Hydroisomerization Catalysts for High-Quality Diesel Fuel Production

Abstract

Upgrading the properties of diesel fractions is considered one of the crucial processes in the petrochemical industry; and for this purpose in laboratory-scale researching it is studied on the base of the hydroisomerization of n-hexadecane as a main model reaction. Recently, zeolite-based bifunctional catalysts have proven their efficiency due to their remarkable acidity, shape-selectivity and relative resistance to deactivation. In this review, different topological-type zeolite-based catalysts, the mechanism of their catalytic effect in n-C₁₆ isomerization, and the principles of shape-selectivity are reviewed. A comparison of their structural-operational characteristics is made. The impact of some feedstock impurities on the catalyst’s performance and deactivation due to carbonaceous deposits as well as various modern eco-friendly cost-effective synthesis techniques are also discussed.

1. Introduction

Currently, a key focus in the global production of petroleum products is to raise the standards for motor fuels. The inadequate quality of such fuels is a leading factor contributing to environmental pollution. To address this issue, improving the chemical composition of the fuel is used to meet high quality requirements [1,2]. One of the most promising technologies is isodewaxing process. Instead of removing n-paraffins by solvent extraction, the is the odewaxing technology modifies the molecular structure of the wax through catalytic isomerization, producing isoparaffins with superior cold flow properties, high viscosity index, and excellent oxidation resistance.

The isodewaxing process comprises four reactions: (i) isomerization of the normal paraffins to obtain their isomers, (ii) hydrocracking of the long-chain paraffins into shorter-chain ones, (iii) hydrogenation of the unsaturated aromatic components, and (iv) hydrodecyclization with the ring-opening reaction. In this review we will focus on the first type of reaction, which usually occurs in the presence of hydrogen, so it is referred to as hydroisomerization. Usually, this reaction is accompanied by hydrocracking in which the extent varies according to the operational conditions of the process and the characteristics of the catalyst used. The rates of hydroisomerization and hydrocracking reactions define the composition of products.

Considerable research efforts have been dedicated towards the production of value-added products from low-grade waxy feedstocks (Figure 1). In particular, the focus has been on converting these feedstocks into branched products to improve the fuel quality. These researches have been centered on catalysts development, with a specific emphasis on micro/mesoporous catalysts to accelerate mass transfer and optimize the residence time of reagents on the active sites, thereby increasing their efficiency in the desired chemical reactions [3,4,5,6].

Hydroisomerization catalysts are generally categorized as bifunctional catalysts, composed of a metal, typically platinum, that is dispersed on an acidic support. The metal serves as a dehydrogenation/hydrogenation agent, while the acidic support aids in the skeletal isomerization reaction of the olefinic intermediates created over the metal sites and in cracking [7,8]. A well-balanced bifunctional catalyst promotes hydrogenation and dehydrogenation reactions, with rearrangements of the hydrocarbon intermediates over acid sites dictating reaction rates [9]. As a result, hydroisomerization and hydrocracking reactions occur in sequence, leading to mono- and multibranched isomers, and cracked paraffins [10,11]. One of the primary challenges in advancing the hydroisomerization technology is the need for highly effective and selective bifunctional catalysts that can minimize the formation of byproducts resulting from hydrocracking [12,13].

In this review, the latest findings in improving the isomerization processes of linear alkanes through the development of catalyst structures, methods of their preparation, and studies related to the operational conditions of the process will be discussed. A comprehensive analysis will be conducted to compare the performance of various catalysts during the hydroisomerization of n-C₁₆. The operating conditions of the hydroisomerization of n-C₁₆ will be discussed in details.

2. Straight-Chain Alkane Hydroisomerization Process

2.1. The Principle of the Process and Its Technical Significance

Hydroisomerization of diesel fraction is considered as a vital chemical process that includes the hydroconversion of normal paraffins into their branched isomers. This reaction comprises breaking and rearranging C-C bonds to obtain more favorable isomers providing the higher cetane number of diesel, thus reducing engine emissions and improving the cold flow characteristics of fuels, making them less prone to clogging at low temperatures.

As shown in Table 1, the presence of linear alkanes improves the cetane number of diesel, however, the melting point also increases. Given the importance of diesel quality, maintaining an adequate cetane number is crucial. According to the 2019 Worldwide Fuel Charter, the minimum cetane number equals 51.0 and up to 55.0 for some markets having high requirements for emission control [14].

It should be noted that branched-chain paraffins typically have lower cetane numbers than their linear counterparts. Among the mono-branched isomers with alkyl substituents or with methyl groups at the end of the hydrocarbon chain, the latter not only have lower melting points but also higher cetane numbers (

Table 2. Characteristics of hexadecane isomers as a fuel component [PubChem database]; [16].

Isomer	Chemical Structure	Boiling Point a, °C	Flash Point a, °C	Melting Point a, °C	Cetane Number
Mono-isomers 2-methylpentadecane		282.0	95.3	−9.2	100
8-methylpentadecane		274.3	94.0	−25.6	75
5-butyldodecane		150.7 b	N/A	−47.4	45
Di-isomers		248.0	N/A	−37.0
(3R,4R)-dimethyltetradecane					61
(3R,4S)-dimethyltetradecane					64
(3S,4R)-dimethyltetradecane					58
(3S,4S)-dimethyltetradecane					62
(5R,6R)-dimethyltetradecane		N/A	N/A	N/A	51
(5R,6S)-dimethyltetradecane					51
(5S,6R)-dimethyltetradecane					48
(5S,6S)-dimethyltetradecane					52
7,8-dimethyltetradecane		270.0	−86.2	−86.2	40
Multi-branched isomers 2,2,4,4,6,8,8-heptamethylnonane		247.0	102.0	N/A	15

^a Measured at 750 mmHg. ^b Measured at 20 mmHg.

). Therefore, the selective hydroisomerizing n-alkanes to these isomers can produce diesel with improved cold flow properties and a shorter ignition delay [15].

Table 1. Characteristics of n-alkanes C₃-C₁₈ as a fuel component (sourced from PubChem database [16]).

Alkane	Chemical Formula	Boiling Point (°C)	Melting Point (°C)	Flash Point (°C)	Cetane Number
n-propane	C3H8	−42.0	−187.6	−104.0	−20.0
n-butane	C4H10	0.5	−138.3	−60.0	20.6
n-pentane	C5H12	36.0	−129.0	−40.0	30.0
n-hexane	C6H14	69.0	−95.4	−22.0	47.9
n-heptane	C7H16	98.4	−90.6	−4.0	56.0
n-octane	C8H18	125.6	−56.7	13.0	64.4
n-nonane	C9H20	150.5	−53.5	31.0	60.9
n-decane	C10H22	174.1	−29.7	46.0	65.5
n-undecane	C11H24	195.9	−25.5	62.0	79.0
n-dodecane	C12H26	216.3	−9.6	83.0	72.9
n-tridecane	C13H28	235.4	−5.4	94.0	88.0
n-tetradecane	C14H30	253.6	5.9	100.0	95.0
n-pentadecane	C15H32	270.6	10.0	N/A	98.0
n-hexadecane	C16H34	286.9	18.2	135.0	100.0
n-heptadecane	C17H36	303.0	22.0	149.0	105.0
n-octadecane	C18H38	316.0	28.2	166.0	110.0

Table 1. Characteristics of n-alkanes C₃-C₁₈ as a fuel component (sourced from PubChem database [16]).

