crystal morphology and/or microstructure of the zeolites [21,22,23,24,25,26,27]. Such a recognition is rooted in the understanding of natural and synthetic crystallization processes. For example, some proteins or other biomolecules often act as modifiers of crystal nucleation and growth during the formation processes of shell and bone [28,29,30,31,32,33,34]. In general, ZGMs possess two moieties, namely a “binder” that interacts with the surface sites of zeolite crystals and a “perturber” that sterically hinders the attachment of the zeolite growth units [28]. Effective ZGMs may closely mimic crystal surface features and orient in solute vacancies by hydrogen-bond, van der Waals, or electrostatic interactions [27,35]. To date, a number of compounds have been used as ZGMs for zeolite synthesis, including inorganic salts/oxides (e.g., GeO2, Na2SO4, and NaCl) [20,36,37,38], small organic molecules (e.g., lactams, amino acids, and alcohols) [18,21,39,40], and polymer compounds [41,42,43,44]. Among them, some low-cost and easily available polymers, such as polyethylene glycol (PEG), polyethylenimine (PEIM), poly-diallyldimethylammonium chloride (PDDA), and polyacrylamide (PAM), have shown some nice features as effective ZGMs for the synthesis of various important zeolites. In comparison with other kinds of crystal growth modifiers, the main advantages of polymer-based ZGMs are their relatively high potency and better recyclability, making them commonly effective at low concentrations and easily removed during post-synthesis rinsing for recycling [16,43]. Previous studies of biominerals have revealed that polymer-based ZGMs are more effective growth inhibitors than smaller molecules, possibly related to their proximal binding moieties, which can block layer nucleation and decrease step propagation through step-pinning mechanisms [6,21,28].
In this review, we mainly highlight the recent progress in the usage of several kinds of polymer-based ZGMs (as listed in Table 1), including PEG, PEIM, PDDA, PAM, etc., in the synthesis of microporous zeolites, as well as the enhanced catalytic application of the resultant zeolite crystals in various catalysis processes. In addition to presenting a variety of characterization results, crystallization kinetic experiments, and theoretical analyses performed by different research groups, we also try to summarize the main modulation mechanisms of the polymer-based ZGMs in changing the morphology and microstructure of zeolite crystals during hydrothermal synthesis processes. Finally, brief perspectives are provided, hopefully suggesting future directions and opportunities with respect to the polymer-based ZGM strategy in contributing to the effective synthesis of zeolite crystals with desirable morphology and microstructural characteristics that would support the development of green chemical industries.

