Aromatics Alkylated with Olefins Utilizing Zeolites as Heterogeneous Catalysts: A Review

Abstract

The alkylation reaction of aromatic compounds gains considerable attention because of its wide application in bulk and fine chemical production. Aromatics alkylated with olefins is a well-known process, particularly for linear alkylbenzene, phenyloctanes, and heptyltoluene production. As octane boosters and precursors for various petrochemical and bulk chemical products, a wide range of alkylated compounds are in high demand. Numerous unique structures have been proposed in addition to the usual zeolites (Y and beta) utilized in alkylation procedures. The inevitable deactivation of industrial catalysts over time on stream, which is followed by a decrease in catalytic activity and product selectivity, is one of their disadvantages. Therefore, careful consideration of catalyst deactivation regarding the setup and functioning of the process of catalysis is necessary. Although a lot of work has been carried out to date to prevent coke and increase catalyst lifespan, deactivation of the catalyst is still unavoidable. Coke deposition can lead to catalyst deactivation in industrial catalytic processes by obstructing pores and/or covering acid sites. It is very desirable to regenerate inactive catalysts in order to remove the coke and restore catalytic activity at the same time. Depending on the kind of catalyst, the deactivation processes, and the regeneration settings, each regeneration approach has pros and cons. In this comprehensive study, the focus was on discussing the reaction mechanism of 1-octene isomerization and toluene alkylation as an example of isomerization and alkylation reactions that occur simultaneously, shedding light in detail on the catalysts used for this type of complex reaction, taking into account the challenges facing the catalyst deactivation and reactivation procedures.

1. Introduction

Alkenes’ alkylation reaction may be viewed as a post-treatment strategy due to its ability to be carried out with minimal or no hydrogen, while also serving to preserve the octane number properties [1]. There are numerous utilizations of the process of alkylation involving aromatics and linear alkenes, including sectors within petrochemical, chemical, and refining industries [2]. The benefits of alkylation conducted in a “liquid phase” include an extended lifespan of the catalyst and the straightforward management of the process through thermal regulation [3]. Furthermore, performing reactions in the “liquid phase” facilitates the management of temperature during the process and prolongs the operational durability of the “zeolite catalyst” [4]. Despite the utilization of an acid catalyst such as zeolite, achieving a notable conversion rate poses a considerable challenge due to the necessity of attaining a high selectivity for the intended products [5,6,7].

Recently, due to the spread of the Coronavirus and the high global need for detergent, the production of linear alkylbenzene (LAB) is essential [8]. LAB production has experienced continuous growth because of the increased per capita consumption worldwide as a result of improved health care and generally increased per capita income [9]. Nearly all LAB is a major surfactant in household and industrial detergents, it is a cost-effective and biodegradable intermediate in the production of detergents and surfactants [10]. Therefore, the global LAB market size reached 4.4 million tons in 2023, demand is expected to increase to 6.1 million tons in 2032 [11]. Generally, industrial production of LAB involves the use of alkylating reagents in the presence of Lewis acids to create benzene alkylation with α-olefin. (such as AlCl₃, ZnCl₂) or Brönsted acids (such as HF, H₂SO₄) serving as catalysts [12,13,14]. However, because of their high levels of pollution, lack of process safety, severe equipment corrosion, and labor-intensive catalyst separation, homogenous catalysts pose a serious threat to the environment. Among the LAB isomers, the 2-phenylalkane, 2-phenylalkane sulphonate 2-alkylbenzene, or 2-alkylmethylbenzene are the most biodegradable, and are actively sought after as they have the finest solubility [15]. In order to replace the traditional AlCl₃ and HF catalysts while maintaining excellent selectivity of alkylation products, the detergent industry has been looking for an ecologically benign alkylation method.

The literature has reported on a number of solid acid catalysts for LAB synthesis, including solid metal oxides like zeolites [10,16], non-zeolite-based catalysts [17], clays and zirconia-pillared montmorillonite [18], sulfated zirconia [19], mesoporous zirconia [20], MCM-41 [21], and mesoporous silica (SBA-15) [22]. “Zeolite catalysts” are being researched extensively as an alternative to anhydrous HF or AlCl₃ catalyst among readily available solid acid catalysts [23,24,25]. Zeolite Y and mordenite catalysts, which have large pores, have been shown to be effective and selective catalysts for the alkylation of benzene by linear long chain alkenes [26,27,28,29]. The existence of mesopores produced by dealumination in the specific instance of mordenite significantly improves internal diffusion characteristics, boosting activity and maintaining shape selectivity [4,30,31]. Therefore, beta zeolites are potentially advantageous catalysts for the production of LAB since they are often generated with significantly smaller crystallites [32,33].

The pursuit of alternative materials has emerged as a fundamental requirement in current research. Consequently, the appeal of heterogeneous catalysts has been on the rise, particularly owing to their possession of acid properties, a characteristic of significant interest in this realm of study. Despite the numerous advantageous attributes associated with zeolites, a number of challenges persist, including their elevated activity and acidity levels, capacity reusability is necessary to handle big compounds like heptyltoluene, and there is susceptibility to various treatment methods [34,35]. These challenges stem from either the inherent structure of zeolite, characterized by micropores or the hindrance of the spread of heavy products due to coke formation resulting in catalyst deactivation [36].

The primary byproducts of aromatic alkylation with olefins over a variety of zeolite catalysts are monoalkylation and olefin isomerization [37]. 2-, 3-, and 4-alkylltoluene are monoalkylated derivatives, whereas double bond shifts result in 2-, 3-, and 4-olefin. 1-heptene isomerization happens readily on a solid catalyst such as zeolite, but it is not the study’s goal [38]. During the aromatic alkylation with 1-octene, 1-phenyloctane is not detected because it is unstable; therefore, 2-phenyloctane is the most biodegradable surfactant with a high solubility that is the first stable product that can be identified [9,39].

Several factors affect the alkylation reaction, such as reaction temperature, aromatic/olefin ratio, amount of catalyst, catalyst properties, and reaction time [40]. Temperature has almost no effect on the distribution of products; however, with the increase in temperature, the lifetime increases but the selectivity of alkylation product decreases; therefore, the temperature cannot be too high [41]. Selectivity towards mono-alkylates increased with higher toluene content in the initial reaction mixture [42]. Reactor effluent contained traces of di-alkylated products at low-benzene/-1-octene feed molar ratios. [43]. In addition, processes involved olefin oligomerization and transalkylation–disproportionation of the alkylated products within the zeolite’s pores. On the other hand, the overall reaction rate was found to increase with an increase in the catalyst loading in the range employed in this work, which was due to the proportional increase in the number of acidic sites and surface area of the catalysts [44]. Furthermore, in order to prevent n-olefin dimerization and cracking reactions, as well as the formation of heavy products such as polyalkylbenzenes and diphenylalkanes, it is necessary to achieve an ideal ratio between the strength of acid sites and the characteristics of the porous superficial structure of the catalyst in order to effectively run the alkylation of benzene with higher olefins in the presence of solid catalysts [34].

Generally, “zeolite catalysts” deactivate either via pore obstruction, which occurs when the large molecules of coke prevent reactants from reaching the active sites, or by site covering, which occurs when specific components, such as coke, firmly and irreversibly adsorb on the acid sites [39]. The cause of catalyst deactivation is the formation of the so-called liquid coke formed by side reactions, e.g., isomerization and oligomerization of 1-alkenes, transalkylation, and polyalkylation, or even dealumination and structure alteration [45,46]. The liquid coke can poison active sites or block their access [47].