Alkane	Chemical Formula	Boiling Point (°C)	Melting Point (°C)	Flash Point (°C)	Cetane Number
n-propane	C3H8	−42.0	−187.6	−104.0	−20.0
n-butane	C4H10	0.5	−138.3	−60.0	20.6
n-pentane	C5H12	36.0	−129.0	−40.0	30.0
n-hexane	C6H14	69.0	−95.4	−22.0	47.9
n-heptane	C7H16	98.4	−90.6	−4.0	56.0
n-octane	C8H18	125.6	−56.7	13.0	64.4
n-nonane	C9H20	150.5	−53.5	31.0	60.9
n-decane	C10H22	174.1	−29.7	46.0	65.5
n-undecane	C11H24	195.9	−25.5	62.0	79.0
n-dodecane	C12H26	216.3	−9.6	83.0	72.9
n-tridecane	C13H28	235.4	−5.4	94.0	88.0
n-tetradecane	C14H30	253.6	5.9	100.0	95.0
n-pentadecane	C15H32	270.6	10.0	N/A	98.0
n-hexadecane	C16H34	286.9	18.2	135.0	100.0
n-heptadecane	C17H36	303.0	22.0	149.0	105.0
n-octadecane	C18H38	316.0	28.2	166.0	110.0

From a thermodynamic point of view, the formation of iso-paraffins is more favorable at lower temperatures; therefore, to achieve better selectivity towards isomers, catalysts are usually designed for low temperature exploitation [17,18].

2.2. Illuminating the Pathways of Molecular Transformation

Zeolites composed entirely of silica have a neutral charge, however, if Si⁴⁺ is replaced with Al³⁺, the framework becomes negatively charged. Protons can neutralize the framework charges, resulting in the development of acid sites that accelerate the main processes of oil refining: hydrocarbon isomerization and cracking. Based on the zeolite topology and acidity, these processes can yield desirable products by converting linear alkanes into diverse branching isomers and breaking down higher hydrocarbons into smaller ones [19]. Before digging into the influence of shape selectivity on these reactions, it is crucial to look at the case where shape selectivity is omitted.

Conventional hydroisomerization proceeds with the formation of an olefinic intermediate via dehydrogenation on the metal site. As a result of the highly endothermic nature of the dehydrogenation step, the concentration of olefins in the hydroisomerization reaction mixture is typically low. Thus, it is crucial to convert these olefinic compounds promptly to carbocations on the acid sites of the catalyst to ensure efficient conversion. Due to the weak basicity of the hydrocarbons, the equilibrium concentration of carbocations on the acid sites of the zeolite is relatively low [20,21].

According to Jacob et al. [22], there are two types of carbocations:

• Tricoordinated carbenium ion;
• Pentacoordinated carbonium ion (extremely unstable).

Carbenium ions can be formed via three distinct routes depending on feedstock properties and the catalyst acidity. These routes include (i) hydride elimination on Lewis acid sites (Figure 2a), which can occur through a variant pathway of the carbenium ion R⁺ adsorbed on active sites of the zeolite, known as “bimolecular hydride transfer” (Figure 2b); (ii) alkane protonation followed by hydrogen abstraction (Figure 2c); and (iii) olefin protonation, leading to the formation of an alkylcarbenium ion (Figure 2d). The last route comprises the protonation of olefinic intermediates instead of the energetically unfavorable direct protonation of alkanes. Thus, the isomerization via route (iii) accelerates as compared to the direct activation of the alkanes [22].

As per the classical bifunctional mechanism of the isomerization, the metal component of the catalyst facilitates the dehydrogenation of alkanes to form alkenes that are protonated to transitional alkylcarbenium ions on the Brønsted acid sites. These ions undergo rearrangement and scission reactions to obtain ultimately obtain, after hydrogenation on the metal sites, the saturated product—mono-substituted isoalkanes. In the case of availability of potent Brønsted acid sites, mono-substituted tertiary carbenium ions may undergo additional isomerization, finally leading to the formation of carbenium ions containing multi-branches [23].

Aluminosilicate zeolites that incorporate platinum or palladium metal typically operate based on the classical bifunctional mechanism (Figure 3), particularly at high conversions, elevated hydrogen pressure, and a sufficiently high carbon number of the feedstock. The pathway of converting hydrocarbons on metals often compete with the bifunctional mechanism, and the significance of the former increases in the case of more severe conditions and the weak acidity of zeolite. The presence of CH₄ and C₂H₆ in the final reaction products is indicative of metal-catalyzed cracking, a process commonly referred to as hydrogenolysis.

Weiss demonstrated that alkenes displayed high reactivity in the isomerization on Pt-free aluminosilicate, whereas alkanes did not [24]. Several studies have reported an increase in the isomerization rate with rising Pt content, which can be attributed to high concentration of the olefinic intermediate [25,26].

Due to the formation of strong covalent bonds, both the enthalpy of protonation (∆H_p) and the activation energy of the isomerization step (E_iso) are expected to be high, as a result of the deformation of the C-O-Z bond (Z-acid site). The skeletal isomerization reaction is postulated to occur through the rearrangement of the carbenium ion via a cycloalkyl intermediate. The resulting isoparaffinic carbenium ion is subsequently transformed into an olefin via the loss of proton at the acid site. Finally, the isoolefin intermediate is rapidly hydrogenated to the isoalkane product [27].

The rate of isomerization has been observed to be heavily influenced by the length of the alkane chain. The longer chains result in the greater stability of bound carbenium ions and the higher rate of the isomerization reaction. However, high isomerization selectivity becomes increasingly challenging as the chain length increase, due to adverse reactions, namely carbocation intermediate hydrocracking on acid sites and hydrogenolysis reactions on metal sites. The balance between hydrogenation and acid function is a key factor that governs the prevalence of acid- or metal-catalyzed reactions, ultimately determining the composition of products [28,29].

Three mechanisms of alkane cracking have been established: (a) non-catalytic thermal cracking that proceeds at high temperatures as radical process, (b) proteolytic α-cracking including carbonium ion and (c) β-cracking via the carbenium ion. The commonly acknowledged mechanisms of catalytic cracking in hydroisomerization conditions can be described using two fundamental steps: the formation of carbenium ions and β-cracking.

In contrast to acid-catalyzed cracking, the product of thermal cracking of alkanes via free-radical intermediates possesses a the distinct composition (

Table 3. Comparison of thermal and catalytic cracking processes.

Hydrocarbons	Thermal Cracking Products	Catalytic Cracking Products
n-Hexadecane	Primary products—alkanes C1-C3; significant quantity of n-olefins C4-C15; relatively low concentrations of branched aliphatics	Primary products—alkanes C3-C6; minor amounts of n-olefins; high concentrations of branched aliphatics
Aliphatics	Minor conversion to aromatics at 500 °C	Large conversion to aromatics at 500 °C
n-olefins	Slow sigmatropic rearrangement of double bond with a low isomerization	Rapid sigmatropic rearrangement of double bond with a significant isomerization conversion
Branched olefins	Non-selective hydrogen transfer and lower rate of cracking as compared to that of the corresponding alkanes	Selective hydrogen transfer and higher rate of cracking than that of the corresponding alkanes
Naphthenes	The rate of cracking is lower than that of the corresponding alkanes.	The rate of cracking is comparable to that of the corresponding alkanes.
Alkyl aromatics	Cracking happens within the side of alkyl functional groups	Cracking happens within the side of aromatic ring itself
Alkyl aromatics with larger functional groups	The rate of cracking is lower than that of the corresponding alkanes	The rate of cracking is higher than that of the corresponding alkanes

).