2. Usage of Polymer-Based ZGMs for the Synthesis and Catalytic Application of Zeolite Crystals

2.1. Polyethylene Glycols (PEGs)

Polyethylene glycols (PEGs) are a family of linear polymers formed by a base-catalyzed condensation reaction with repeating ethylene oxide units being added to ethylene [45,46]. Due mainly to the nice features like high flexibility and a hydrophilic nature, PEGs have been widely used in various pharmaceutical preparations and particle modifications [47,48,49,50]. A few literature works demonstrated that PEGs can also be used as crystal growth inhibitors/modifiers in zeolite synthesis, since they can be adsorbed on the surface of the zeolite precursors, whose space barrier plays an important role in affecting the nucleation and growth of zeolite crystals [51,52].
In an early work reported by Hosokawa and coworkers, they found that nanosized A-type zeolites with primary particle sizes of 30–40 nm could be obtained by using PEG 600 as a crystallization inhibitor. The resultant zeolites exhibited higher cationic exchange rates than commercially available, microsized A-type zeolites, mainly due to the decrease in the primary particle size of the zeolites [51]. In 2015, Muraza and coauthors reported that elongated cross–hexagonal crystals of ZSM-12 zeolites with an average size of 1200 nm composed of many small primary particles (around 150 nm) could be synthesized by using PEG and polyoxyethylene Brij-35 surfactant as crystal growth modifiers. They proposed that the usage of the two modifiers could suppress the nucleation of impurities, in addition to reducing the synthesis time due to the promotion of the crystallization rate of the zeolite [52].
In 2021, Dan and coworkers synthesized a unique cubic structure, SAPO-34 zeolites with intact hierarchical crystals (around 3–7 μm), by introducing a moderate amount of PEG as a crystal growth modifier [42]. By combining a variety of characterization results, the authors tried to investigate the modulation mechanism of PEG-mediated SAPO-34 synthesis. They proposed that PEG could act as a steric barrier in the growth stage of SAPO-34 crystals, which can induce the rapid formation of large, intact crystals, in addition to small crystals. During the hydrothermal synthesis process, chain-like PEG molecules could adhere to the crystal surface with increases in temperature. In this case, the resultant bent micelle could wrap a number of crystal nuclei, offering a place for the growth of zeolite crystals. Finally, the crystal nuclei of zeolite with high surface energy could easily aggregate and generate large, intact SAPO-34 crystals (Figure 1a). As revealed by the characterization results of 29Si and 27A MAS NMR, the condition-optimized catalyst named S-PEG(0.012)-F, which was derived from the addition of a suitable amount of PEG modifier, exhibits higher framework Si content (78%) and lower disordered Si content (22%) than conventional SAPO-34 zeolites (S-F) (Figure 1b,c). After high-temperature hydrothermal aging, the resultant S-PEG(0.012)-HT sample loses just 3.3% of the framework Si(1-4OAl) structure, which is much lower than that of S-HT sample (27.5%), indicating the enhanced structural stability of S-PEG(0.012)-HT (Figure 1d).
After supporting with Cu, the authors found that the resultant Cu/SAPO-34 catalyst (referred to as CS) showed enhanced hydrothermal stability and durability for the NH3-assisted selective catalytic reduction of nitrogen oxides. In comparison with the conventional Cu-based SAPO-34 zeolites (CS-F), the CS-PEG(0.012)-F catalyst could convert NOx into N2 more efficiently in a wide temperature region after hydrothermal aging treatments at 70 °C for 10 h or at 800 °C for 12 h (Figure 2a,b,d). The resulting catalysts of CS-PEG(0.012)-F/HT also display higher reaction rates and lower Ea values than CS-F/HT according to Arrhenius plots (Figure 1c). The robust hydrothermal stability of Cu/SAPO-34 is better suited to catalytic applications under harsh reaction conditions. The authors believed that the usage of PEG modifier plays a vital role in constructing more large, intact zeolite crystals of SAPO-34 with fewer structural defects. The feature of fewer defects may weaken the opportunity for H2O attack during hydrothermal treatment and could also help Cu ions distribute into the ion-exchange sites more uniformly, thus generating catalytically active sties with excellent stability and redox properties. These advantageous redox sites ultimately result in the formation of a highly stable and efficient Cu/SAPO-34 catalyst for NOx removal from a diesel vehicle.
By using PEG as a growth inhibitor, Li and coauthors reported that a unique, plate-like SAPO-34 zeolite with a smaller particle size (0.9–1.2 μm) and improved MTO (methanol-to-olefins) performance could be hydrothermally synthesized in the presence of a triethylamine template via two-step crystallization of a concentrated gel system (Figure 3a) [53]. They found that PEG may play different roles in the formation of SAPO-34 crystals when different synthesis/crystallization conditions are adopted. In the normal gel system, PEG acted as a soft template to fabricate mesopores, while in the concentrated gel system, PEG mainly functionalized as a growth inhibitor, which can considerably reduce the crystal size of SAPO-34 zeolites. They also found that the two-step crystallization method makes the particle size of SAPO-34 more uniform in comparison with the one-step crystallization method.
Based on the characterization results of 27Al, 29Si, and 31P MAS NMR, they proposed that the crystallization processes of SAPO-34 are quite different in the two gel systems. In the normal system, silicon species are activated in the crystal center upon heating; then, aluminum and phosphorous species gradually polymerize to produce the zeolite crystals. In contrast, in the concentrated gel systems, the aluminophosphate framework is constructed first from aluminum and phosphorus species; then, silicon species gradually enter the zeolitic framework to substitute aluminum or phosphorus species. Compared with conventional SAPO-34 (N1), hierarchical SAPO-34 (NG1), and other reference samples, the resulting plate SAPO-34 (CG2) shows a much longer lifetime (326 min) and 80% olefin (ethylene and propylene) selectivity for the MTO catalysis reaction (Figure 3b). The excellent catalytic performance of the CG2 sample should be mainly attributed to its distinct features, such as its plate-like morphology, smaller particle size, hierarchical porosity, and lower Si content and acid amount, which are mainly derived from the usage of a PEG modifier in the concentrated gel synthesis system.
Very recently, Huang et. al. reported that hierarchical β-zeolite nanocrystal aggregates with intra-mesopores and inter-mesopores could be fabricated by using PEG 2000 and cetyltrimethylammonium bromide (CTAB) as dual templates in the presence of tetraethylammonium hydroxide (TEAOH) [54]. They found that the morphology and porosity of the resulting β-zeolites could be adjusted by tuning the amount of PEG 2000. They proposed that CTAB and PEG 2000 may work synergistically to tailor the particle size of aggregates and the nanocrystal size of β-zeolite crystals. During the hydrothermal crystallization process, the dissolved Si and Al species first undergo co-condensation to generate amorphous TEA+-aluminosilicates composites through interaction with the TEAOH structure-directing agent. After that, the β-zeolite nuclei are formed by the so-called solid-state reorganization of the amorphous composites. In the case without PEG 2000, the CTAB molecules could be transformed into ordered micelles, which could act as a mesoporous template for the formation of abundant intracrystalline mesopores. CTAB cations could form a hydrophobic layer around the zeolite nuclei, which can inhibit the transport of nutrients and the further growth of zeolite crystals, leading to the generation of aggregates with enriched intercrystalline mesopores. When PEG 2000 is added into the initial synthesis system, it may act as a swelling agent to accelerate nucleation, resulting in the formation of small crystals. The surrounding PEG molecules can also act as crystal growth inhibitors to decrease the size of nanocrystals by attaching to the surface of the zeolite crystals and repel zeolite crystals from contact with each other. In addition, the authors suggested that some PEG 2000 and CTAB molecules might be encapsulated inside of the zeolite nanocrystals, thus helping to form intracrystalline mesopores (Figure 4a).
The obtained β-zeolite nanocrystal aggregates (β-5CxP) showed outstanding catalytic activity and stability under the harsh reaction conditions for the alkylation of benzene and cyclohexene. As shown in Figure 4b, the lifetime of the optimized β-zeolite aggregates (β-5C4P) could be extended by 4.3 times in comparison with that of the commercial nanosized β-zeolite (β-Nano). The authors proposed that the additional PEGs play the role of crystal growth inhibitors that can decrease the size of nanocrystals and, therefore, may effectively increase the accessible Brønsted acid sites and enhance the diffusion properties of the resultant β-zeolite nanocrystal aggregates. In addition, the resulting β-5C4P also exhibited extraordinary regeneration performance, which is a crucial factor for industrial applications with desirable stability (Figure 4c).

2.2. Polyamines Modifiers

Several polyamine compounds have also been employed as ZGMs for the synthesis of various zeolite crystals, including PEIM (polyethylenimine), PDDA (polydiallyldimethylammonium chloride), and PAM (polyacrylamide). All of them are commonly characterized by high hydrothermal stability (no decomposition under hydrothermal conditions), good water solubility (compatibility with zeolite gels), and high charge density (with either positive or negative charge). By rationally utilizing these polyamine-based ZGMs, some industrially important zeolites with desirable morphology, appropriate crystal size, and enhanced catalytic performance, like SSZ-13, ZSM-5, LTL, ZSM-23, and TS-1 have been synthesized successfully, demonstrating their great potential in practical applications [21,41,43,55,56,57].