In this review, we discuss the reaction mechanism of 1-octene isomerization and alkylation of toluene as an example for the isomerization and alkylation reactions. Based on the reaction mechanism, the recent progress on catalysts was summarized to provide an update on recent developments. Furthermore, the effect of reaction conditions and catalyst deactivation were emphasized. In addition, there are some possible challenges that need to be overcome and explored for future industrialization.

2. Previous Studies

Aromatic alkylation with linear olefins over “zeolite catalyst” has been the subject of several investigations.

Liu et al. [48] pointed out that the activity of bulk and MCM-41 supported Keggin-type heteropoly acids (HPA), such as tungstophosphoric acid (HPW), tungstosilicic acid (HSiW), and molybdophosphoric acid (HPMo), for the 1-octene alkylation of toluene in the “liquid phase”. Among these catalysts, the supported catalysts—in particular, HSiW and HPW supported on MCM-41 (HSiW/MCM-41 and HPW/MCM-41)—exhibited more activity than bulk HPA. After two hours of reaction at 120 °C over HSiW/MCM-41, the conversion of 1-octene was 100% and the selectivity for monoalkylation products was 99.9%.

Craciun et al. [39] investigated the production of phenyloctanes by three USY zeolites using bulk Si/Al ratios of 6, 13, and 30 for the liquid-phase alkylation of benzene with 1-octene at temperatures between 70 and 100 °C, with benzene/1-octene feed molar ratios ranging from 1 to 10. There were two different kinds of reaction products: isomers of octene and phenyloctane. However, the major products were simply 2-octene and 2-phenyloctane. They discovered that when the Si/Al ratio rises, so does the catalytic activity, with the 1-octene conversions between 10% and 99%, while the selectivities are not affected.

Fernandes and Lachter [44] examined how octyltoluene is produced and how different cation-exchange resin catalyst structures affect the “liquid phase” alkylation of toluene with 1-octene (Lewatit SP112, Amberlyst 15, Amberlyst 35). Amberlyst 15 and Amberlyst 35 resins demonstrated strong selectivity for monoalkylation products (>90%) and high conversions (>90%). But between 80 and 110 °C, which are very moderate reaction conditions, Lewatit is essentially inert. The best catalyst, according to the data, is Amberlyst 35, which also increases the selectivity of 2-octyltoluene synthesis.

Mayerová and co-workers used various zeolites (ZSM-5, ZSM-11, ZSM-12, beta, mordenite, zeolite Y), micro/mesoporous composites in 1-octene alkylation with toluene, and mesoporous molecular sieves (AI)MCM-41 [49]. It was demonstrated that the activation of olefin for the alkylation process is significantly influenced by the acid strength of the acid sites. It was shown that the conversion of 1-octene significantly increases shifting from medium to large pore zeolites (USY~Beta >> ZSM-5). Mesoporous molecular sieves showed the lowest conversion because they included Lewis acid sites with comparatively weak acid strength, whereas zeolites with a large concentration of Broensted acid sites yielded the highest conversion in all alkylation processes.

Lachter et al. [38] pointed out the limiting reactant’s conversion and the yield of monoalkylation products through the toluene alkylation with 1-octene, 1-octanol, 2-octanol, and isopropanol over macroreticular cation-exchange resin (Amberlyst-15) using the batch reactor and at 80 °C, in the “liquid phase”. They concluded that the best results were achieved with octene, and the alkylation products showed that the reaction predominantly gave 2-octyltoluene and the selectivity for the monoalkyltoluene was 100%; nevertheless, the conversion of octene was 75% after 4 h of reaction.

Cowley et al. [50] investigated the impact of the molar ratio between aromatic and olefin during the process of alkylating toluene with 1-pentene over Y and beta zeolite catalysts at temperatures between 100 and 200 °C. They came to the conclusion that at high aromatic to olefin ratios, there was less chance for a carbocation to connect with an olefin or an alkylated toluene molecule, which led to a decrease in oligomerization and amyltoluene alkylation selectivity. Furthermore, compared to Y zeolite, beta zeolite was shown to have less alkylation activity.

Al-Shathr et al. [32] studied the effect of fresh and modified H-mordenite and H-beta “zeolite catalyst” on the toluene alkylation with 1-heptene at 90 °C inside a batch reactor. It indicates that the H-beta zeolite had the lowest conversion, which is most likely due to the coke that generated acting to immediately deactivate the zeolite by obstructing its pore openings. On the other hand, the dealuminated H-beta and parent H-mordenite catalysts had greater 1-heptene conversion rates, at 84.67% and 85.3%, respectively. Due to its smaller pore size than other catalysts employed in this kind of reaction, the H-beta zeolite exhibits the lowest selectivity of around 10% when utilized in 2-heptyl-methylbenzene reactions. On the other hand, because the parent H-mordenite sample’s framework has an appropriate pore size that allows it to desorb the 2-heptyl-methylbenzene isomers, it exhibits the best selectivity.

Magnoux et al. [51] investigated the producing of linear alkylbenzene, and the effect of the pore structure and of the acidity of large pore zeolites: HFAU (framework Si/Al ratio from 4 to 100), HMOR (Si/Al from 10 to 80), HBEA (Si/Al = 10) and of an average pore size zeolite on the toluene alkylation with 1-heptene. They found that the target product’s desorption from these pores and the pH of the solution affect the activity. Therefore, HFAU with a Si/Al ratio of 30, which has a relatively high acidity and contains mesopores, is the more active catalyst.

Da et al. [26] utilized an HFAU zeolite in a fixed-bed reactor at 90 °C, a molar toluene/dodecene ratio of 3, and WHSV = 10 h to generate linear alkylbenzene by alkylating toluene with 1-dodecene. Bidodecyltoluenes only emerge in trace amounts once dodecene is fully converted, whereas monododecyltoluenes are generated selectively. Although they cannot desorb from the zeolite, tridodecyltoluenes are also generated inside the supercages. Catalyst deactivation is caused by chemicals that are mostly found on the outside of the crystallites. Nevertheless, by treating under toluene flow, tridodecyltoluenes may be entirely eliminated, allowing the catalyst to regenerate entirely. Tridodecyltoluenes and toluene molecules transalkylate to remove this compound, resulting in the creation of monododecyltoluenes. In a connected setting, Da et al. [28] utilized HFAU and HBEA zeolites to create linear alkylbenzene by alkylating toluene with 1-heptene and 1-dodecene. Despite having tiny pores, the zeolite beta is either inactive when reacting toluene with 1-dodecene or slowly active when reacting with 1-heptene. The sluggish desorption of products from the zeolite pores and the subsequent production of coke are caused by this low activity. Coke forms over HBEA in greater quantities than it does over HFAU. Furthermore, compared to faujasite, the amount of coke generated over zeolite HBEA is either smaller (1-heptene) or similar (1-dodecene). Since the primary product trapped in the HBEA zeolite is monoheptyltoluene, the transalkylation process cannot occur over this zeolite.

Yuan et al. [52] noted that the temperature of the pre-treating step, which lowers the amount of coke, affects the activity and stability of Ultra-Stable Y (USY) zeolite for the alkylation of benzene with 1-dodecene by using FBR at high pressure. This is because these factors act to increase the strength of the acid sites and decrease their number and density, as well as occasionally create mesopores.

Wang et al. [53] studied the influences of reaction conditions such as temperature and the molar ratio of benzene to 1-dodecene during benzene alkylation with a 1-dodecene molar ratio of 8.7:1 over three kinds of faujasite-type zeolites HY (SiO₂/Al₂O₃ = 4.8), USHY (SiO₂/Al₂O₃ = 80) and DAY (SiO₂/Al₂O₃ = 200) were used in this work using the batch reactor. The results of the alkylation of benzene with 1-dodecene over various catalysts showed that the conversion of 1-dodecene was 90%, 100%, and 50% for the three catalysts. The selectivity for 2-phenyl isomer of 32%, 27%, and 26% with the order of DAY > HY > USHY are in inverse trend with the conversions.