Typically, the formation of free-radical intermediates in the process is of kinetic significance at temperatures higher than 530 °C [30,31].

According to the traditional catalytic cracking mechanism (also known as the carbenium ion mechanism), an alkane is subjected to hydride abstraction by a carbenium ion, resulting in the formation of another carbenium ion, which then undergoes β-scission, or the cleavage of the C-C bond β-situated to the trivalent carbocation [32]. Carbenium ions are known to act as chain carriers, with the most stable ones, typically tertiary ions like the t-butyl cation, being the predominant carriers [33,34].

3. n-Hexadecane Hydroisomerization on Bifunctional Catalysts

3.1. Development of Hydroisomerization Catalysts

To improve the formation and isomerization of the carbocation intermediate in the hydroisomerization reaction, the catalyst must have a significant acidity [35,36]. Since the discovery of aluminum chloride in 1933 as a catalyst for hexane and heptane isomerization, four generations of hydroisomerization catalysts have been developed over the course of several decades (

Table 4. Catalysts generations for hydroisomerization of n-alkanes.

Generation	Catalysts Composition	Operating Temperature, °C	Advantages	Disadvantages
1st	Friedel-Crafts, AlCl3	80–100	High activity	Sensitivity to trace water; fast deactivation
2nd	Metal/support	350–500	Easy to apply; corrosion issues are mitigated; sensitivity problems are less severe	Thermodynamic limitations on conversion
3rd	Metal/halogenated support	120–160	Enhanced acidity due to halogenation (Cl, F) of alumina support	Sensitive to impurities; continuous feeding chlorine to maintain catalyst activity; corrosion of equipment
4th	Bifunctional zeolite-based catalysts	250–340	Easy to use; tolerant to poisons; no feedstock pretreatment	Selectivity issues; more expensive to produce

) [37,38,39]. Currently, only third and fourth generations are used in the industry, although the academic interest in these generations of catalysts has increased dramatically over the past 10 years (Figure 4), and this review focuses on the last generation. A critical challenge in the research and development of innovative hydroisomerization catalysts is to achieve high selectivity of isomerization and considerable n-paraffin conversion, constraining the formation of cracking byproducts. This parallel reaction reduces the yield of the intended products [40].

Typical zeolites-based catalysts are prone to poisoning and deactivation in the hydroisomerization process of linear alkanes due to the presence of sulfur-containing substances in the feed. Currently, the major developments in the design of catalysts for n-paraffin hydroisomerization are the high efficiency at temperatures 100–200 °C and tolerance to both sulfur and water.

Synthetic zeolites have been recognized as solid acid catalysts and are widely used in a variety of applications in industry, notably in petrochemical synthesis and oil refining [41]. The topology and morphology of the zeolites defines their suitability for the particular application process (

Table 5. The standard structural properties of zeolites [Database of Zeolite Structure].

Zeolite	Topology	Member Ring (MR)	Channel Dimensionality	Accessible Volume, %
ZSM-5	MFI	10, 6, 5, 4	3D	9.81
ZSM-12	MTW	12, 6, 5, 4	1D	9.42
ZSM-22	TON	10, 6, 5	1D	8.04
ZSM-23	MTT	10, 6, 5	1D	7.98
ZSM-35	FER	10, 8, 6, 5	2D	10.01
ZSM-48	MRE	10, 6, 5, 4	1D	6.55
SAPO-11	AEL	10, 6, 4	1D	6.77
Beta	BEA	12, 6, 5, 4	3D	20.52
Mordenite	MOR	12, 8, 5, 4	2D	12.27
EMM-23	EWT	21, 10, 6, 5, 4	3D	21.95

), which is defined by interlinked channel structures or a hierarchical structure that integrates micro- and mesopores, in addition to acid-base properties [42,43,44,45,46].

3.2. Analysis of the Relationship between Catalyst Characteristics and Activity

3.2.1. Shape-Selectivity

The study of the structural effect of zeolites on their selectivity deals with the concept of “shape-selectivity” that is classified into two basic categories: configurational selectivity and transition state selectivity (Figure 5). Typically, zeolites acquire the requisite shape selectivity at the expense of restricted mass transfer inside micropores to and from the active sites. To overcome this issue, several approaches have been used to improve the accessibility of active sites inside zeolites. One successful way is to take advantage of molecular shape selectivity, which acts inside zeolite’ channel systems and has shown higher efficiency and selectivity in various chemical reactions.

Configurational selectivity arose from the insight that molecule cannot pass through the channels of the zeolite. In the case where the molecule will not be adsorbed it will keep staying intact in the resultant mixtures (reactant shape selectivity) (Figure 5a). In the case where the molecule will not be desorbed, it can exist as an adsorption transitional-state, and only molecules originating from this state will be included in the product via the following reactions (product shape selectivity) (Figure 5b). In addition, the effective pore diameter may strongly affect the formation of transition-state intermediates and direct them towards forming states that fit its topology or towards participating in secondary intermediate interactions (transitional-state shape selectivity) (Figure 5c).

In some cases, transition state intermediates approximating the topological framework of the zeolite are quickly formed (inverse shape selectivity) (Figure 6a), this phenomenon was caused by sorption shape selectivity instead of transition state shape selectivity, as previously thought [47,48].

Transition state selectivity can be influenced by restrictions that could occur either on the surface or within the pores of the framework of the zeolite. In the locations where the paths of the pores happen to intersect, this case can affect the diffusion of the reactant molecules—whether due to geometric reasons or the characteristics of molecular diffusion. As a result, molecules of the specific reactant may enter one of the pores and do not enter to the other one (molecular traffic control) (Figure 6b). Otherwise, the ends of the straight-chain alkanes may enter simultaneously into two (or more) adjacent pores (Pore mouth & key-lock selectivity) (Figure 6c) [49,50,51,52,53].

In addition to the previous two classifications, some effects related to the interactions on the outer surfaces of zeolites were studied and formulated by Chen et al. [54], including the window or cage effect (Figure 6d), to understand how the shape selectivity of erionite affects the product distribution of cracking of linear alkanes (C₁₆H₂₂–C₃₆H₇₄). This phenomenon attempts to explain the differences in diffusion rates of n-paraffin having various chain lengths inside a particular zeolite structure, which are referred to as “up and down” diffusivity variations. Generally, as the chain length of the n-paraffin increases, its diffusivity within the zeolite structure decreases due to the higher steric hindrance [55]. Derouane [56] also formulated the nest effect (Figure 7), which tried to explain the non-shape selectivity of zeolite at transition-state intermediates formation that occur within the cavities located at the ends of the pores.

Various approaches have been established to manipulate the morphological characteristics of zeolites based on using and developing new structure-directing agents in order to gain a better distribution of products of n-hexadecane hydroisomerization. Several of these approaches have led to acceleration of hydrocarbons’ transport within the crystals, and eventually to decreasing the residence time of intermediate alkylcarbenium in microspores and the rate of secondary cracking.