2.2.1. Polyethyleneimine (PEIM)

Polyethyleneimine (PEIM) has been widely used in the fields of CO2 adsorption, water treatment, and wet-end chemistry due to its polycationic character [41,58,59]. In 2015, Rimer and co-workers reported that PEIM may also act as an effective growth modifier/inhibitor in zeolite synthesis [41]. They found that the morphology and particle size of SSZ-13 could be adjusted to a certain extent when different amounts of PEIM were introduced into the synthesis gels. The introduction of a small amount of PEIM (1.0 wt%) in the synthesis system could result in about 1.0 μm crystals and generate more monodisperse crystals with smoother surfaces relative to the sample obtained in the absence of a modifier (Figure 5a). When the PEIM concentration reaches 1.6 wt%, the particle size of SSZ-13 crystals is apparently reduced 10-fold to 133 ± 30 nm (Figure 5b). Further increasing the PEIM concentration to 3.2 wt% could result in the formation of crystal polymorphs, indicating that there is an upper limit of PEIM concentration for the preparation of growth solutions. Under similar operational conditions, the authors found that the use of two additional modifiers with smaller molecular weights, like 1,2-hexanediol (D61,2) and cetyltrimethylammonium bromide (CTAB), can also reduce the particle sizes of SSZ-13 to a certain extent (Figure 5c,d). However, it was found that the addition of D61.2 modifier could not fully inhibit the formation of SSZ-13 crystals with larger particle sizes, possibly due to its smaller molecular size relative to the polymer modifier. The usage of CTAB modifier resulted in the formation of more aggregated SSZ-13 crystals with particle sizes around 192 ± 26 nm. One disadvantage of CTAB is that it has a stronger binding affinity to SSZ-13 crystals, which may bring difficulty in removing the residual CTAB from the zeolite samples. These results suggest that PEIM is a more efficient growth inhibitor of SSZ-13 crystallization than small-molecule modifiers. By combining a variety of characterization results, the authors proposed that PEIM may function as a colloidal stabilizer to inhibit crystal growth by adsorbing on the surface of the crystal/precursor. Under the adopted synthesis conditions, PEIM is charge-neutral and, therefore, may have relatively weak van der Waals or hydrophobic interactions with the precursors/crystals of SSZ-13, which can lead to a decrease in the kinetic rate of crystal growth and ultimately alter the morphology and particle size of SSZ-13 zeolite crystals.
In a subsequent work carried out by the same group [44], it was reported that the crystallization time of SSZ-13 crystals was reduced by 2-fold when a lower concentration (0.1 wt%) of PEIM was introduced. It was further demonstrated that PEIM is a potent modifier and can exert an effective modulation effect at a very low concentration. The above results clearly show that it is possible to considerably shorten the crystallization time and tailor the textural properties of zeolite SSZ-13 by judiciously controlling the concentration of PEIM, implying its great potential for industrial application.

2.2.2. Polydiallyldimethylammonium Chloride (PDDA)

As a commercially available and low-cost cationic polymer, polydiallyldimethylammonium chloride (PDDA) has been widely used in many industrial fields, such as sewage treatment, papermaking, metal electrodeposition, etc. [60,61,62,63]. PDDA has also received considerable attention in the field of zeolite synthesis, since it may act as an effective polymer-based ZGM in modulating the morphology and microstructure of various zeolite crystals [41,57,64].
In 2013, Rimer’s group reported that the rod-like LTL crystals with high aspect ratios could be synthesized by using PDDA as a growth modifier [21]. The formation of rod-like LTL crystals is mainly attributed to the fact that the electrostatic interactions contribute to the adsorption of positively charged quaternary amines on negatively charged LTL crystal surfaces. During the hydrothermal synthesis process, PDDA may preferentially bind to the [100] surface, thus shifting the dominant growth rate along the axial [001] direction and ultimately leading to the formation of LTL crystals with high aspect ratios (Figure 6c,f). In contrast, organic small compounds (like butylamine) added into the LTL solutions can preferentially bind to the basal [001] surface, thereby generating thin platelets with short diffusion paths along the c-axis [100] plane (Figure 6b,e). The authors also found that increasing PDDA concentrations may result in a monotonic change in aspect ratio, whereas the aspect ratio reaches a plateau at a threshold PDDA weight percent (e.g., 0.2 wt%). They believe that the rod-like LTL crystals with high aspect ratios may be useful in the future for the rational design of improved zeolite catalysts and other applications in biomedical or photonic devices.
In a later work, the same group also employed this approach to the synthesis of cube-like SSZ-13 crystals with large particle sizes of 23 ± 8 μm (Figure 7a), which is an order of magnitude larger than that of reference samples [41]. The resultant SSZ-13 crystals are faceted cubes with [100], [010], and [001] crystallographic faces (Figure 7b). The high-resolution AFM image reveals the presence of hillocks with a step height of ca.1.2 nm, which is approximately equal to the unit cell dimension of the CHA framework (Figure 7c). The terraced surfaces are features of crystals that grow via a classical two-dimension layer-by-layer mechanism. The enlarged SEM images on the corners of the cubes also demonstrate the existence of distinct facets on the corners of the cubes with macroscopically rough surfaces (Figure 7d). Further studies revealed that SSZ-13 grows by two concerted mechanisms, namely nonclassical growth involving the attachment of amorphous particles and classical layer-by-layer growth via the incorporation of molecules in advancing steps on the crystal surface. Their works demonstrated that both the morphology and crystal size of SSZ-13 can be selectively tailored in a wide range through the rational use of a PDDA modifier, which may provide more possibilities to establish quantitative structure–performance relationships for commercially relevant applications.
Recently, by studying the effects of several kinds of polyquaternary amines on the nucleation of SSZ-13 zeolites, Rimer and coworkers found that PDDA has the most pronounced impact on the kinetics of SSZ-13 formation, leading to a 4-fold reduction in crystallization time [44]. The dynamic light scattering (DLS) characterization results demonstrated that PDDA could accelerate the aggregation of precursors at a critical concentration (Figure 8a). In addition, oblique illumination microscopy (OIM) studies (Figure 8b) revealed that the addition of the optimal concentration of PDDA to the synthesis system could facilitate the assembly of soluble silicates into denser phases. The authors proposed that the optimal scenario at intermediate PDDA concentrations may lead to precursor clustering, akin to bridging flocculation. This is a characteristic of polymer–colloid interaction at low-to-moderate polymer concentrations. The precursor particles within these clusters could maintain a high level surface area contact with the solution, thus creating confined regions of entrained supernatant solution. These confined regions might speed up the nucleation via pathways that seem to include more facile mass transfer and exchange of aluminosilicate species among solid and solution phases, thus creating a localized high concentration of soluble species where precipitation can be accelerated by the introduced PDDA (Figure 8d).
In another work, Rimer and coauthors demonstrated that the crystallization rates of MOR and MFI zeolites could also be considerably improved when an optimal PDDA-modifier was introduced into the synthesis system [64]. By analyzing solid samples extracted at different stages, they proposed that the addition of PDDA could inhibit the evolution of amorphous particles into worm-like particles and induce the aggregation of precursors with concomitant reductions in crystallization time. In this case, a working hypothesis on the role of PDDA in the synthesis of MOR and ZSM-5 was also put forward that is similar to the previous explanation of PDDA-modulated SSZ-13 [44]. They posited that the interstitial regions between the aggregated precursors may enhance the crystallization rates, coupled with the existence of PDDA, which can enhance the nucleation rate.
By using PDDA as the growth inhibitor and secondary porogen, Tontisirn and coworkers hydrothermally synthesized nanosized tubular ZSM-23 crystals with hierarchical structures under a rotational condition (Figure 9b) [57]. A plausible mechanism scheme was proposed as follows. During the initial crystallization stage, the formed negatively charged aluminosilicate zeolitic subunits can be attracted by the cationic polymers PDDA with positive charges. The rapid migration of PDDA to the zeolitic subunits may bring a repulsion force acting on the zeolitic subunits. These fragments then grow to the nanosized tubular crystals along the [001] direction. After that, the nanosized tubular crystals grow further around the mesoscale PDDA to form a hierarchical structure with irregular mesopores and macropores. The pore size can also be altered by the flexible chains of PDDA and its free movement in the gel under the rotational synthesis condition (Figure 9a).
In comparison to the conventional HZSM-23-supported Pt catalyst, the PDDA-modifier derived Pt/HZSM-23 exhibits improved maximum isomer yield (54% vs. 37%), higher n-heptane conversion (79% vs. 69%), and lower cracked product yield (25% vs. 31.2%) in the hydroisomerization of n-heptane (Figure 9d,e). The excellent catalytic performance of the PDDA-derived Pt/HZSM-23 catalyst is mainly attributed to its special nanosized tubular morphology with preferred growth along the [001] direction and hierarchical pore structure, which can provide a higher number of pore mouths with fewer constraints on the diffusion of the reactants.