Cao et al. [9] produced LAB (linear alkylbenzene) isomers by the alkylation of benzene with 1-dodecene over FAU, BEA, and EMT zeolites in the presence of decane as solvent. They obtained that the residual Na content, Si/Al ratio, and porous structure of the catalysts were discussed in relation to the activity and stability. There was a significant rise in the selectivity to the most desirable 2-phenyl dodecane isomer with increasing porosity limitations. Despite a significantly lower conversion over BEA catalysts, the preferred isomer yields were remarkably similar to those offered by the FAU open structure catalysts.

de Almeida et al. [10] examined the process of converting benzene with 1-dodecene into linear alkylbenzene using three different types of zeolite catalysts HZSM-5, HZSM-12, and HY. They found that whereas the activity of zeolite HY is dependent on the quantity of aluminum, or the Si/Al ratio, the HZSM-5 and HZSM-12 zeolites had low activity. Using HY zeolite increased the linear alkylbenzene’s selectivity to 97–98%.

Benzene alkylation with 1-dodecene over HY zeolite investigated in a batch reactor was studied by Liang et al. [46]. A special feature of the effect of the benzene to 1-dodecene ratio on the reaction conversion was found. The zeolite’s production of coke causes the catalyst to deactivate fast. A unique technique for renewing the deactivated catalyst was created. It involves using benzene as one of the reactants to extract the deactivated catalyst. The activity of the regenerated catalyst is significantly influenced by the regeneration temperature. Additionally, it was discovered that the product distributions of the regenerated and deactivated catalysts are the same for a comparable reaction conversion.

The alkylation of benzene with 1-alkenes in a stainless steel batch reactor at autogenous pressure in a “liquid phase” at the temperature of 120 °C for Y zeolite, 180 °C for beta zeolite, and 200 °C for mordenite was investigated and achieved by Horňáček et al. [54]. 1-hexene through 1-octadecene were employed as 1-alkenes. The alkyl chain length reduced the conversion of 1-alkenes in the alkylation of benzene across all zeolites. The wide-pore Y zeolite showed the lowest decline in activity with chain length, whereas mordenite showed the biggest decrease. Additionally, Peregoa and associates, Ref. [55], observed that in the “liquid phase”, a benzene alkylation reaction with alkenes, zeolite Y, and beta exhibit created more activity than ZSM-5, particularly at low temperatures.

3. Mechanism of Alkylation Reaction

When the Y and beta zeolites used in this study are present, the double bond isomerizes to produce the octene isomers (2-octene, 3-octene, and 4-octene), and the alkylation of aromatic with these octene isomers forms the phenyloctanes (2 phenyloctane, 3-phenyloctane, and 4-phenyloctane) [39]. Bhore et al. [56] found that 2-octene and 2-phenyloctane are the sole major products when the feed only contains 1-octene and aromatic; 3-octene and 3-phenyloctane are secondary products; and 4-octene and 4-phenyloctane are tertiary products.

The examination of the experimental data also showed that the internal equilibrium distribution of the octenes is not obtained, despite the fact that double-bond isomerization proceeds more quickly than benzene alkylation [43]. As a result, it is necessary to characterize the double-bond isomerization and the alkylation as happening simultaneously rather than as two successively disconnected processes since they occur on a comparable time scale.

Understanding the origin of the reactivity of zeolite materials necessitates the development of the molecular-level understanding of the structure and chemical nature of catalytic centers [57]. A network of elementary steps involved in the alkylation of benzene and toluene with octenes over solid acid catalysts was developed, as shown in Figure 1 and Figure 2, respectively [43,44,48]. With benzene responding from the “liquid phase” and olefins reacting as adsorbed species on the catalyst’s surface, the reaction network’s underlying mechanism is of the Eley–Rideal type. Adsorbed benzene molecules are not regarded as reactive species in the suggested mechanism because contact with an acid site reduces the electrical density on the aromatic ring, making an electrophilic assault more challenging. Eley–Rideal-type methods for the alkylation of benzene over large pore zeolites like Y and beta have previously been postulated [58].

This reaction network is based on carbenium–ion chemistry, which is thought to happen on the catalyst surface [39,43]. If not, the widely known process involving the creation of an alkylcarbenium ion by the interaction of the olefin with the acid catalyst may be used to explain the synthesis of these products from benzene or toluene and 1-octene. In most situations, this step determines the pace at which the thus-formed carbocation attacks the aromatic ring and generates an arenium ion, or so-called sigma complex. The last step involves the sigma complex removing the hydrogen linked to the attacked aromatic carbon as a proton, resulting in the creation of the final alkylated product and catalyst renewal. Given that the production of the 1-phenyl-isomer would require the highly active creation of a primary carbenium ion, this mechanism explains why it is absent from the reaction products.

4. Catalysts for Alkylation Reaction

Berzelius, in 1835, introduced the term “catalysis” derived from the Greek term καταλειν, which signifies to loosen or dissolve [59]. A catalyst is a substance that facilitates a chemical reaction by lowering the activation energy required, thereby enhancing the reaction rate [60,61]. The interest in catalysts has significantly surged over the past century. In the contemporary era, catalytic technology is a pivotal component in 80–90% of the industrialization procedures and is emphasized by Guisnet and Pinard [62]. Industrial catalysts were initially incorporated into fixed-bed catalytic cracking systems, utilizing natural bentonite clays, specifically of the montmorillonite type [63,64]. These clays consist primarily of montmorillonite, which is a hydrated aluminum silicate that also contains traces of magnesia [65].

Catalysts are commonly categorized into two distinct groups, with the initial group consisting of heterogeneous and homogeneous catalysts as outlined by Hagen [66]. The selection of a specific catalyst is of paramount importance as it profoundly influences the rates of conversion, selectivity, and yield of the intended product [67].

Homogeneous catalysts offer numerous benefits, such as elevated activity and exceptional selectivity towards alkylation products [68,69]. Nonetheless, they are accompanied by drawbacks, including the generation of pollution, posing industrial hazards, leading to equipment degradation through corrosion, and it is quite challenging to effectively distinguish the catalyst from the resulting products. Moreover, the homogeneous processes discussed are commonly linked to issues such as decomposition, corrosion, high quantities of catalysts, and challenges in catalyst separation [70,71].

Acid-treated natural aluminosilicate catalysts were superseded in the 1940s by synthetic amorphous alumina–silica catalysts thanks to the development of moving-bed and fluidized-bed processes [72]. The most important advancement in catalyst development was during the five-year period between 1962 and 1967 when crystalline silica–alumina catalysts known as zeolites or molecular sieves like faujasite (FAU-type) were employed [73]. Several factors have contributed to the utilization of heterogeneous catalysts in the industrial sector [74,75]. Among these factors is their ability to enhance the rate of the intended chemical reaction. Furthermore, their ease of separation from the liquid reactants distinguishes them from liquid catalysts. In terms of environmental impact, heterogeneous catalysts are deemed more sustainable as they can be reused multiple times, contrasting with homogeneous catalysts that require disposal after each use [76]. The safety profile of heterogeneous catalysts surpasses that of liquid catalysts by mitigating issues related to corrosion [77]. Moreover, in scenarios involving endothermic reactions, these catalysts facilitate the continuation of the reaction process despite elevated temperatures. Conversely, for exothermic reactions, they ensure that the reaction persists even as temperatures decrease. The disadvantage associated with solid catalysts lies in their susceptibility to deactivation, primarily attributed to the build-up of heavy byproducts within the catalysts’ channels and reactive sites [46]. Catalyst deactivation is instigated by the creation of what is commonly referred to as liquid coke [78].