Romero D. et al. [57] observed the effect of member ring (MR) size of the zeolite framework on the selectivity of n-C₁₆ hydroisomerization over 3D-10MR ZSM-5 (bulk and nanostructured), 1D-12MR ZSM-12, 1D-10MR ZSM-22 and extra-large intersecting pores (10MR + 21MR) EMM-23. Zeolite ZSM-5 having hierarchical structure and shorter intercrystalline length (higher diffusion rate) demonstrates an upgraded shape-selectivity for di-branched isomers (dimethyl) cracking owing to a snug fit of isomers at (straight-zigzag) channel intersections in the ZSM-5 framework. This shape of intersections is absent in the framework of ZSM-22; however, the cracking of di-branched isomers could be explained by the pore mouth type of shape-selectivity. The wide size of the 12MR pores provides an acceptable performance of ZSM-12, whilst in the case of EMM-23 the 10MR pores prevailed over the framework at the expense of 21MR, as a result of molecular defects of silanol groups.

Table 6. The morphological properties of zeolite-based catalysts of hydroisomerization prepared with using various organic structure-directing agents (OSDAs).

Zeolite Support Type	OSDAs	Si/Al	Specific Area a, m2·g−1	Total Pore Volume b, cm3·g−1	Total Acidity c, µmol/g	Ref.
ZSM-5	Tetrapropylammonium hydroxide	32	371	0.232	238	[58]
ZSM-12	Tetraethylammonium bromide (TEABr)	45 60 90 120	310 270 220 260	N/A	195 92 80 54	[59]
ZSM-12	Methyltriethylammonium chloride (MTEACl)	25	280	N/A	720	[60]
		40	293		530
		50	287		430
		75	314		300
		100	323		270
ZSM-12		45	301	0.160	601	[61]
ZSM-12	Tetraethylammonium hydroxide (TEAOH)	40	323	N/A	N/A	[62]
		50	331
		71	300
		100	291
ZSM-12	Benzyltrimethylammonium chlorid (BTMACl) TEABr Methyltriethylammonium bromide (MTEABr)	~90	260 220 260	N/A	107 80 115	[63]
ZSM-12	TEAOH MTEACl	61 94	258 380	0.120 0.230	N/A	[64]
ZSM-12	P-Phenylenedimethylene-bis(N-methylpyrrolidinium) dibromide P-Phenylenedimethylene-bis(N-methylpiperidinium) dibromide	20 50	297 306	0.370 0.340	898 550	[65]
ZSM-22	1,6-diaminohexane (+Organosilane) (+Silanized silica beads)	76 72 72	258 257 251	0.210 0.190 0.230	N/A	[66]
ZSM-22	1,6-diaminohexane (with acid treatment)	46	229	0.320	N/A	[67]
ZSM-22	1,6-diaminohexane	40 60 100	240 255 244	N/A	192 145 95	[68]
ZSM-22	6-amino-1-hexanol	35 50 100 150	110 137 134 144	0.210 0.190 0.140 0.120	150.2 121.7 83.2 59.1	[69]
ZSM-23	N,N-dimethylformamide	40	150	N/A	N/A	[70,71]
ZSM-23	Pyrrolidine (PY)	100 150	305 228	0.280 0.230	68 * 52 *	[72]
ZSM-23	Isopropylamine (IPA)	200	206	0.260	49 *
ZSM-23	Dual-OSDA (PY + IPA)	100 150 200	278 282 251	0.190 0.200 0.200	22 * 19 * 17 *
EU-1	Hexamethonium bromide	25	325	0.160	847	[61,73]
IM-5	1,5-bis(methylpyrrolidinium)- pentane bromide	33	338	0.257	289	[58]
IM-5 (micro- pores)		21	381	0.412	252
SAPO-11	Dipropylamine phosphate (DPA.H3PO4)	0.24	209	0.134	245.3	[74]
SAPO-31	Di-n-butylamine	0.6	222	0.144	57.9	[75]
BETA	(TEAOH)	20 30 40 50	648 614 591 553	0.341 0.314 0.277 0.271	850 830 580 490	[76,77]

^a Calculated by BET method. ^b Obtained using t-plot method at p/p₀ = 0.99. ^c Obtained using FTIR data of pyridine absorption considering bands at 1545, 1490, and 1445 cm⁻¹. * Obtained by NH₃-TPD considering peaks centered at 441–445 °C, 313–348 °C, and 220–230 °C.

shows ZSM-12 prepared using various types of organic templates giving different morphological and acidic properties of zeolite. For instance, ZSM-12 with high specific area (>300 m².g⁻¹) and good total acidity (~200 µmol/g) with low Si/Al ratios (~40) is synthesized applying template comprising tetraethylamonium cations (like TEABr and TEAOH). In contrast, samples prepared with MTEACl and MTEABr as templates have high specific area with Si/Al ratios near to 70, to increase their acidity up to 300–700 µmol/g. As for ZSM-22, the majority of researchers use 1,6-diaminohexane as the template, giving zeolite with good crystallinity and optimal textural properties. With regard to ZSM-23, it may be produced using template comprising amide and amine functional groups, however, the last type has proven its superiority with Si/Al = 100–150 despites of the weak acidity of the zeolite produced.

The wide pores are not always preferred, where it could allow secondary reactions to happen. For instance, 12MR zeolites have the capability to accommodate mono-, di- and multi-branched hydrocarbons. However, multi-branched hydrocarbons are prone to undergoing cracking reactions [78,79].

An optimal hydroisomerization catalyst provides a high yield of isomerized products alongside the decreasing of cracking products formation (Figure 8). As a result, 10MR zeolites-based catalysts have been identified as the preferred catalyst because they allow unhindered diffusion of monobranched hydrocarbons while discouraging diffusion of multi-branched hydrocarbons via shape selection, promoting superior isomerization selectivity. Zhang M. et al. [80] studied different 10MR zeolites (ZSM-22, -23, -35 and -48)—corresponding topologies (TON, MTT, FER and MRE respectively)—as catalysts’ supports n-C₁₆ hydroisomerization reaction. The products distribution of n-C₁₆ hydroisomerization over 10MR zeolites was significantly influenced by tiny distinctions in the channel configuration.

The predominated hydroisomerization product was primarily 2-methylpentadecane. This likely occurred due to fast diffusion of 2-methylpentadecane. 4-Methylpentadecane and 5-methylpentadecane were not observed in products obtained over the ZSM-22-based catalysts, due to the smallest pore size in zeolite of this type.

The hydroisomerization mechanism might be explained by the pore mouth reaction pathway due to the lack of detection of multi-branched isomers. These results are supported by the study of Zhang et al. [79], in which three different types of zeolites were tested (ZSM-12, ZSM-22 and Beta) as supports of 0.5 wt. % Pt catalysts. 1D-10MR- ZSM-23 gave the highest concentration of mono-branched C16 isomers, whereas ZSM-12 and ZSM-48 zeolites gave the highest concentrations of multi-branched isomers. These could be described by pore channel sizes for ZSM-12 and ZSM-48 (0.57 × 0.61 mm and 0.53 × 0.56 mm respectively). Significant isomerization yields have been reported with non-interconnected mesopore zeolites and zeotypes, such as ZSM-12, IZM-2 and ZSM-23 [81,82].