2.2.3. Polyacrylamide (PAM)

Polyacrylamide (PAM) is a water-soluble polymer with a three-dimensional crosslinked network structure and has been applied in wastewater treatment for its flocculation among suspended particles [65,66,67]. A few recent works showed that PAM could also be used as a growth modifier or inhibitor in zeolite synthesis, mainly due to the fact that the large numbers of amide groups existing in PAM may form strong interactions with the zeolitic species (such as silicon or titanium species) in the synthesis gels [43,55,56].
In 2019, Lin and co-workers reported that cuplike ZSM-5 nanocrystal agglomerates could be obtained by self-assembly of a preformed MFI precursor and polyacrylamide (PAM) through prolonged high-temperature (448 K) crystallization [55]. Cuplike agglomerates with a thin-wall structure (around 200 nm thickness) are composed of monolayer ZSM-5 nanocrystals. The introduced PAM plays very important roles in nucleation promotion, crystal growth restriction, and morphological evolution during the prolonged high-temperature crystallization process. The authors proposed that the building units in the center nanocrystals continuously dissociate, migrate, rearrange, and assemble onto the nanocrystals close to the external surface of the preformed agglomerates, thereby producing shell-like structural intermediates. With further prolonging of the hydrothermal treatment time, the “shell” collapsed randomly, leading to the formation of a cuplike morphology (Figure 10a).
Based on the acid characterization results, it was found that the acidity of ZSM-5 zeolites is governed by synthesis parameters such as crystallization temperature and time (Figure 10b). Under the same crystallization temperature (448 K), the strong acid ratio of the ZSM-5 sample (S2) crystallized for four days is much higher than that of the ZSM-5 sample crystallized for two days (S12). The catalytic properties of the ZSM-5 nanocrystal agglomerates were investigated for the cracking reaction of low-density polyethylene (LDPE). As shown in the differential scanning calorimetry (DSC) curves of various samples (Figure 10c), the S2 sample shows enhanced catalytic activity, and the Td-max (maximum cracking rate) is as low as 661 K, which is much lower than that of other samples. The excellent catalytic performance can be mainly attributed to the abundantly accessible active sites and the strong acidity, which are benefits of the special cuplike particle morphology of the resultant ZSM-5 nanocrystal agglomerates.
Through the introduction of crosslinked PAM hydrogels (C−PAM) into the synthesis gel, Han and co-workers obtained c-axis-oriented HZSM-5 zeolites [56]. The growth orientation and acidity of ZMS-5 crystals can be controlled to some extent by tuning the amount of PAM. With increasing C-PAM content, the length along the a-axis and b-axis decreases obviously, accompanied by a considerable decrease in length along the c-axis. By analyzing the FT-IR spectra results in different periods of zeolite synthesis, the presence of an interaction between the enriched polar groups (−CONH2) in the C-PAM hydrogel and the initial synthesis gel could be verified. The authors proposed that the C-PAM molecules are more easily dispersed and absorbed on the [010] and [100] facets (rather than the [101] facet) owing to the properties of lower energy and more stability, leading to oriented growth during the crystallization process (Figure 11a). Compared with the conventional ZSM-5 and other samples, the resulting c-axis-oriented ZSM-5 modified with 2% C-PAM (Z-2) exhibits the highest catalytic activity and light olefin selectivity for the MTO (methanol-to-olefin) reaction. In addition, Z-2 has a lifetime of more than 23 h under the tested conditions, which is much longer than that of the other ZSM-5 catalysts (Figure 11b). The enhanced catalytic performance is mainly attributed to Z-2 sample having the highest medium and strong acid amounts and the shortest b-axis length.
Recently, Yu and coauthors reported that the crystallization pathway of zeolite TS-1 could be switched from a classical to a non-classical mechanism when PAM was used as growth modifier in a two-step hydrothermal crystallization process [43]. The introduction of PAM may considerably accelerate the nucleation rate and result in the formation of TS-1 zeolite with enriched active Ti sites and intragranular mesopores. By combining a variety of characterization results, the authors proposed that the acyl groups on PAM can form hydrogen bonds with the Si/Ti species, which can quicken the achievement of the critical aggregation concentration in the network region constructed by PAM chain entanglement, then facilitate the formation of worm-like particles (WLPs) through self-assembly of the amorphous nanoparticles. The generated WLPs may provide more sites for nucleation and promote the evolution of ordered structural fragments, ultimately leading to the formation of TS-1 crystals through the non-classical dominated crystallization pathway. In addition, a large number of Ti species could be stabilized in WLPs owing to coordination with the amino groups of PAM, and they may participate in the formation of crystal nuclei and are easily transformed into the framework Ti species (Figure 12a).
The PAM-regulated TS-1 catalyst named P-10-80-120 exhibits enhanced catalytic activity for oxidative desulfurization of dibenzothiophene (DBT) with tert-butyl hydroperoxide (TBHP) that is much better than that of the conventional microporous TS-1 zeolite (Figure 12b). The excellent catalytic performance of P-10-80-120 can be mainly attributed to the greatly increased content of active TiO6 species (mononuclear hexacoordinated titanium). P-10-80-120 also displays the highest catalytic activity in the epoxidation of 1-hexene (Figure 12c), showing 26.5% conversion under the test conditions, which is higher than that of TS-1 zeolite without the addition of PAM (P-0-80-120). In addition, the spent TS-1 catalyst also showed excellent cycling stability in the above two reactions after high-temperature calcination, which demonstrates the great potential of PAM-based ZGMs in the synthesis and catalytic application of heteroatom-containing zeolite catalysts.