5. Zeolites

Zeolites have been proposed as ideal materials for applications in catalysis, adsorption, and biomedicine due to their notable intrinsic features such as accessible void space, large surface area, and strong acidity [79,80,81]. Zeolites play a fundamental role in various chemical sectors, particularly in catalytic processes and oil refineries, due to their significance in the field of catalysis science [82]. The discovery of the initial zeolite mineral, characterized as “an unknown kind”, dates back to 1756, and is credited to the Swedish mineralogist Alex Cronstedt [73]. The term “zeolite” was derived from a combination of two Greek terms, “Zeo” signifying to boil, and “lithos” representing rock. In the 1940s, Barrer and Milton opened up the avenue to the synthesis of zeolites [83]. Since then, there have been considerable efforts in the synthesis of new zeolite materials. The first classification system for zeolites was introduced by Barrer in 1945 based on molecule size and absorption rate, with the synthetic zeolite being defined in 1948 [84]. A significant advancement in zeolite production occurred in the 1980s with the discovery of a new type of molecular sieve with distinct structures and compositions [72]. Additionally, the late 20th century saw the development of secondary synthesis modifications for zeolites.

Catalysts containing zeolites are employed in the industry for a variety of processes, including aromatization, alkylation, hydrocracking, hydrotreatment, proton exchange, and isomerization [85,86,87,88,89,90]. They are also used in the olefin-mediated alkylation of aromatic and paraffinic hydrocarbons. These catalysts have great selectivity, regeneration ability, and enough activity to produce alkylates of superior quality. A range of acidity levels of monofunctional or bifunctional catalysts are chosen, depending on the process and hydrocarbon feedstock composition [23].

The inclusion of a catalyst results in a decrease in the temperature required for decomposition and an acceleration of the decomposition rate. Consequently, it enhances the efficiency of a chemical reaction and decreases pollution by conserving energy and minimizing the production of unnecessary byproducts and products [84,91]. The substantial pore dimensions, exemplified by Y zeolite, are governed by two primary factors that influence the activity of the zeolite: the level of acidity and the efficiency of product desorption from the zeolite’s pores [51].

5.1. Zeolite Classification

Zeolites are categorized not only using the IUPAC nomenclature but also based on the number of channel systems that are present in the structure, such as uni-, bi-, and tri-dimensional systems [92]. For instance, zeolite ZSM-5 has two sets of intersecting channels—one straight and the other sinusoidal—while zeolite type A has straight intersection channels along three dimensions. Mordenite, on the other hand, has a one-dimensional channel system.

Another classification of zeolites concerns the molar ratio of Si/Al; there are three levels for this classification. Low silicon (Si/Al ratio is 1–1.5), e.g., zeolite A; medium silicon (Si/Al ratio is 1.5–10); e.g., zeolite Y and zeolite mordenite; and high silicon (Si/Al ratio is higher than 10), e.g., zeolite beta and zeolite ZSM-5 [93].

The classification of zeolites based on the number of the T atoms of the open window is classified into the following categories: small pore size by 7, 8 membered rings (MR), e.g., zeolite A; medium pore made by 9, 10 MR, e.g., zeolite MFI; large pore made by 11 and 12 MR such as FAU and MOR; and extra-large pore made by more than 12 MR such as VFI and CFI [94]. Accordingly, Lin et al. [95] recently employed ZEO-1 zeolite which has an extra-large pore opening, high stability, and high surface area in catalytic cracking. The zeolite structure is classified based on the dimensions and arrangement of its channels, meaning that its pore system might have one, two, or three dimensions [96]. Other classifications, however, distinguish between two kinds of zeolites: hydrophilic and hydrophobic [97]. Breck [98] states that zeolites can be categorized into two groups based on their dehydration behavior from thermal analyses: (a) those that exhibit continuous dehydration curves as a function of temperature upon dehydration and do not show major structural changes, and (b) those that undergo structural changes with dehydration and display steps or other discontinuities in the dehydration curves. Figure 3 shows some of the well-known zeolite catalysts.

5.1.1. Zeolite Y

The characteristics of synthetic faujasite were uncovered at a later stage, leading to the emergence of rare earth-exchanged zeolite X and/or Y as prominent contenders in the petroleum refining sector [104]. Zeolites play a crucial role in various chemical industries, particularly in catalytic reactions and oil refineries, due to their significance in the field of catalysis science [105]. Zeolite-based catalysts showcased superior catalytic attributes, particularly zeolite type Y, which exhibited significantly elevated levels of activity and selectivity in comparison to the options presented by natural catalytic materials during that era. Additionally, zeolite type Y is considered to be a significant variant among zeolites, bearing resemblance to the faujasite (FAU) structure [106,107]. The first synthesis of zeolite Y was documented by Breck in 1964 as reported by Lutz in 2014 [108]. This type of zeolite can be created by linking small sodalite cages using 6-rings, forming hexagonal prisms as explained by Kulprathipanja in 2010 [72] and Busca in 2014 [103]. Large cages known as “Supercages”, with a diameter of around 13 Å, are the outcome of the link and are reachable by three dimensions, as seen in Figure 3A. It has a diameter of approximately 7.4 Å and is made up of twelve silicate rings.

Another variation of zeolite Y is the Ultra Stable Y (USY), known for its enhanced hydrothermal stability and a Si/Al ratio exceeding 30 [109]. Moreover, water holds a significant role in the composition of zeolites with high aluminum content like zeolite Y, contributing to increased stability as highlighted by Byrappa and Yoshimura in 2001 [110]. The initial utilization of zeolite Y in the synthesis of alkylbenzene in the “liquid phase” was pioneered by Universal Oil Products (UOP) [4]. The first utilization of zeolite as a catalyst in an industrial process was declared by UOP in 1995 [111,112].

5.1.2. Zeolite A

Zeolite A, also known as LTA (Linde Type A), belongs to the family of aluminosilicate molecular sieves [113]. It has distinct capabilities of adsorption by offering ion exchanges. It is synthetic, hydrous, alkali aluminosilicates, with exceptionally advantageous structural channels and cavities [84]. Sodalite cages are the main structural components of zeolite A. They are joined by rings with four members to create a three-dimensional (3D) network (Figure 3B) [114]. These cages are made up of eight ring apertures with a 4.1 Å aperture that links the center, 11.4 Å diameter supercage cavities. This creates an incredibly open zeolite framework with a high void volume percentage of 21.43%.

Smaller molecules, such as H₂S and H₂O, may be selectively separated from organic molecules using type A zeolite structures. This technique is frequently used to separate light normal from branched paraffins, as the latter (iso-alkanes) are somewhat bigger than the former. Traces of sulfur compounds can also be eliminated with the help of type A zeolite [115]. Moreover, the cubic framework structure is commonly used as a selective sorbent for drying gases and solvents. Type A zeolites with 4 to 8 mesh sieves are normally used in gas-phase applications [116].

5.1.3. Zeolite Mordenite

Mordenite zeolite is a siliceous molecular sieve with two pore channels [101]. Leonard in 1927 was the first scientist that declared the synthesis of mordenite [117]. However, in 1961, Meier described the structure of mordenite with two-dimensional pores [118]. The hydrothermal method is used to synthesize the mordenite. It is employed in the adsorption and separation of gas or liquid mixtures that have acid components. It is considered a microporous zeolite, and the Si/Al ratio is approximately ten. The mordenite pores consist of main channels of 6.5 × 7.0 Å, that are connected by tortuous pores of 2.6 × 5.7 Å that form the so-called “side pockets” (see Figure 3C) [119].

