In recent decades, many studies have been performed on the hydroisomerization of long-chain paraffins over zeolite catalysts thanks to their suitable acidity or hierarchical structures that combine microporous and mesoporous channel systems. In fact, due to their peculiar pore channel, zeolite-based catalysts are used to improve the degree of branching of product and decrease the tendency toward cracking reaction through the transition state selectivity (TSS) and product shape selectivity (PSS) effects. On the other hand, the acidity of the zeolite greatly influences the hydroisomerized yield. The acid strength distribution and acid site density both influence this yield, and the relation between metal sites and protonating sites is critical in determining the performance of the bifunctional catalysts. Many authors have concluded that weak or medium-strength Brønsted acid sites are responsible for promoting isomerized selectivity, whereas strong Brønsted acid and Lewis acid sites tend to promote cracking [19
]. Hence, modifying the Brønsted acidity of the zeolite is essential to improve its hydroisomerization selectivity.
The acidity of the zeolite is a crucial factor in determining the overall catalytic performance in the upgrading of vegetable oils to produce low-pour-point insulating and lubricating oils. The degree of acidity and the acid site types (Brønsted or Lewis) determine the degree to which the cracking or isomerization reaction is favored. Mériaudeau et al. [40
] studied the performance of 1%Pt/SAPO-41, 1%Pt/SAPO-31, and 1%Pt/SAPO-11 catalysts in n
-octane hydroisomerization through a fixed-bed continuous flow reactor at 200–400 °C under atmospheric pressure. The catalytic activity decreased in increasing order of the amount and strength of acid sites: SAPO-41 > SAPO-11 > SAPO-31. However, the reaction rate was most strongly restricted by diffusion in the microporous channels of the SAPO-type zeolite. The low activity of the catalyst based on SAPO-31 zeolite resulted from the combined effect of its diffusion limitations and low acidity. Verma et al. [39
] synthesized hierarchical mesoporous crystalline HZSM-5 zeolite catalysts in high-surface-area and low-surface-area types for the hydroconversion of algae and Jatropha seed oil to jet fuel. According to 27
Al MAS NMR analysis, the concentration of extra-framework Al present in the high-surface-area zeolite sample was higher than that in its low-surface-area counterpart, representing an increased number of Lewis acid sites in the former [41
]. The former catalyst type also had more strong acid sites, favoring cracking and thus decreasing its isomerization selectivity. SAPO-11 and ZSM-22 have been used widely in paraffin long-chain hydroisomerization catalysts because of their moderate acidity and straight channels of 10-membered rings. The drawback of this zeolite type is its lower acid site density at the pore mouths where the main isomerization occurs, according to the pore mouth and key-lock mechanism [42
The acidity of zeolite can be also controlled by modifying its Si/Al ratio by means of ion exchange or post-synthesis dealumination or desilication treatments, or by creating a siliceous border over the pore mouth of the channel system. The advantages of these tailoring methods are that they change both the total number of acidic sites and the density of electrons on the linking hydroxyl group, thereby changing the Brønsted acidity. Parma et al. [28
] studied the effect of the zeolite Si/Al ratio in the Pt/ZSM-22 catalyst system upon the branched isomer selectivity for n
-hexadecane hydroisomerization (Table 4
). Decreases in the total number of acid sites and in the number of Brønsted acid sites correspond to increases in the Si/Al framework ratio. At the constant conversion level of 90%, a catalyst having a lower Si/Al ratio showed excellent selectivity and maximum isomer yield at lower reaction temperatures, 300–320 °C, compared to the reaction temperature range of 330–350 °C, providing maximum isomer yield for a catalyst with a higher Si/Al ratio. This result can be attributed to the mild Brønsted acidic strength of ZSM-22 zeolite, which favors isomerization over cracking at lower temperatures. Furthermore, relative to CAT-1 (having the Si/Al ratio of 30), CAT-2 (having the Si/Al ratio of 45) has a lower reaction temperature that maximizes isomer yield, indicating that it has optimal acid function and metal site balance over the mouths of its zeolite pores.
However, many authors have also stated that the number of acid centers of this zeolite type and their strength mainly determine their activity as hydroisomerization catalysts while having virtually no effect upon their selectivity, which instead depends more upon the structure and behavior of their pore systems.