2.3. Other Modifiers

2.3.1. Fatty Alcohol Polyoxyethylene Ether Ammonium Sulfate (AESA)

As a commercially available anionic surfactant, fatty alcohol polyoxyethylene ether ammonium sulfate (AESA) has an amphiphilic structure, which possesses a negatively charged sulfate head and a hydrophobic carbohydrate chain on the other end [68]. Recently, Hao and coauthors reported that AESA could be used as a ZGM for the synthesis of zeolite β crystals with improved yield and increased framework Si/Al ratios [68]. During the crystallization process, the AESA anion affected the condensation reaction of two Si(OH)4 molecules by hydrogen bonding interactions, thus facilitating the formation of Si-O-Si bonds and leading to the improvement of the efficiency of Si incorporation. In addition, by optimizing the usage amount of AESA, the enrichment of polymorph-A was achieved, with the proportion of polymorph-A as high as 77.9% (Figure 13). These results suggest that AESA might act as a crystal growth modifier, which can bind selectively to certain faces of zeolite β during crystallization and change the anisotropic rates of surface step growth. The authors believe that the resultant zeolite β with a chiral structure should have both shape selectivity and enantioselectivity, which is of significance in enantioselective catalysis and separation.

2.3.2. Polydopamine (PDA)

Polydopamine (PDA) is a mussel-inspired amphiphilic polymer containing outer exposed amine and phenolic hydroxyl groups [69]. The enriched functional groups and good biocompatibility and degradability of PDA make it an ideal polymer material for applications in various fields [70,71]. Recently, by studying the role of different types of ZGMs, Li and coworkers found that phenolic hydroxyl is an effective functional group to guide the growth of ZSM-48 zeolites [72]. Through the addition of PDA to the synthesis gels, hollow ZSM-48 aggregates consisting of needle-like nanocrystals (above 10 μm) were obtained with a prolonged crystallization time. Compared with other phenolic ZGMs such as phenol, pyrocatechol, and tyramine, the PDA-modified ZSM-48 shows improved dispersion and properly suppressed acidity. In the presence of phenolic ZGM, the phenolic hydroxyl groups may induce a firmer hydrogen bond interaction with the Si–OH group due to the formation of the p-π conjugation of the phenolic structure. Such an interaction may separate the growth of crystal nuclei and ultimately lead to the formation of highly dispersed crystals (Figure 14a). In addition, the PDA modifier could decrease the pH of the synthesis gel due to the acidity of phenolic hydroxyl group, which may cause fewer alumina tetrahedrons to be incorporated into the ZSM-48 crystals, resulting in fewer acid sites on the zeolite.
After loading Pt nanoparticles, the resultant Pt/ZSM-48-DA exhibits excellent catalytic performance in the hydroisomerization of n-hexadecane. In comparison with the conventional Pt/ZSM-48 catalyst, the catalyst derived from the PDA modifier obtained higher isomer selectivity (97% vs. 92%) and isomer yield (89% vs. 80%) when the n-hexadecane conversion was lower than 80% (Figure 14b–e). The authors believe that abundant voids among ZSM-48-DA crystals and suppressed acidity could reduce the possibility of overcontact of reactants and products with acid sites, thus reducing cracking and improving the isomer selectivity for hydroisomerization of n-hexadecane.

2.3.3. Multifunctional Polymer (PK3)

By using a polymer named poly(1,4-cyclohexylacetone dimethylene ketal)–poly(1,5-pentylacetone dimethylene ketal (denoted as PK3) as a multifunctional template, Yue and coauthors synthesized aluminum-rich hierarchical ZSM-5 zeolite (Si/Al = 8) with abundant mesopores, comprising oriented and assembled nanocrystals (Figure 15a–d) [73]. The authors proposed that the PK3 polymer could serve as a template for the construction of the microporous/mesoporous structure and also act as a crystal growth modifier to construct a single-crystalline zeolite. The fast Fourier transform diffraction pattern shows that the resultant zeolite particle has clear and regular diffraction spots, suggesting the [010] orientation of the nanocrystal (Figure 15e,f). The fact that the nanocrystals are assembled in the same orientation is an indication that PK3 could act as a modifier to interact with the specific facets of ZSM-5 zeolite crystals. To verify this point, the authors probed the interaction of PK3 with different MFI surfaces by means of USMD (umbrella sampling molecular dynamics) simulations to calculate the adsorption free-energy profile (F(z)). The adsorption free energy of PK3 for the [010] facet is the lowest, suggesting that PK3 preferentially binds to the MFI [010] facet through the hydrogen bond between alcohols on PK3 and the exposed Si-OH or Al-OH groups presented on the [010] facet (Figure 15g). In this case, crystal growth in the [010] direction is considerably inhibited, leading to an identical orientation. The hydroxyl groups existing in PK3 can also chelate with Al3+ cations, which favors a higher Al incorporation rate in the final zeolites.
Compared with the reference zeolites (e.g., C-Z5), the PK3-modified ZSM-5 nanocrystals (i.e., H-Z5-PK3) exhibit much higher catalytic activity for the LDPE (low-density polyethylene) cracking reaction, which can efficiently convert LDPE to hydrocarbons at the lowest temperature (Figure 15h–j). In addition, the spent H-Z5-PK3 catalyst could be easily regenerated by simple calcination, and no obvious loss in catalytic activity could be detected in the four cycles, which is much better than the C-Z5 reference sample. The enhanced catalytic cracking performance of the PK3-modified ZSM-5 nanocrystals can be mainly attributed to its unique single-crystalline structure, hierarchical porosities, and more accessible strong acidic sites.