Mordenite is widely used in catalysis and in separation and purification because of its uniform, small pore size, high internal surface area, flexible framework, and controlled chemistry [118]. It is employed as a catalyst in a number of significant industrial processes, including reforming, hydrocracking, hydro-isomerization, alkylation, and cracking [104]. Remarkably, due of its porous nature and great stability, it is employed in alkylation processes [103].

5.1.4. Zeolite MFI

H-ZSM-5 zeolite with MFI topology belongs to the pentasil family [120]. Many pentasil units are joined through oxygen bridges to form chains in MFI zeolites [121]. The framework of H-ZSM-5 zeolite contains two types of intersecting 10-membered ring channels, as shown in Figure 3D [122]. The straight channels (5.4 × 5.6 Å) run along the crystallographic b-axis and the sinusoidal channels (5.1 × 5.5 Å) parallel to the a-axis [102]. In general, the high selectivity and extending the durability of the catalyst (i.e., lifetime) are two primary motives for modifying the physicochemical properties of ZSM-5 catalysts [123,124]. Modifying the synthesis protocols of ZSM-5, including its post-synthetic manipulations, is vital to change the physicochemical properties.

Zeolite ZSM-5 characterized by uniform pore size, adjustable acidity, and high-temperature resistance has a broad application prospect in catalytic reactions [125]. In addition, it has been extensively studied and applied in a series of chemical processes due to its abundant well-defined microporous structures and the intrinsic moderate Brønsted acidities [120].

5.1.5. Zeolite Beta

The categorization of the zeolite beta family falls under the classification of a high-silica framework [126]. Its initial discovery dates back to 1967 by Wadlinger and his colleagues. Characterized by a delicate and disorganized framework, zeolite Beta comprises two distinct channel types, each featuring three-dimensional 12-rings, albeit with differing pore sizes (medium and large) (see Figure 3E) [127].

The Si/Al ratio of zeolite beta typically exceeds 10, as ratios below ten result in a lack of crystalline structure [126]. Noteworthy attributes of zeolite beta include its high silica content and large pores, leading to diverse applications such as aromatic transalkylation, alkylation, hydroisomerization, and cracking [28,32,128]. Beta zeolites, which are for the most part integrated with a lot more modest crystallites, are hence likely great impetuses for the combination of LAB. Uses of beta zeolites were accounted for with Brønsted causticity somewhat like Y zeolite [129] and positive pore structure [130], however, with more modest movement and more quick deactivation [33,131].

6. The Utilization and Applications of Zeolites as a Catalyst

Zeolites, as microporous materials, have unique physicochemical properties and a unique structure, making them widely used in many modern scientific and industrial fields (see Figure 4) [132].

They serve primarily as catalysts in various industries, including petroleum refining, petrochemical production, and synfuel manufacturing [133,134,135]. Moreover, the significance of zeolites extends to the petrochemical sector, notably in the alkylation process, where prominent types include Y, mordenite, and beta [107]. Apart from their catalytic roles, zeolites find applications as absorbents for gas purification, clean-up, and drying purposes [104]. Furthermore, zeolites function as desiccants in wastewater treatment operations and are employed in ion-exchange processes, particularly in detergent and soap production.

7. Structures of Zeolite

Extra-framework species and a host framework make up a typical zeolite. Despite the fact that all zeolites are constructed from TO₄ tetrahedra (where T can be any of the following: Si, Al, or P), they connect to form various forms of tetrahedral frameworks in distinct ways [136,137]. A collection of primary building units (PBUs) that are created by merging secondary building units (SBUs) make up the zeolite framework structure [103]. As seen in Figure 5, the final ones are formed by Al or Si atoms joined by four oxygen atoms.

Up to 16 PBUs, or T-atoms, can be found in SBUs, and the SBU composition is used to categorize zeolite types [73]. A unit cell is defined as the smallest repeating unit selected according to specific criteria to reflect all the properties of a lattice. It always has the same number of SBUs, even when other materials have varying combinations of SBUs inside the zeolite framework. Therefore, the lattice may be rebuilt from the SBU using the so-called lattice parameters, also known as lattice constants, which are the edge lengths (a, b, and c) and inter-edge angles (α, β, and γ).

(SiO₄)⁴⁻ tetrahedral units and their corners link all these tetrahedral units together [84,91]. Since silicon has a valance of four and aluminum a valance of only three, the AlO₄ tetrahedron carries a net negative charge. Therefore, the best way to balance this negative charge is by the addition of a positive cation like (Na⁺, K⁺, and Ca²⁺) [107]. Additionally, according to Löwenstein’s rule, oxygen bridges connect the Al and Si atoms such that no two aluminum atoms attach to the same oxygen atom. This means that four Si atoms can surround each Al atom and up to four Al atoms can surround the Si atom.

Since zeolites are microporous structures, the size of the oxygen ring that defines the pore—that is, the size of the SBU—determines the typical pore opening for each zeolite topology [139]. As a result, describing a zeolite structure usually includes describing its pore openings and the dimensionality of its internal channel system. Zeolite usage has increased to a global market of several million tons annually due to its special porous qualities.

The Structure Commission of “International Zeolite Association” (IZA) has allocated a three-lettered code, such as BEA, MOR, LTA, MFI, and FAU, for each form of zeolite framework that currently exists, based only on the framework topology and not on the chemical composition. The IZA has so far certified over 258 different kinds of zeolite frameworks [140,141].

Every sort of framework can be associated with several distinct zeolite materials. For instance, the MFI-type framework structure of the pure-silica zeolite silicalite-1 and the aluminosilicate zeolite ZSM-5 are the same, yet their chemical characteristics are different [142]. ZSM-5′s negatively charged aluminosilicate framework allows for variable Brønsted acidity and hydrophilicity, while silicalite’s charge-neutral pure-silica framework allows for mild acidity and significant hydrophobicity. Typically, alkali metal ions or organic amines are the extra-framework species found in zeolites. These species crystallize alongside the host frameworks throughout the zeolite material production process [143]. They are commonly referred to as templates or structure-directing agents for zeolites since they are required to be introduced in order to synthesize specific zeolite framework types. The International Union of Pure and Applied Chemistry (IUPAC) has the following categorization; with dp being the pore diameter, there are three types of pores: micropores (dp ≤ 2 nm), mesopores (dp ≤ 50 nm), and macropores (dp > 50 nm) [107,144]. Adsorbed throughout the reaction are the dissolved organic molecules in these holes that have the right size to fit into the catalyst pores.

Numerous corner-sharing TO₄ tetrahedra create rings that serve as the opening windows of micropores [72]. The number of TO₄ tetrahedra in the appropriate ring determines the size of each window. The most commonly seen windows are the 8-, 10-, and 12-rings, which are made up of 8, 10, and 12 TO₄ tetrahedra, respectively. A hierarchical structure is generated if a zeolite crystal has mesopores or macropores in addition to its inherent micropores [145,146]. There are a plethora of different varieties of zeolite material when you combine the variations in zeolite framework types, chemical compositions, extra-framework species, and porous topologies [147].

Powder X-ray diffraction (XRD) is an efficient technique used to identify the crystallographic structure of soiled states [148,149]. It is used to study the crystal structure of zeolite, crystal size of zeolites, and presence of defect in zeolites. In the context of zeolites, its applications encompass the examination of crystal structures and the distinction between amorphous and crystalline entities. On the other hand, X-ray diffraction cannot be used to obtain information about the structural environment, or chemical surrounding, of an atomic nucleus present within a zeolite framework or outside of one [150]. Instead, atomic nuclei can be studied using the nuclear magnetic resonance (NMR) technique. Nonetheless, the framework Si/Al ratio determined from the ²⁹Si-spectra using the NMR approach is consistently reported to be more accurate than any other computed ratio from other characterization techniques that are currently available.