3.2.2. Effect of the Active Metal Site

In the literature, much attention is given to the impact of the metal precursor and its loading on the catalyst selectivity (

Table 7. The performance of zeolite-based catalysts of hydroisomerization prepared with using various metal precursors.

Bifunctional Catalyst	Metal Loading, %	Precursor	Si/Al	Conversion, %	Selectivity to i-C16, %	Ref.
Pt/ZSM-12	0.50	[Pt(NH3)4](OH)2	45 60 90 120	88 88 86 85	86 87 87 92	[59]
Pt/ZSM-12	0.50		90	82	87	[63]
Pt/ZSM-22	0.50	Pt(NO3)2 H2PtCl6 Pt(NH3)4Cl2	100	33 32 30	52 72 45	[88]
Pt/ZSM-22 (Pt nanocrystals) Octahedral Spherical Cubes	0.50	H2PtCl6	100	30 35 32	71 56 53	[92]
Pt/ZSM-22	0.46	[Pt(NH3)4](NO3)2	N/A	60	85	[93]
Pt/BETA	0.70			26	76
Pt/SAPO-11	0.90			90	75
Pt/SBA-15 + BETA	0.50	H2PtCl6 [Pt(NH3)4](NO3)2 Pt(NH3)4Cl2	SBA(7) BETA(75)	80 80 80	70 85 60	[94]
Pt/Fe/ZSM-23	0.98:0.98 1.00:1.90 0.99:3.66 1.03:7.00	H2PtCl6 Fe(NO3)·9H2O	50	57 70 64 62	80 82 65 64	[95]
Pt/ZSM-22/ZSM-23 (40:60)	0.50	H2PtCl6	100	58	78	[96]
Ni/ZSM-48	N/A	Ni(NO3)2·6H2O	200	60	50	[97,98]
Pt/SAPO-11	0.50	H2PtCl6	0.24	96	58	[74]
Pd-Ni2P/SAPO-31	0.05 Pd 4.00 Ni2P	Pd(NO3)2 (NH4)2HPO4 + Ni(NO3)2	0.6	83	72	[75]
Pt/(SAPO-11/Al2O3) SAPO-11:Al2O3 = 0.43 0.67 1.00	1.00	H2PtCl6	0.015 0.020 0.025	50 56 57	99 99 99	[99]

). The metal precursor utilized in synthesis can also contribute to heterogeneities in the metal structure obtained on both bulk and nanometer scales, which are referred to as precursor effects [83,84,85]. Distinct characteristics of each metal precursor, such as its decomposition temperature and interaction with the support, can significantly affect the distribution of metal particles across the support and the particle size distribution [86]. As a result, the metal to acid site ratio (n_M/n_A) within the sample varies. Ultimately, this can have a substantial impact on the performance of a catalyst [87,88,89,90]. In contrast, when considering the specific n-paraffin and operating conditions, the performance of bifunctional catalysts depends not only on the zeolite utilized but also on the ratio of metal/acid sites [25,91]

Table 7 shows that changing Si/Al—using [Pt(NH₃)₄](OH)₂ as a metal precursor—does not effect the performance of ZSM-12 especially on the side of conversion, but on the other hand, the selectivity slightly improves with increasing ratio Si/Al. Also the use of different metal precursors greatly affects the selectivity of zeolite ZSM-22, as the case of using Pt(NO₃)₂, Pt(NH₃)₄Cl₂ and H₂PtCl₆, it is clear that the latter offers the best selectivity. In general, it is obvious that the best metal loading for all zeolite-based catalysts is 0.50% achieving an optimum balance between metal and acidic functions along with obtaining and guarantee the high dispersion of metal on the support.

3.2.3. Effect of the Acidity

The zeolite framework contains exchangeable cations, which play an important role on the neutralization of the charge produced by isomorphous substitution (Figure 9). For example, the substitution of Si⁺⁴ with Al⁺³ leads to the formation of bridging [Si-OH-Al] hydroxyls, which acts as a Brønsted acid sites (BAS) providing the catalytic activity of zeolites [100]. The removal of the 4-coordinate Al associated with the BAS via dealumination results in the formation of extra-framework Al (EFAL) species within the pores, which often function as Lewis acid sites (LAS) [101,102,103]. These EFAL species have been found to modify the thermal stability of zeolites and enhance their catalytic activity. Notably, the hydrothermally dealuminated Y zeolite (USY) serves as a general catalyst in the fluid catalytic cracking process.

Moreover, extra-framework or framework-incorporated metals, such as Ga, B, Ti, and Sn, may form LAS. These metals frequently provide zeolites with bifunctionality characterized by both redox property and acidity, which extends their application beyond petrochemistry to biomass transformation [104,105,106].

Achieving a balance between the density and strength of acidic sites is critical for the reactivity and selectivity of hydroisomerization reactions, and catalysts with optimal hydrogenation capacity and suitable acidity are preferred [107,108]. Aluminosilicate zeolites with medium pores have significant activity and selectivity for the isomerization of long-chain n-alkanes. However, excessive acidity may lead to cracking reactions, and extensive efforts have been made to modify the acidity via improved synthetic methods or post-treatments, such as altering acid site distribution using mixed zeolitic catalysts, among others [109].

A catalyst with high acidity could cause both hydroisomerization and hydrocracking processes, resulting in lower isoparaffin yields and the breakdown of long-chain n-paraffins to obtain less valued and lighter products. To promote hydroisomerization and suppress hydrocracking, a catalyst with the strong hydrogenation activity and moderate acidity is ideal. Furthermore, the size of the pore aperture in zeolites may have a substantial impact on catalyst selectivity. In the case where pore sizes are small enough to prevent larger iso-paraffin formation at the acid sites, the catalyst will be good on converting n-paraffins [110].

Over the past few decades, researchers have emphasized the significant influence of the zeolite acidity of hydroisomerization catalysts on their activity and selectivity [111,112,113]. The general consensus is that a proper balance between hydrogenation and acid functions is essential to maximize isomer yields. However, the role of acid site density and strength remains a subject of debate due to variations in zeolite structure and reaction conditions [114]. For instance, in the case of hydrogen spillover highlights, the olefinic intermediates formed on acidic sites can be hydrogenated with H adsorbed over the far Pt cluster, and as a result, the physical mixture of Pt/SiO₂ and zeolite can exhibit good selectivity. Batalha et al. [115] propose that catalyst selectivity is determined by the intimacy between metal and protonic sites, which is the number of acid sites that olefinic intermediates encounter during diffusion between two metallic sites.

The acidity of zeolites can be adjusted by altering the ratio of SiO₂/Al₂O₃. Furthermore, some alkaline earth metals (Ba [116], Mg [117]) and transition metals (Fe [118]) can be included into the structure to decrease the acidity; the same effect exerts coating the zeolite with alumina [119].

4. Impact of Operating Parameters on Process Efficiency

The operational conditions of hydroisomerization of the diesel fraction are largely influenced by the composition of feedstocks and the desired quality of the final product. However, the choice of catalyst is equally critical. Initially, the hydroisomerization process was performed at high hydrogen pressures ranging from 10 to 25 MPa. Upon the development of more active catalysts, however, the pressure requirements have been reduced to 4–5 MPa.