3. Conclusions and Future Outlook

In this review, we summarize the recent progress in the utilization of several easily available polymer compounds, like PEG, PDDA, and PAM, as ZGMs (zeolite growth modifiers) for the synthesis of various microporous zeolites, including SAPO-34, Beta, SSZ-13, LTL, ZSM-5, mordenite, ZSM-23, ZSM-48, etc. By rationally selecting polymer-based ZGMs with different organic functional groups and optimizing the hydrothermal synthesis parameters, the morphologies, porosities, and particle sizes of the resultant zeolite crystals, which are mainly governed by a range of modifier–precursor/crystal interactions, such as van der Waals, hydrogen bonding, electrostatic, and π-π stacking interactions, can be altered. It has been proposed that these multiple interactions may play crucial roles in adjusting the nucleation and growth kinetics during the different crystallization stages, ultimately achieving morphological/microstructural control of the zeolite crystals to a certain extent. Significantly, a number of zeolite-based catalysts, which are derived from polymer-based ZGM approaches, have shown enhanced catalytic activity, selectivity, stability, and/or recyclability for some industrially important catalytic processes, including NH3-SCR of nitrogen oxides, MTO (methanol-to-olefins), alkylation, hydroisomerization, catalytic cracking, etc. This progress clearly demonstrates the great potential of the polymer-based ZGM strategy in the field of zeolite synthesis and catalytic application.
Concerning the complexity of the synthesis system of the zeolites and the diversity of polymer-based ZGMs, that there is still very broad space in the future to utilize the polymer-based ZGM approach for the synthesis of various zeolite crystals with desirable morphological and physicochemical properties, presenting new opportunities and challenges in the development of more efficient zeolite-based catalysts to meet the needs of various industrially important catalysis processes. An interesting subject might be the combined utilization of a polymer-based modifier with other kinds of modifiers like inorganic salts or small organic compounds (e.g., amino acids), which may further optimize the efficiency of synthesis in generating zeolite crystals with controlled morphological and microstructural characteristics. In this case, two or multiple ZGMs may play a synergistic role in regulating nucleation and growth processes by building different interactions with the precursor/crystal of the zeolites. In addition, continued studies are still required to further explore the modulation mechanisms of polymer-based ZGMs in various zeolite synthesis systems, providing potential guidance for the design of more effective synthesis approaches that can achieve precise control and efficient synthesis of zeolite crystals with specific morphologies, microstructures, and enhanced performance for a given catalytic reaction.

Author Contributions

J.Z.: investigation and writing—original draft; C.B. and X.D.: formal analysis; T.L. and M.J.: writing—review, editing and supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Nature Science Foundation of China (22172058) and PetroChina Company Limited (2020A-1818).

Data Availability Statement

No new data were created or analyzed in this study.

Conflicts of Interest

Authors Xiaohui Du and Tao Liu were employed by the PetroChina. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The authors declare that this study received funding from the company PetroChina. The funder was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.