Zeolites are intricate materials characterized by high concentrations of silicon dioxide (SiO₂) and aluminum oxide (Al₂O₃). Consequently, the ratio of silicon to aluminum (Si/Al) is of paramount significance, as it affects the operational characteristics and behavior of a zeolite by influencing the density, quantity, and strength of acid sites, along with the inherent nature and stability of the zeolite. The application of non-destructive analytical methodologies, such as X-ray fluorescence (XRF), is instrumental in determining the Si/Al ratio [151]. XRF evaluates the elemental composition of the sample by examining the interactions of atoms with X-ray radiation. Thus, the identification and/or quantification of elements within any sample is contingent upon the energy emitted from that specimen. In addition, the EDX technique is utilized for elemental analysis as it can yield information regarding the overall Si/Al ratio present in zeolite [152].

Zeolite structures typically exhibit significant properties, which can be succinctly outlined as follows [153,154]:

• The stability of the crystal structure remains intact upon dehydration (i.e., the elimination of water from the zeolite crystals), a trait commonly observed in numerous zeolite types, with dehydration occurring at temperatures below 400 °C.
• The adsorption of gases, vapors, and various molecules within the microporous channels is facilitated by their adequate size to accommodate guest species. Alongside a substantial void volume, the majority of zeolite materials are characterized by low density and uniformly sized molecular channels.
• Additionally, a range of physical properties such as electrical conductivity, cation exchange, and catalytic capabilities are observed in zeolite materials.

8. The Zeolite Features

Zeolites are now the go-to catalyst for many significant industrial processes for a number of reasons. The qualities listed below call for extra consideration:

(a)

Catalytic activity:

Zeolites exhibit much higher acidity than the earlier amorphous silica–alumina catalysts [155,156]. The formation of acid sites occurs when trivalent aluminum replaces the tetrahedral silicon atom in the zeolite lattice, creating a negative charge that must be counterbalanced by a cation [157]. “Brønsted acid sites” (BAS) are formed by proton compensation, whereas Lewis acid sites (LAS) are formed when metallic cations are present as compensating species. Furthermore, extra-framework aluminum species (EFAL) that are present and cannot be completely removed during the synthesis of aluminosilicate are another source of Lewis acidity [158]. The reactivity of “zeolite catalysts” can be tailored by introducing heteroatoms either into the framework or at the extra-framework positions that gives rise to the formation of versatile Brønsted acid, Lewis acid, and redox-active catalytic sites [159].

Alkali metal ions are frequently added to zeolite syntheses in order to balance the framework’s charge [66]. Without a doubt, the zeolites cannot function as a catalyst in their current state and can be converted to a protonic form in order to accomplish the intended result. While protons can be used to directly replace the alkali metal, the optimal solution is thought to include substituting ammonium ions for the alkali metal. As seen in Figure 6, the resultant ammonium ions are next heated to 500–600 °C to drive off ammonia and create the proton [82].

The distribution, concentration, and strength of the catalytic activity of acidic zeolites is primarily determined by three factors, namely, “Brønsted acid sites” (BAS) [160,161]. The catalytic activity of BASs is also significantly influenced by the dimensions and shape of the pores and/or empty spaces that they are confined to. The density, spatial configuration, and local environment of Al atoms inside the zeolite framework all have a direct bearing on the reactivity of BAS. In addition to influencing Bronsted acidity, Lewis acidity is a crucial characteristic that characterizes the wide range of catalytic uses for zeolite materials. Either by changing the lattice composition or by stabilizing reactive complexes at zeolite cation sites, the particular Lewis acidic sites can be added to the zeolite matrices. The density of these acid sites is related to the silica-to-alumina ratio in the zeolite framework. With an increase in aluminum content, this density rises. However, the strength of the isolated sites increases and reaches a maximum when the silica-to-alumina ratio rises, and the number of sites falls. The acidity of a zeolite is generally influenced by the following factors [162,163]: the type or nature of the acid sites, their density or concentration, their strength distribution, where they are located within the zeolite framework, and their geometric distribution over the zeolite crystals.

Different techniques such as Fourier transform infrared (FTIR) spectroscopy and the Brønsted and Lewis sites in a solid acid system have been measured using a variety of techniques, including thermogravimetry, UV–visible spectroscopy, temperature-programmed desorption (TPD) of amines, microcalorimetry, and solid-state NMR spectroscopy; however, most of these methods struggle to distinguish between the two [164,165]. In other words, a mixture of two or more approaches is needed because no one characterization can clearly offer all the information.

(b)

Shape selectivity:

There are size limitations on the reactants, products, or transition state intermediates since a catalyzed chemical reaction usually occurs inside the zeolite pores, internal channels, or cavities [166]. Thus, the shape selectivity phenomena must be strongly influenced by the maximal free pore diameters (Ø). The selectivity of the processes that the zeolite frameworks catalyze is strongly influenced by the size of the pores inside them, which are frequently comparable to the sizes of the molecules involved in those reactions [146,167]. Zeolites come in a variety of structural forms, such as BEA, MFI, MWW, FAU, with varying pore sizes as well as variations in the quantity and arrangement of channels [125,153]. High-silica zeolites are very selective in alkylation processes due to their large pore volume, high acidity, superior hydrothermal stability, and molecular sieve characteristics. However, controlling the product distribution of zeolite as a catalyst is still confronting great challenges and applications.

Weisz and Csicsery [168,169] have demonstrated that there are three primary categories for zeolite shape selectivity, each having a mechanism illustrated in Figure 7. Restricted transition state selectivity occurs when certain reactions are prevented because the corresponding transition state would require more space than is available in the cavities [103]. Neither reactant nor potential product molecules are prevented from diffusing through the pores. Smaller transition state reactions continue uninterrupted. When just a portion of the reactant molecules are tiny enough to diffuse through the catalyst pores, reactant selectivity happens [170]. On the other hand, the interaction will not happen if the beginning material is too big to fit through the aperture diameter. Product selectivity occurs when some of the products formed within the pores are too bulky to diffuse out as observed products [66]. Using this kind of shape selectivity has a number of disadvantages, particularly when the molecule size is greater than the aperture size of the pore; in this case, the product is still inside the pore, which causes the formation of cock, side products, and catalyst deactivation.

The micropore volumes and surface areas of the zeolites were calculated from the nitrogen isotherms recorded at 77 K [172]. Measurements of surface area using Brunauer, Emmett, and Teller (BET) are typically based on the gas adsorption–desorption isotherm phenomena [173]. The same idea underlies both procedures, although one is carried out in reverse. When gas molecules come into contact with a solid material’s surface, adsorption occurs and an adsorbate layer is created [174,175]. Adsorption refers to a volume rather than a surface because absorption, in which molecules flow into a liquid or solid to produce a solution, is distinct from adsorption, which takes up the gas atoms by the solid surface, such as the accumulation of N₂-gas molecules on the zeolite surface. To put it simply, sorption is the process that includes both adsorption and absorption at the same time.

(c)

Thermal stability:

A new parameter, namely, the stability index, was introduced to quantify the thermal stability of zeolites. One of the most important reasons behind the selection of zeolites as catalysts for high-temperature reactions is their good thermal stability [176]. The significance of the Si/Al ratio in regulating thermal stability of zeolites was validated by the association seen between the stability index and the ratio. Zeolites with a Si/Al ratio of 3.80 or less are reported to be highly stable; zeolites with a ratio of 1.28 or less are quite unstable; and (iii) it is not possible to anticipate the stability of zeolites in the intermediate Si/Al range based only on the Si/Al ratio [177].