The temperature of the process is determined by both the boiling range of the feedstock and the activity of the catalyst. Usually, higher boiling fractions necessitate higher process temperatures. The application of highly efficient zeolite-based catalysts has enabled a significant temperature reduction in recent years. For example, kerosene fractions are processed at temperatures ranging from 240 to 280 °C, as opposed to previous temperatures as high as 450 °C. Thus, the progress of the hydroisomerization process could be defined as a trend to significant reduction of temperature and pressure due to the use of more active catalysts. Despite numerous investigations in the field of knowledge, there is a relatively limited understanding of the interconnected influence of these operational parameters. Usually, experiments are performed by varying temperature within a predetermined range while keeping the remaining parameters fixed. Consequently, the predominant impact of temperature variation on catalyst performance can be observed. In general, with rise in temperature, conversion increases, but selectivity decreases (Figure 10).

Typically, recent studies of n-hexadecane hydroisomerization have performed at temperature range of 200–380 °C, operating pressure up to 6 MPa, the weight hourly space velocity (WHSV) between 1–6 h⁻¹, and H₂/n-C₁₆ ratios ranging from ~10 up to 1000.

As

Table 8. The performance of zeolite-based catalysts of hydroisomerization at optimal operating conditions.

Bifunctional Catalyst	Active Metal Loading, %	Si/Al	Optimal Conditions				Conversion, %	Selectivity, %	Ref.
			T, °C	P, MPa	WHSV, h−1	H2/n-C16
Pd/ZSM-5	1.00	40	280	6.0	10.0	20	95	4	[57]
Pd/ZSM-5 a	1.00	61	290				58	16
Pt/ZSM-5	0.50	32	240	2.0	6.0	10	33	4	[58]
Pt/ZSM-12	0.50	90	310	6.0	1.1	10	86	87	[63]
Pd/ZSM-12	1.00	62	310	6.0	10.0	20	85	92	[57]
Pt/ZSM-22	0.46	N/A	320	4.0	N/A	N/A	80	85	[93]
Pd/ZSM-22	1.00	61	330	6.0	10.0	20	85	56	[57]
Pt/ZSM-22	0.45	45	305	6.0	0.7	10	94	76	[120]
Pt/Ca-ZSM-22	0.42		312				91	82
Pt/Ba-ZSM-22	0.44		312		1.1		90	86
Pt/ZSM-22	0.90	100	300	4.0	N/A	600	15	33	[81]
Pt/ZSM-23	0.91		290				70	80
Pt/ZSM-23	0.60	60	270	3.0	1.0	6	91	82	[117]
Pt/ZSM-23	0.50	58	290	3.0	2.0	1000	85	85	[121]
Pd/EMM-23	1.00	60	290	6.0	10	20	80	13	[57]
Pt/ZSM-35	0.95	40	300	4.0	N/A	600	92	48	[80]
Pt/ZSM-48	0.93	200					20	54
Pt/EU-1/ZSM-48	0.97	200	300	4.0	N/A	600	80	55	[122]
Pt/SAPO-11 b	0.15	0.6	310	2.0	3.1	650	94	96	[123]
Pt/SAPO-11	0.90	N/A	350	4.0	N/A	N/A	90	75	[93]
Pd/SAPO-31	0.15	0.3	340	2.0	3.7	500	90	90	[124]
Pd/SAPO-41	0.80					8	90	96	[125]
Pt-Pd/SAPO-41	0.15:0.35						96	94	[126]
Pd/SAPO-41	0.10	0.3	360	2.0	3.7	500	87	95	[127]
Pd-Ni2P/SAPO-31	0.05/4.00	0.3	380	2.0	3.7	500	85	87	[75]
Pt/IM-5	0.50	33	240	2.0	6.0	10	43	5	[58]
Pt/BETA	0.40	14	200	0.5	3.0	13	50	63	[128]
Pt/BETA	0.79	-	280	4.0	N/A	N/A	50	70	[93]
Pt-Pd/BETA	0.40/0.40	25	200	0.1	3.0	750	78	93	[129]
CoMo/Al2O3-Y	3.50–13.00	2.5	360	1.5	1.0	150	11	55	[130]
Pt/BETA	1.00	14	200	0.5	3.0	13	50	68	[117]
Pt/USY	0.70	17	215					61
Pt/USY+ Pt/BETA	0.70/1.00	14/17	205					65
Pt/Al-SBA-15 + BETA	0.50	75	300	5.0	3.5	5	84	71	[131]
Pt/Al-SBA-15 + MOR		45					43	81

^a Nanostructure. ^b Vacuum-assisted synthesis.

shows, the majority of catalysts operate at temperature of at least 270 °C and pressure exceeding 2 MPa. Analysis of the performance of catalysts above reveals that zeolites ZSM-12, ZSM-22, and SAPO-11 at temperatures above 310 °C and ZSM-23 at temperatures between 270 and 290 °C are superior supports for platinum-impregnated catalysts. In contrast, zeolites SAPO-11, SAPO-31, and SAPO-41 at temperatures exceeding 340 °C are preferable for palladium-impregnated catalysts, while catalysts supported with BETA, USY, and mordenite have the modest performance.

However, ZSM-5 supports demonstrated the worst selectivity (whatever type of active metal) even in the nanostructural form. As for the effect of pressure change, it depends on the type of zeolites. For example, in the case of ZSM-5, the increase in pressure from 2 to 6 MPa has affected remarkable effect on the conversion of n-alkanes. On the other hand, in the case of ZSM-22, the effect of pressure is somewhat proportionally limited on the reaction conversion and selectivity. However, the effect of WHSV and H₂/n-C₁₆ is still unclear considering that they have not ever been the main objectives of any investigation.

Parmar et al. [116] studied the impact of increasing WSHV on the performance of bifunctional catalysts comprising Pt/ZSM-22 modified with Group-II cations (Ca⁺², Ba⁺², Mg⁺²). Their findings revealed that higher WSHV and temperatures > 300 °C led to a decrease in the conversion and an increase in selectivity towards iso-paraffins C₁₆. This observation can be due to the shortened reaction time, which has a negative impact on both the main (isomerization) and secondary (cracking) reactions. Subsequently, Zhang et al. [80] investigated the impact of WHSV at temperatures below 300 °C on the performance of different Pt-impregnated 10MR zeolites. These data confirmed an advantageous effect of increasing WHSV for both MTT and FER zeolite types (Pt loadings ~0.93 wt %), resulting in improved selectivity. This phenomenon can be attributed to the suppression of cracking iso-paraffins C₁₆. In contrast, increasing WSHV led to a decrease in selectivity for both TON and MRE zeolite types (Pt loading ~0.91 wt %), due to the lesser content of Brønsted acid sites.

Upon the investigation of n-hexadecane hydroisomerization over Pd-Pt/β-zeolite catalyst, Bauer et al. [129] observed a decline in the conversion from 89.0% under pressure of 3 MPa to 53.0% at 7.5 MPa. This result can be attributed to the competition of olefinic intermediates with hydrogen adsorbed over active sites under high-pressure conditions. Although higher hydrogen pressures have a negative impact on the conversion of n-paraffins and provide minimal enhancements in selectivity and catalyst stability, it should be noted that industrial specifications are usually required to exploit catalysts within the pressure range of 3–5 MPa.