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Figure 1. (a) The proposed crystallization process of intact SAPO-34 crystals in the presence of a PEG modifier; (b) 29Si solid NMR spectra of S-F, S-HT, S-PEG(0.012)-F, and S-PEG(0.012)-HT; (c) quantified results of Si species on S-F and S-PEG(0.012)-F; (d) the loss of framework Al species on S-HT and S-PEG(0.012)-HT. (Reprinted with permission from [42]. Copyright: 2021, Royal Society of Chemistry, London, UK).
Figure 1. (a) The proposed crystallization process of intact SAPO-34 crystals in the presence of a PEG modifier; (b) 29Si solid NMR spectra of S-F, S-HT, S-PEG(0.012)-F, and S-PEG(0.012)-HT; (c) quantified results of Si species on S-F and S-PEG(0.012)-F; (d) the loss of framework Al species on S-HT and S-PEG(0.012)-HT. (Reprinted with permission from [42]. Copyright: 2021, Royal Society of Chemistry, London, UK).
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Figure 2. (a) The catalytic performance of NH3-SCR of NOx over (b) fresh and (c) hydrothermally treated SAPO-34 catalysts; (c) Arrhenius plots of CS-F/HT and CS-PEG(0.012)-F/HT; (d) the NH3−SCR performance and amount of N2O over the low-temperature hydrothermally aged samples. (Reprinted with permission from [42]. Copyright: 2021, Royal Society of Chemistry, London, UK).
Figure 2. (a) The catalytic performance of NH3-SCR of NOx over (b) fresh and (c) hydrothermally treated SAPO-34 catalysts; (c) Arrhenius plots of CS-F/HT and CS-PEG(0.012)-F/HT; (d) the NH3−SCR performance and amount of N2O over the low-temperature hydrothermally aged samples. (Reprinted with permission from [42]. Copyright: 2021, Royal Society of Chemistry, London, UK).
Catalysts 14 00375 g002
Catalysts 14 00375 g003
Figure 3. (a) SEM images of different samples and (b) the catalytic activity and selectivity variations with time on stream over various SAPO-34 catalysts in MTO reaction. Reaction conditions: WHSV = 2.0 h−1, T = 673 K, and catalyst weight = 300 mg. (Reprinted with permission from [53]. Copyright: 2021, Royal Society of Chemistry, London, UK).
Figure 3. (a) SEM images of different samples and (b) the catalytic activity and selectivity variations with time on stream over various SAPO-34 catalysts in MTO reaction. Reaction conditions: WHSV = 2.0 h−1, T = 673 K, and catalyst weight = 300 mg. (Reprinted with permission from [53]. Copyright: 2021, Royal Society of Chemistry, London, UK).
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Catalysts 14 00375 g004
Figure 4. (a) Plausible route for the formation of β nanocrystal aggregates; (b) conversion of cyclohexene and selectivity variations of CHB over various β-zeolites with time on stream; (c) conversion of cyclohexene and selectivity variations of CHB over fresh and regenerated β-zeolite with time on stream (reaction condition: temperature = 353 k, pressure = 0.1 MPa, benzene/cyclohexene ratio = 20, WHSV = 12 h−1). (Reprinted with permission from [54]. Copyright: 2023, American Chemical Society, Washington, DC, USA).
Figure 4. (a) Plausible route for the formation of β nanocrystal aggregates; (b) conversion of cyclohexene and selectivity variations of CHB over various β-zeolites with time on stream; (c) conversion of cyclohexene and selectivity variations of CHB over fresh and regenerated β-zeolite with time on stream (reaction condition: temperature = 353 k, pressure = 0.1 MPa, benzene/cyclohexene ratio = 20, WHSV = 12 h−1). (Reprinted with permission from [54]. Copyright: 2023, American Chemical Society, Washington, DC, USA).
Catalysts 14 00375 g004
Catalysts 14 00375 g005
Figure 5. (a) SEM images of SSZ-13 crystals synthesized in the presence of growth inhibitors. (a,b) SEM images of SSZ-13 crystals derived from two synthesis systems with different concentrations of PEIM: (a) 1 wt% and (b) 1.6 wt%; (c) SSZ-13 crystals synthesized in the presence of 13.5 wt% 1,2-hexanediol (D61,2), exhibiting a bimodal distribution of small and large crystallites; (d) image of calcined SSZ-13 crystals that were synthesized with 7.5 wt% CTAB. (Reprinted with permission from [41]. Copyright: 2015, American Chemical Society, Washington, DC, USA).
Figure 5. (a) SEM images of SSZ-13 crystals synthesized in the presence of growth inhibitors. (a,b) SEM images of SSZ-13 crystals derived from two synthesis systems with different concentrations of PEIM: (a) 1 wt% and (b) 1.6 wt%; (c) SSZ-13 crystals synthesized in the presence of 13.5 wt% 1,2-hexanediol (D61,2), exhibiting a bimodal distribution of small and large crystallites; (d) image of calcined SSZ-13 crystals that were synthesized with 7.5 wt% CTAB. (Reprinted with permission from [41]. Copyright: 2015, American Chemical Society, Washington, DC, USA).
Catalysts 14 00375 g005
Catalysts 14 00375 g006
Figure 6. SEM images of zeolite L crystals synthesized under the following conditions: (a) control (absence of ZGMs); (b) butylamine, A4 (10 wt%); and (c) PDDA (2.1 wt%). The schematic in (d) illustrates that the path length for sorbate diffusion in zeolite LTL channels is proportional to the [001] length (L), while sorbate access to pore openings is dependent on the [001] surface area or diameter (D). (e) ZGMs that preferentially bind to the [001] surface reduce the diffusion path length along the c-axis. (f) ZGM binding to [100] surfaces increases the LTL crystal aspect ratio, L/D. (Reprinted with permission from [21]. (Copyright: 2013, American Chemical Society, Washington, DC, USA).
Figure 6. SEM images of zeolite L crystals synthesized under the following conditions: (a) control (absence of ZGMs); (b) butylamine, A4 (10 wt%); and (c) PDDA (2.1 wt%). The schematic in (d) illustrates that the path length for sorbate diffusion in zeolite LTL channels is proportional to the [001] length (L), while sorbate access to pore openings is dependent on the [001] surface area or diameter (D). (e) ZGMs that preferentially bind to the [001] surface reduce the diffusion path length along the c-axis. (f) ZGM binding to [100] surfaces increases the LTL crystal aspect ratio, L/D. (Reprinted with permission from [21]. (Copyright: 2013, American Chemical Society, Washington, DC, USA).
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Catalysts 14 00375 g007
Figure 7. (a,b) SEM images of SSZ-13 crystals synthesized with PDDA. (c) AFM image of the crystal surface (side of the cube), showing the presence of hillocks comprising single layers with a ca. 1.2 nm step height. (d) High-resolution SEM image showing a rough, porous surface of the crystal (corner of the cube). (Reprinted with permission from [41]. Copyright: 2015, American Chemical Society, Washington, DC, USA).
Figure 7. (a,b) SEM images of SSZ-13 crystals synthesized with PDDA. (c) AFM image of the crystal surface (side of the cube), showing the presence of hillocks comprising single layers with a ca. 1.2 nm step height. (d) High-resolution SEM image showing a rough, porous surface of the crystal (corner of the cube). (Reprinted with permission from [41]. Copyright: 2015, American Chemical Society, Washington, DC, USA).
Catalysts 14 00375 g007
Catalysts 14 00375 g008
Figure 8. (a) DLS measurements of colloidal silica particle hydrodynamic diameter. (b,c) OIM studies in clear solutions prepared from a soluble silica source (b) with 0.05 wt% PDDA after 30 min and (c) 0.5 wt% PDDA after 60 min. (d) Cartoons illustrating how increased coverage of polymer on amorphous precursors with increasing polymer concentration putatively impacts the precursor–solution exchange, precursor aggregation, and the generation of confined interstitial pockets of solution between aggregated precursors at Copt. (Reprinted with permission from [44]. Copyright: 2022, John Wiley and Sons, Hoboken, NJ, USA).