Depending on temperature, up to 50% of the aluminum initially present in the zeolite structure was lost to form extra-framework species that restrict the diffusion of reactants and products inside the catalyst particles [178]. Nonetheless, temperatures as high as 650 °C have no effect on the majority of zeolites. Only at temperatures as high as 1000 °C does structural collapse become serious for zeolites with high silica-to-alumina ratios [179,180]. Moreover, zeolite’s thermal stability is raised by the addition of rare earth ions. As a result, these zeolites work well in FCC processes, which frequently include high temperatures and other challenging working circumstances.

9. Role of Coke and Catalyst Deactivation

Catalysts are used in more than 80% of chemical processes across a range of industrial industries, either to increase reaction speeds or to specifically synthesize certain chemicals [181]. The inevitable deactivation of industrial catalysts over time on stream, which is followed by a decrease in catalytic activity and product selectivity, is one of their disadvantages. Over time, the deactivation of heterogeneous catalysts leads to a reduction in catalytic rate and is a common issue [182]. The majority of catalysts incorporating zeolite deactivate rapidly [57], a process linked to the development of high-molecular hydrocarbons that obstruct the surface’s active centers and porous structure [183]. Prior research has indicated that pentasil-type zeolites, characterized by a narrow pore size, encourage the production of branched hydrocarbons less and in lesser quantities than highly porous zeolites [184]. Furthermore, zeolites deactivate rapidly, and at 200 °C, they facilitate the synthesis of resins and aromatic, polycyclic hydrocarbons [185,186]; the catalyst is deactivated by the resin formation, which is why it is unwanted. Therefore, special consideration must be given to catalyst deactivation in relation to the setup and operation of the catalysis process.

Despite the large number of mechanisms involved in solid catalyst deactivation, they can be categorized into six intrinsic mechanisms of catalyst decay: (1) poisoning; (2) fouling; (3) thermal degradation; (4) vapor compound formation and/or leaching along with transport from the catalyst surface or particle; (5) vapor–solid and/or solid–solid reactions; and (6) attrition/crushing [60,187]. Deactivation has three main causes: chemical, mechanical, and thermal. Mechanisms 1, 4, and 5 are chemical in nature, whereas mechanisms 2 and 6 are mechanical.

Table 1. Catalyst deactivation mechanisms, adapted from [182].

Mechanism	Type	Brief Definition/Description
Poisoning	Chemical	Strong chemisorption of species on catalytic sites which block sites for catalytic reaction
Fouling	Mechanical	Physical deposition of species from fluid phase onto the catalytic surface and in catalyst pores
Thermal degradation and sintering	Thermal Thermal/chemical	Thermally induced loss of catalytic surface area, support area, and active phase-support reactions
Vapor formation	Chemical	Reaction of gas with catalyst phase to produce volatile compound
Vapor–solid and solid–solid reactions	Chemical	Reaction of vapor, support, or promoter with catalytic phase to produce inactive phase
Attrition/crushing	Mechanical	Loss of catalytic material due to abrasion; loss of internal surface area due to mechanical-induced crushing of the catalyst particle

provides a brief definition of each of the six fundamental mechanisms. The subsequent subsections address each mechanism in more detail, with a focus on the first three. Since mechanism 4 is a subset of mechanism 5, they are discussed together [182].

Solid catalyst deactivation can be caused by a variety of intrinsic mechanisms, but the main six can be broadly classified as follows [188]: (1) poisoning from certain species’ chemisorption on the active sites; (2) fouling from coke deposition; (3) thermal degradation; (4) vapor compound formation and/or leaching accompanied by molecular transport from the catalyst surface; (5) vapor–solid and/or solid–solid reactions; and (6) attrition and/or crushing. While some of these methods may only result in a brief loss of catalytic activity, others may induce irreversible deactivation.

Coke deposition can lead to catalyst deactivation in industrial catalytic processes by obstructing pores and/or covering acid sites [45]. It is very desirable to regenerate inactive catalysts in order to remove the coke and restore catalytic activity at the same time. The removal of coke may be achieved by a variety of chemical reactions and techniques, but industrial practice still struggles with the development of cost-effective, dependable regeneration strategies for catalytic processes [36].

Coke has the potential to impact catalytic activity in two ways: by causing pore blockages, which prevent reactants from accessing active sites, or by covering active sites and poisoning them [23]. A lot of effort to date goes into coke prevention to extend catalyst lifetime, but catalyst deactivation is still inevitable [189].

10. Catalysts Regeneration (Deactivated by Coke Deposition)

It is very desirable to regenerate inactive catalysts in order to remove the coke and restore catalytic activity at the same time [190]. Despite the fact that coke may be removed using a variety of chemical reactions and techniques, industrial practice still struggles with the development of cost-effective, dependable regeneration strategies for catalytic processes [181]. Regeneration techniques differ according to the catalytic processes involved, since coke type is determined by catalyst and reaction circumstances [191]. Furthermore, the pace at which coke forms has a significant impact on how long it takes to renew a particular catalyst. If the coke creation is quick, then continuous catalyst regeneration becomes economically necessary [182].

In the petrochemical sector, deactivation of catalyst by coke deposition is a key problem. In addition to the chemical processes involved in coke production, coke species are also retained in the pores and on the outside of catalysts [45,192]. Despite the fact that coke may be removed using a variety of chemical reactions and techniques, industrial practice still struggles with the development of cost-effective, dependable regeneration strategies for catalytic processes [181]. The three most frequent regeneration techniques for removing coke were hydrogenation (hydrogen), gasification (carbon dioxide and water vapor), and oxidation (air, ozone, and oxynitride) (see Figure 8) [193]. Each technique required the design and optimization of catalysts as well as associated processes.

The type of catalyst used during air combustion determines how quickly coke is removed. When Magnoux and Guisnet [47] examined the rates of coke oxidation of several catalyst types in air, they discovered that HY and H-mordenite had quicker rates of coke oxidation than HZSM-5. Since the pore structure of catalysts influences O₂ diffusion, which in turn changes the contact between coke and O₂, they concluded that the catalysts’ pore structure is responsible for the rate difference. This leads to shape selectivity in the coke oxidation process. During regenerations, acid sites can have an impact on coke removal [194]. The regeneration of coked HZSM-5 ethylbenzene conversion catalysts in air was investigated by S. Jong et al. [195]. They discovered that coke deposited close to the “Brønsted acid sites” in intracrystalline channels was eliminated before that on the outside. In this instance, coke oxidation transformed alkylated polyaromatic carbonaceous molecules into a more condensed aromatic structure. The density of acid sites, or HY framework aluminum atom density, has the ability to modify the rate of coke oxidation, as demonstrated by Guisnet et al. O₃ can remove coke from catalysts at a considerably lower temperature (50–200 °C) because of its potent oxidizing qualities. Even though coke oxidization is frequently used to renew industrial catalysts that have become deactivated, a significant quantity of CO₂ is produced, and the coke itself is worthless. One greenhouse gas that significantly affects the environment, such global warming, is CO₂. Utilizing gasification with CO₂ or H₂O to renew the used catalyst is an additional strategy to lower CO₂ emissions [181].

11. Benefits of Coke

Catalyst deactivation by coking is generally linked to carbonaceous or hydrocarbonaceous deposits that accumulate on the surface of heterogeneous catalysts during reaction [32,196]. Nonetheless, in a few circumstances, these deposits could improve catalytic activity. This includes the direct catalytic action of coke deposits, as in the case of alkane dehydrogenation reactions; the selective deactivation of nonselective surface sites, leading to an increase in catalytic selectivity; and the involvement of deposits in the reaction mechanism, such as the transfer of hydrogen and hydrocarbons and the well-established hydrocarbon pool in the conversion of methanol to hydrocarbons.