5. Tolerance of Bifunctional Catalysts to Poisons and Deactivation

5.1. The Effect of Impurities-Containing Feed

Commercial feedstock is frequently impaired with nitrogen- and sulfur-containing compounds, which could damage the metallic and acidic active sites of bifunctional catalyst [132]. Nitrogen poisoning is a serious issue in catalysis, especially with zeolite-based catalysts employed in n-hexadecane isomerization. Understanding the forms of nitrogen adsorption and the further blocking of active sites allows us to understand the mechanisms that govern catalyst deactivation and selectivity loss. The deactivation of bifunctional catalysts on hydroisomerization of n-hexadecane with sulfur and nitrogen compounds has got some attention, but the understanding of their impacts on catalytic isodewaxing is unclear [133,134].

According to Lee et al. [135] the polymetallic bifunctional Pt-Pd-Mg/ZSM-23 catalyst exhibited a high stability of product properties for more than 30 days in the presence of sulfur impurities (1–2 wt.%).

Pimerzin et al. [136] examined the stability of two type of SAPO-11-supported bifunctional catalysts (transition metal sulfides and noble metal) for the isomerization of hexadecane. Nitrogen-containing feedstock inhibits isomerization catalytic activity more strongly, and whether the quantity of sulfur in the feedstock is equivalent to or higher than 100 ppm, transition metal-based bifunctional catalysts surpass Pt-catalyst in n-hexadecane isomerization (Figure 11).

Later, the effect of nitrogen-containing feed was retraced by Vinogradov et al. [130] with different zeolite-supported catalysts (ZSM-5, Beta, and Y). It was observed that the nitrogen impurities (quinoline) could play a role as a selective-controlling agent, as a result of the predominant adsorption of quinoline on strong acid sites performing the cracking process.

5.2. The Impact of Water

The effect of water-containing feed on the process of hydroisomerization of n-alkanes was considered recently [137,138,139]. Controlled amounts of water and iso-pentanol promoted the performance of FAU-type structured zeolite (with rare-earth metal—REM) in the hydroconversion of n-hexadecane [140]. According to the results, the support was activated by creating new protonic active centers via the hydration of REM cations. However, due to competing adsorption and hydrating protons located in the zeolite, water decreases acidic strength and deactivates the catalyst.

The impact of competing water adsorption on the acid sites of Pt/MFI, Pd/MFI, and Pt/HY was investigated by Brosius et al. [141]. Authors discovered that water prevented n-hexadecane cracking products from the secondary isomerization. Moreover, in the case of using Pt(0.9 wt%)/HY products with a great extent of branching were obtained. In contrast, Pt(0.9 wt%)/MFI produced a wider extent of linear products (Figure 12).

5.3. Deactivation by Coke Deposition

Upon the hydroisomerization of n-alkanes over zeolite-based catalysts, carbonaceous deposits are obtained as a result of complicated chain of reactions, namely the oligomerization, cyclization, and condensation of alkylaromatics to generate polycyclic aromatics (Figure 13) [142,143,144,145]. Coke is composed of heavy components that are extremely deposited as a byproduct on the surface of the zeolite, poisoning the active sites and clogging the pores. It was observed that the interaction between intermediates of coking and other reactions intermediates impacts on the distribution and selectivity of the products. Coking occurs faster on strong acid sites than on weak acid sites, because the last are less sensitive to coke formation [146,147].

The coke can be formed in one of three ways: (a) synchronously with the desired reaction (competitive phase), (b) consecutively to the directed reaction (successive phase), or (c) both of (a + b) simultaneously [145]. Coke combustion can renew the catalyst, however, the intense regeneration conditions and the presence of water prevent full catalytic activity from being regained [148].

Bauer and Karge [149] proposed a classification system that differentiates between two forms of coke obtained. Condensation, oligomerization, and alkylation processes involving reactants and their intermediates produce “Coke-type I” at lower temperatures. “Coke-type II” is found at higher temperatures and comprises chemical stages requiring greater activation energies, such as dehydrocyclization, thermal cracking, and dehydrogenation.

The coke formation rate is influenced by the same parameters as the rate of the catalytic reaction itself. These parameters encompass the nature of the reactants [150,151], the pore structure of the catalyst [152], the density of the active sites [151,153], and the operating conditions [153].

Carbonaceous deposits are thought to disable the catalyst by poisoning the active sites or by pore blockage [154], but the latter causes more severe deactivation than active site poisoning [155], because one active site per coke molecule can be deactivated, but pore clogging with a single carbonaceous molecule could limit or prohibit reactant molecules from accessing all active sites inside the clogged pore.

The coke may accumulate via three ways: (a) uniform surface deposition [156], (b) pore-mouth blockage [157], and (c) bulk-phase blockage [156,158]. Each mode has a particular effect on the diffusivity of reactants and, as a result, on catalytic activity.

In the case of uniform surface deposition, it has been hypothesized that carbonaceous deposits are uniformly distributed throughout the micropore channels of catalyst. Nonetheless, this distribution is likely to shorten the micropore diameter decreasing the diffusion rate and catalytic activity. According to the pore-mouth blockage scenario, carbonaceous deposits tend to build at the pore entrances, limiting access for the reactant to the active sites within the pore. Although the pores are not totally blocked, this process reduces the diffusion rate. In this situation, carbonaceous deposits cover a tiny fraction of active sites, in which productivity will be proportional to the number of accessible active sites [157]. The bulk-phase blocking scenario incorporates both of the aforementioned carbonaceous deposition mechanisms. However, in this instance, a large quantity of coke is generated on the outer surface of catalyst, resulting in full clogging of the pores.

During the catalytic cracking of n-hexane and 1-hexene over ultrastable Y zeolite, Chen and Manos [159,160] differentiated coke into coke precursors and hard coke. Coke precursors are easily volatilized in inert nitrogen and removed off the catalyst sample, whereas hard coke stays on the catalyst even at high temperatures (600 °C) and is removed by burning. Bauer et al. [129] investigated the formation of coke depositions during the hydroisomerization of n-hexadecane on the Pt-Pd/H-beta bifunctional catalyst. It was revealed that both the volume and surface area of micropores were decreased by roughly 8% after 62 days on-stream as a result of pore occlusion with coke.

Bifunctional catalysts including mesopores have enhanced the lifetime. In the most cases, deactivation is caused by pore-mouth occlusion by carbonaceous deposition. Numerous studies have claimed that the favorable effect on the lifetime is due to an increased tolerance to the impacts of coke rather than a lower rate of the coke formation. Shortening diffusion lengths combined with increased pore openings in the mesoporous makes the pores more resistant to blocking, which may explain the reduced effects on coking. This simple reasoning, however, may not fully describe the situation. Holm et al. [161] reasoned that, due to mesoporosity, products obtained will have a shorter retention time in the micropores, potentially suppressing subsequent secondary coking.