Figure 8. (a) DLS measurements of colloidal silica particle hydrodynamic diameter. (b,c) OIM studies in clear solutions prepared from a soluble silica source (b) with 0.05 wt% PDDA after 30 min and (c) 0.5 wt% PDDA after 60 min. (d) Cartoons illustrating how increased coverage of polymer on amorphous precursors with increasing polymer concentration putatively impacts the precursor–solution exchange, precursor aggregation, and the generation of confined interstitial pockets of solution between aggregated precursors at Copt. (Reprinted with permission from [44]. Copyright: 2022, John Wiley and Sons, Hoboken, NJ, USA).
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Catalysts 14 00375 g009
Figure 9. (a) Plausible mechanism of formation of hybrid nanotube assembly and hierarchical structure of derived ZSM-23 modified with PDDA; SEM images of (b) calcined ZSM-23 modified with PDDA and (c) calcined conventional ZSM-23 crystals; conversion, isomer yield, and cracked product yield in hydroisomerization of n-heptane at varying of reaction temperatures over bifunctional catalysts of (d) derived Pt/H-ZSM-23 and (e) conventional Pt/H-ZSM-23 (X = conversion; Yiso = isomer yield; Ycr = cracked product yield). (Reprinted with permission from [57]. Copyright: 2020, John Wiley and Sons, Hoboken, NJ, USA).
Figure 9. (a) Plausible mechanism of formation of hybrid nanotube assembly and hierarchical structure of derived ZSM-23 modified with PDDA; SEM images of (b) calcined ZSM-23 modified with PDDA and (c) calcined conventional ZSM-23 crystals; conversion, isomer yield, and cracked product yield in hydroisomerization of n-heptane at varying of reaction temperatures over bifunctional catalysts of (d) derived Pt/H-ZSM-23 and (e) conventional Pt/H-ZSM-23 (X = conversion; Yiso = isomer yield; Ycr = cracked product yield). (Reprinted with permission from [57]. Copyright: 2020, John Wiley and Sons, Hoboken, NJ, USA).
Catalysts 14 00375 g009
Catalysts 14 00375 g010
Figure 10. (a) Schematic representation of morphology variation of ZSM-5 nanocrystal agglomerates; (b) acid amount characterized at various treatment temperatures: 523, 623, and 723 K for S10 (open square), S11 (solid square), S12 (open triangle), and S2 (solid circle), respectively; (c) residual weight curves of LDPE cracking over (a) blank, (b) S10, (c) S11, (d) S12, and (e) S2. (Reprinted with permission from [55]. Copyright: 2019, Elsevier, Amsterdam, The Netherlands).
Figure 10. (a) Schematic representation of morphology variation of ZSM-5 nanocrystal agglomerates; (b) acid amount characterized at various treatment temperatures: 523, 623, and 723 K for S10 (open square), S11 (solid square), S12 (open triangle), and S2 (solid circle), respectively; (c) residual weight curves of LDPE cracking over (a) blank, (b) S10, (c) S11, (d) S12, and (e) S2. (Reprinted with permission from [55]. Copyright: 2019, Elsevier, Amsterdam, The Netherlands).
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Catalysts 14 00375 g011
Figure 11. (a) The possible crystallization process of c-axis-oriented HZSM-5 zeolites in the presence of C-PAM; (b) catalytic performance of Z-0, Z-1, and Z-2 in the MTO reaction. (Reprinted with permission from [56]. Copyright: 2020, Springer Nature, New York, NY, USA).
Figure 11. (a) The possible crystallization process of c-axis-oriented HZSM-5 zeolites in the presence of C-PAM; (b) catalytic performance of Z-0, Z-1, and Z-2 in the MTO reaction. (Reprinted with permission from [56]. Copyright: 2020, Springer Nature, New York, NY, USA).
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Catalysts 14 00375 g012
Figure 12. (a) Scheme of the putative crystallization pathway of TS-1 in the presence of PAM; (b) catalytic oxidation of DBT with TBHP; (c) epoxidation of 1-hexene over synthesized TS-1 samples. (Reprinted with permission from [43]. Copyright: 2022, The Royal Society of Chemistry, London, UK).
Figure 12. (a) Scheme of the putative crystallization pathway of TS-1 in the presence of PAM; (b) catalytic oxidation of DBT with TBHP; (c) epoxidation of 1-hexene over synthesized TS-1 samples. (Reprinted with permission from [43]. Copyright: 2022, The Royal Society of Chemistry, London, UK).
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Catalysts 14 00375 g013
Figure 13. Graphical abstract of β zeolite in the presence of AESA. (Reprinted with permission from [68]. Copyright: 2022, The Royal Society of Chemistry, London, UK).
Figure 13. Graphical abstract of β zeolite in the presence of AESA. (Reprinted with permission from [68]. Copyright: 2022, The Royal Society of Chemistry, London, UK).
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Catalysts 14 00375 g014
Figure 14. (a) Schematic diagrams of the crystallization of conventional ZSM-48 and phenolic ZGM-modified ZSM-48. (b) Conversion of n-hexadecane as a function of reaction temperature and (c) isomer selectivity; (d) molar ratio of the mono-branched isomers in all isomers and (e) the molar ratio of central methyl-substitutions in all methyl-substituted C16 as a function of n-hexadecane conversion on Pt/ZSM-48 and Pt/ZSM-48-DA. (Reprinted with permission from [72]. Copyright: 2022, Elsevier, Amsterdam, The Netherlands).
Figure 14. (a) Schematic diagrams of the crystallization of conventional ZSM-48 and phenolic ZGM-modified ZSM-48. (b) Conversion of n-hexadecane as a function of reaction temperature and (c) isomer selectivity; (d) molar ratio of the mono-branched isomers in all isomers and (e) the molar ratio of central methyl-substitutions in all methyl-substituted C16 as a function of n-hexadecane conversion on Pt/ZSM-48 and Pt/ZSM-48-DA. (Reprinted with permission from [72]. Copyright: 2022, Elsevier, Amsterdam, The Netherlands).
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Catalysts 14 00375 g015
Figure 15. (a) SEM image and (b–e) TEM images and (f) FFT diffraction pattern of H-Z5-PK3; (g) free energy as a function of the PK3 center-of-mass distance from the [100], [010], and [101] surfaces of ZSM-5 computed using USMD; weight loss curves of LDPE pyrolysis over (h) H-Z5-PK3−1, (i) H-Z5-PK3, and (j) C-Z5 in the four consecutive pyrolysis regeneration cycles.
Figure 15. (a) SEM image and (b–e) TEM images and (f) FFT diffraction pattern of H-Z5-PK3; (g) free energy as a function of the PK3 center-of-mass distance from the [100], [010], and [101] surfaces of ZSM-5 computed using USMD; weight loss curves of LDPE pyrolysis over (h) H-Z5-PK3−1, (i) H-Z5-PK3, and (j) C-Z5 in the four consecutive pyrolysis regeneration cycles.
Catalysts 14 00375 g015
Table 1. Molecular structure of some representative polymer-based ZGMs.
Table 1. Molecular structure of some representative polymer-based ZGMs.
ModifierMolecular Structure
Polyethylene glycol (PEG)Catalysts 14 00375 i001
Polyethylenimine (PEIM)Catalysts 14 00375 i002
Poly-diallyldimethylammonium chloride (PDDA)Catalysts 14 00375 i003
Polyacrylamide (PAM)Catalysts 14 00375 i004
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Zhan, J.; Bi, C.; Du, X.; Liu, T.; Jia, M. Morphology and Microstructural Optimization of Zeolite Crystals Utilizing Polymer Growth Modifiers for Enhanced Catalytic Application. Catalysts 2024, 14, 375. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060375

AMA Style

Zhan J, Bi C, Du X, Liu T, Jia M. Morphology and Microstructural Optimization of Zeolite Crystals Utilizing Polymer Growth Modifiers for Enhanced Catalytic Application. Catalysts. 2024; 14(6):375. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060375

Chicago/Turabian Style

Zhan, Junling, Chongyao Bi, Xiaohui Du, Tao Liu, and Mingjun Jia. 2024. "Morphology and Microstructural Optimization of Zeolite Crystals Utilizing Polymer Growth Modifiers for Enhanced Catalytic Application" Catalysts 14, no. 6: 375. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060375

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