Coke can be advantageous in a number of ways in heterogeneous catalytic processes [197]. For example, coke deposits may improve selectivity by selectively poisoning non-selective active sites or by enhancing the shape selectivity of zeolite pores. Alternatively, coke may be catalytically active, creating new active sites for the reaction in situ. Alternatively, the coke may serve as an intermediary in a number of processes or help transport hydrogen or hydrocarbons. The following describes the several advantageous functions that coke deposits may play in a variety of processes.

When coke is deposited within or on the surface of the pores, Guisnet and Ribeiro [187] showed that the selectivity rises to para isomers of alkylation by utilizing zeolite HMFI. The primary benefit of coke over zeolite ZSM-5 in the alkylation of toluene and methanol to produce para-xylene, according to Keading and colleagues, is that the catalyst surface coating of carbonaceous polymers increases selectivity from 24% to around 90% [198]. Furthermore, because the coke serves as active sites, Enchigoya and his colleagues argue that the catalyst activity rises with time on stream [199].

12. Coke Characterization

The formation of coke deposits leading to catalyst deactivation has been a challenge for catalytic technology in many hydrocarbon processes [200]. The effective management of catalyst deactivation and catalyst regeneration is the key in many heterogeneously catalyzed processes. The optimization of such complex processes requires the characterization of the coke deposits in order to understand the effect of the operational variables on these deposits and, therefore, minimize its formation and develop effective regeneration strategies. Coke characterization has been included in many papers where deactivation is a major issue. In the case of acid-catalyzed hydrocarbon reactions on “zeolite catalysts”, the last few years has seen a growing interest in this field due to zeolite catalysis’s basic research and practical applications [201]. Several characterization techniques have been used to study coke deposits and to obtain information regarding reaction mechanism, deactivation mechanism, and regeneration conditions.

Temperature-programmed oxidation (TPO) is one of the most often utilized methods. This approach is commonly utilized for characterizing coke in a wide range of catalytic systems due to its ease of use and practicality [202]. Since it offers useful data on coke composition (hydrogen to carbon ratio, or H/C) and the kinetics of coke generation, as well as coke type, shape, location, distribution, and content, it is regarded as both a quantitative and qualitative measurement [32]. Temperature-programmed combustion of coke has, in fact, been successfully applied to coke investigation on the H-forms of several zeolites and has proven to be an effective tool for distinguishing between two types of coke: highly condensed, polyaromatic residues (coke-type II, high-temperature coke), the oxidation of which begins at temperatures of approximately 700 K, and hydrogen-rich carbonaceous residues (coke-type I, low-temperature coke) which are burned at temperatures of approximately 600 K [201].

Clarifying the structure of an unknown chemical compound starts with elementary analysis (CHNS) [201]. Similarly, even though coke’s composition can be complicated and often consists of elements with varying hydrogen contents, the first step in identifying coke residues should be determining the H/C ratio of carbonaceous deposits on catalyst surfaces. Furthermore, the assessment of the nitrogen and sulfur content should be part of the basic analysis of coke on zeolite. In order to calculate the H/C ratio, samples containing coke are typically heated to 1000 °C in the presence of pure oxygen [203]. There is aneed for further information on carbonaceous residues and the wide range of H/C ratios seen in coking investigations. However, the coke’s temperature dependence of content and H/C ratio indicate different types of coke species: formation of hydrogen-deficient residues, or coke-type II (also known as “high-temperature coke”, “hard coke”, or “black coke”) with H/C ratio < 0.8 typically formed at higher temperatures (>550 K), and deposition of hydrogen-rich coke species, or coke-type I (also known as “low-temperature coke”, “soft coke”, or “white coke”) with H/C ratio > 1.0, which are primarily formed at lower reaction temperatures (<500 K).

A useful method for figuring out a specimen’s mass variations over a certain period of time and temperature range is thermogravimetric analysis (TGA) [204,205]. It is used to calculate the temperatures at which polymers degrade, residual solvent levels, absorbed moisture content, and breakdown kinetics. Additionally, it is used to give details on the kind, quantity, and composition of coke. One major element influencing catalyst deactivation is the kind of coke, such as soft or hard coke [32]. The TG and DTG diagram of the spent catalyst decoking, which was obtained during the toluene alkylation process using 1-heptene over H-beta “zeolite catalyst”, is displayed in Figure 9. It has two primary peaks: the soft coke area is found in the first zone, which is between 200 and 400 °C, and the hard coke region is found in the second zone, which is between 400 and 800 °C.

One of the earliest methods used to thoroughly clarify the mechanics of catalyzed reactions was Fourier transform infrared (FTIR) spectroscopy [206,207]. It is possible to gather details on the transition of active centers, the creation of adsorbed intermediates, and the accumulation of carbonaceous deposits during hydrocarbon processes. One of the primary methods used to determine the aliphatic, olefinic, and aromatic chemical components of coke deposits on the catalyst during hydrocarbon reactions is this one [200]. FTIR has numerous benefits, including the ability to examine the mechanism of reaction and coke production with sufficient information and the ability to preserve the sample after analysis [208].

Spectral areas [201]: where carbonaceous deposits deposited on “zeolite catalysts” are studied include stretching modes of free acidic and nonacidic OH groups, or those interacting with adsorbates, range from 3000 to 3800 cm⁻¹; 3000–3200 cm⁻¹ (aromatic hydrocarbon stretching modes). CH stretching modes of paraffinic species range in length from 2800 to 3000 cm⁻¹, while CC stretching modes of unsaturated hydrocarbons and CH bending modes of paraffinic species are between 1300 and 1700 cm⁻¹.

Among the many techniques that have been employed to study coking, NMR occupies a particularly important place because of the many nuclei that can be involved and the development of high-resolution solid-state NMR [209,210]. Catalyst deactivation may be investigated by NMR via the detection of coke-forming nuclei (¹H, ¹³C, ¹⁵N), via zeolite modification due to coke deposition (¹H, ¹⁷O, ²⁷Al, ²⁹Si) and via probe molecules (¹H, ¹³C, ³¹P, ¹²⁹Xe).

13. Conclusions

Given the great importance of aromatic alkylation reactions due to their wide industrial application in various chemical production fields in bulk and fine quantities, the Friedel–Crafts alkylation reactions are the most significant process for the synthesis of alkylated aromatic compounds. Zeolites have different types of framework structures. In this study, the focus was on keeping pace with the development in the field of using zeolites in aromatic alkylation and isomerization reactions that occur simultaneously and taking into account the reasons that made zeolites become the preferred catalysts for many reactions of industrial importance due to their properties such as thermal stability, acidity, and appropriate selectivity. On the other hand, it is noted that heterogeneous catalytic processes result in coke deposition, which leads to the deactivation of the catalyst during the reactions that take place on its surface, as the coke covers the acidic sites or blocks the pores. To solve this problem, it is highly recommended to regenerate the deactivated catalysts to remove coke and restore catalytic activity at the same time, which prolongs the life of the catalyst and reduces production costs. This study has clarified the mechanisms of coke formation, catalyst poisoning, and methods of testing and treating them. It has been concluded that any type of catalyst, deactivation processes, and reactivation settings depend mainly on the nature of the catalyst’s structural framework and the reaction conditions under which it operates. Each regeneration approach has certain pros and cons that have been cautiously explained to represent a summary addition to the published literature and will be used later by researchers. In addition, the progress in catalysis using zeolites cannot be stopped, and there are current and future trends to take further steps towards greener and more powerful catalytic processes and technologies based on zeolites and related materials, making them less expensive and more environmentally friendly.