6. Cost-Effective and Eco-Friendly Synthesis Approaches

Conventional methodologies for zeolites synthesis have been extensively documented in the scientific literature [162]. However, numerous challenges exist, including the high expenses associated with structure-directing agents and starting materials, energy consumption and, recycling wastes. Moreover, the multistage production comprising expensive procedures hinders commercialization [163,164,165]. As a result, the synthesis of hierarchical zeolites using an environmentally friendly approach has gained attention [166,167]. Reliable syntheses aim to accommodate the effectiveness of conventional methods with strategies that reduce environmental impact, such as utilizing alternative starting materials, carrying out solvent-free or template-free syntheses, and reusing water [168,169,170].

Recent studies have reported the successful synthesis of hierarchical zeolites using an eco-friendly approach [171,172]. These works have demonstrated the feasibility of zeolite production through the application of green chemistry principles. However, certain drawbacks have also been observed, including low crystallinity, non-uniform pore structures, and reduced process efficiency (

Table 9. Comparison of traditional and modern approaches of zeolite-based catalyst production.

Approach	Principle	Influencing Factors	Advantages	Disadvantages
Solvothermal method	Using solvents	Temperature, pressure, reactants sources and compositions, Si/Al, aging time, alkalinity, stirring conditions, seeding, solvents type	Simple; cheaper	Environmentally harmful; energetic and time-consuming approach
Alkali-fusion method	Fussing raw materials with alkali before hydrothermal treatment	Si/Al, concentration of the alkali medium, temperature of crystallization	Using low-grade raw materials without purification; offers the zeolite with high purity	High energy consumption; expensive; multistep; time-consuming
Sol-gel method	Formation of an inorganic sol and its gelation in liquid phase gel to form a 3D structure	Hydrolysis rate, temperature, heating rate, PH	High-quality product; homogeneity; does not require expensive equipment	Cost of precursors
Microwave method	Using microwave irradiation	Si/Al ratios, alkalinity, wavelength, temperature, crystallization time	Concise time; high purity product; small particle size	Low crystal aspect ratio
Ultra-sound method	Using ultrasound with frequency 20 kHz–2 MHz	Duration and frequency of ultrasonication	Very simple; rapid reaction; high crystals growth rate; suitable particle size distribution and morphology; control on nucleation process	The mechanism of ultrasonic effect remains unclear

).

Significant efforts have been devoted to establishing the connections between the characteristics and catalytic performance of zeolites, driven by their economic significance. In this regard, the development of innovative syntheses and alternative raw materials have a focal point. An example of a recent eco-friendly approach is template-free synthesis, which has been tested for the production of the MFI framework structure [173,174,175]. Notably, tubular clay nanomaterials, specifically Halloysite, have been utilized as triple-functional materials: the source of alumina and silica, and hard template [176,177,178,179,180]. It should be noted that bioresources, such as water reeds (Phragmites) [181] and rice husk ash [182,183,184,185,186,187], mineral sources including rectorite [188] and diatomite [189,190], and industrial wastes like coal fly ash [191,192], have a minor scale, but employed as sources of silica and alumina in the synthesis above. This advancement represents a notable step forward in green chemistry for zeolite production.

Occasionally, traditional synthesis approaches necessitate substantial quantities of organic or water-based solvents that are economic and ecological drawbacks, particularly evident in the context of silicoaluminophosphate (SAPO) materials [193,194].

Recently, Xiao et al. effectively synthesized a variety of zeolites without the need for extra solvents. The zeolites obtained comprised SAPO-11, SAPO-20, SAPO-34), aluminophosphates (AlPO-11), metal-containing aluminophosphate zeolites (Mg-AlPO-11, Co-AlPO-11, Mg-SAPO-46, and Co-SAPO-46) [195], as well as aluminosilicates (ZSM-5, silicate-1, ZSM-49, SOD, MOR, Beta, and FAU) [196]. This innovation in synthesis indicates that it has the potential to produce a wide range of zeolite frameworks without the need for extra solvents.

Most current studies have focused on the seed-assisted approach to crystal pattern modification as a simple and ecologically benign method [197,198,199]. This method includes the inducing and orientation of crystals growth by adding into the gel a minor amount of the same targeted zeolite structure as a seed. This method has been shown to be successful in syntheses of zeolite frameworks such as MFI [200,201,202,203], MTW [64,204], MEL [205], FAU [206], MOR [207], CHA [208], and MRE [209].

Nonetheless, despite its potential benefits, the seed-assisted approach has several limits, such as the broad particle size distribution, tendency to particle aggregation, and possibility of pore blockage [210,211]. To maximize the efficiency and performance of the seed-assisted synthesis technique, these constraints must be handled and minimized.

Unconventional methodologies for zeolite synthesis and the ability to modify surface properties through post-treatments are two effective strategies employed to improve the zeolite performance. One such approach is microwave-assisted hydrothermal synthesis (MAHyS), which significantly accelerates synthesis. For instance, Muraza et al. [212] successfully increased the rate of synthesis of the TON framework zeolite from 66–72 hours to 24 hours using the MAHyS technique.

Another promising technique is the ultrasound-assisted synthesis of zeolites, which offers several advantages, including increased surface area and shorter synthesis time [213,214]. These effects have been confirmed in the syntheses of MCM-22 [215], ZSM-5 [182,216], and SAPO-11 [217] zeolites, where the ultrasound-assisted method promotes the dissolution of reactants and accelerates the progression of crystallization.

7. Conclusions

The isomerization of n-hexadecane and its multiple isomers is critical to upgrading the low-temperature characteristics of diesel, such as cloud point, pour point and cold filter plugging point, also enhancing the cetane number, therefore, the effective catalysts and the optimal operating conditions have been reviewed.

The concept of main and secondary types of shape selectivity was analyzed in detail. On this basis the requirements for the preparation procedure of zeolites were formulated: mainly, mesopores should possess the product shape, transition-state, and pore mouth selectivity, but pores with the reactant shape, mutual intersections, and nest effect selectivity should be excluded.

The fundamentals of mechanism of n-hexadecane hydroisomerization, characterizing zeolite were highlighted. To improve the yield, the choice of structural and morphological characteristics of catalyst, combined with the optimal balance between metal and acidic sites (n_M/n_A) have the greatest impact on the process. Large external surface area and mesoporous structures minimize the diffusion limitations and unfavorable cracking. Diffusion on a high surface area enhances adsorption at pore-mouths and improves accessibility to acid sites, resulting in better conversion and selectivity.

The effects of nitrogen-, sulfur- and water-containing feedstock on the catalyst performance were also discussed. It was concluded that sulfur plays a lesser deactivation role than nitrogen impurities as a selectivity-control agent, poisoning the acid sites and deactivating the isomerization function. In contrast, water prevents the deactivation of catalysts and suppresses cracking reactions.

The recent trend in the synthesis zeolite-supported catalysts from natural sources—such as clay and natural minerals is considered very attractive from both economic and environmental standpoints. Although it is still limited to the ZSM-5 zeolite, it promises a lot of progress in developing new types of zeolites. The novel methods for synthesis of bifunctional catalysts, and their success were discussed in overcoming the problems that may face traditional methods, despite the presence of some challenges that still exist in environmental and economic areas, and the textural and structural properties of zeolites.

Furthermore, the use of alternative feedstocks, such as renewable sources, as well as the integration of hydroisomerization to other refining processes, such as hydrotreating and hydrocracking, could open up new avenues for producing high-quality diesel fuels.

The authors thank GZ (FSZE-2022-0002).