Pore Structure Effect
Molecular sieve-based catalysts are used widely for hydroisomerization because of their narrow pores and the restricted access to and escape from their inner surfaces; the resulting behavior is termed pore mouth and key-lock catalysis [44
]. The effects of the pore channels upon the metal/zeolite catalyst performance and the product distribution have been studied in depth in various efforts. The ZSM-22 zeolite, having TON (Theta-One)-framework topology, and SAPO-11, having the AEL (Aluminophosphates with sequence number ELeven)-framework structure consisting of one-dimensional channels each comprised of 10-membered rings, have been studied extensively for hydroisomerization [19
]. Martens et al. [43
] demonstrated that the specific confined space channels of these two zeolite structures allow their peculiar methyl branching selectivity and hinder decomposition reactions inside the pores. These authors studied the formations of monomethyl and dimethyl branching groups of long-chain paraffins from decane to tetracosane during hydroisomerization over the Pt/ZSM-22-bifunctional catalyst, based on the pore mouth and key-lock mechanism [46
]. The maximal isomers yields were 77%–90%, with the increase of mono- and multibranched isomer yields depending on the length of the paraffin chain. In detail, the mono- and multibranched isomer yields are 55% and 22% for n
-C10 transformation, respectively, compared to over 80% and 70% in the case of higher carbon number paraffin chains as the initial material (n
-C22, and n
-C24). Nghiem et al. [52
] investigated Pt-based catalysts supported on ZSM and SAPO zeolite types with one-dimensional tubular and non-intersection medium pore structure for n
-octane hydroisomerization. The catalyst performance and selectivity based on these zeolite types are shown in Table 5
At 15% conversion, each of the listed catalysts has an isomerized selectivity of over 98%, excepting those with large pore structures, ZSM-12 and SAPO-5, with 89% and 79% selectivities. Moreover, the maximum isomerized yields versus n-octane conversion are only 50% for ZSM-12 and less than 30% for SAPO-5, whereas the other molecular sieves in this studied series gave 77–81 wt % isomerized yields.
Chi Kebin et al. [53
] investigated the performance of platinum supported on ZSM-22/ZSM-23 zeolite mixtures as hydroisomerization catalysts. The ZSM-22/ZSM-23 zeolites were prepared to have a uniform needle-shaped particle size from spindle-shaped ZSM-22 and nest-shaped ZSM-23. These catalysts displayed unique molecular shape selectivity with a pore cross-section of 0.45 × 0.55 nm and had similar physical properties. At low conversions, the principal hydroisomerized products were monobranched isomers for all catalysts. This primary product transformed to multibranched isomers as secondary products when the degree of conversion was above 80%, due to competitive adsorption on the catalyst surface favoring n
-alkanes over monobranched isomers. In order of decreasing acidity, namely ZSM-23 > ZSM-22 > ZSM-22/ZSM-23, the hydrocracked products were decreasingly predominant at high conversions (>85%), with >35%, 27%, and 19% yields over the respective supported platinum catalysts. However, as is common to all 10-membered ring zeolites, a drawback of these catalysts is that they hinder the hydrocracking reaction, thereby also inhibiting the generation of multibranched isomers due to increasing product diffusion limitations with increasing paraffin length. Actually, the hydroisomerization reaction would take place at the pore mouth while the carbon bears the positive charge of the stable carbenium ion localized inside the pore. This means that only the external surface of a ZSM-22 or SAPO-11 crystal contributes to the catalytic activity, whereas the remaining part of the crystal is catalytically inactive. Moreover, the diffusion limitations and confined access may also result in micropore blockage by large molecules or catalyst deactivation by means of coke deposition. Therefore, tailoring the pore architecture of zeolite-based catalysts to solve their diffusion limitations is highly desirable.
Vandegehuchte et al. [54
] investigated the effect of mixing a ZSM-22 zeolite (Si/Al = 45) with a non-shape-selective Y zeolite (Si/Al = 2.6) in a Pt-based catalyst for n
chain hydroisomerization via single-event microkinetic model simulations. The aim of this research was to optimize the synergy between primary monobranching on the more active ZSM-22 and secondary dibranching on Y. The best performance was obtained by using a zeolite mixture containing 75% ZSM-22. The maximal isomerized yields in this case reached 80% for n
as the initial feed and 63% for a commercial mixture of n
paraffin at the conversion level of 90%.
A series of Pt/ZSM-22 catalysts with various siliceous degrees were synthesized for n
-dodecane isomerization by Niu et al. [29
]. Their work showed that the highly siliceous ZSM-22-based catalyst performed better than silica-alumina ZSM-22 analogues in terms of isomerized selectivity and hindering of the cracking reaction. The effect of Brønsted acidity upon isomerization was investigated by introducing acidic silanol groups. It was demonstrated that the acidic silanol group in pure silica zeolite is consider as a second kind of acid site, thereby improving the activity and isomer selectivity of the ZSM-22-based hydroisomerization catalyst when the amount of typical Brønsted acid sites is inadequate. In fact, the catalysis selectivity slightly increased with the Si/Al ratio; also, the selectivity of a catalyst based on siliceous ZSM-22 was higher than that based on a silica-alumina mixture by 5%–7% over conversion, reaching the highest isomerized selectivity of 95.7% and maximum yield of 80.2%.
In recent years, zeolites exhibiting hierarchical porosity, with their inherent microporous system and additional mesoporosity, have been investigated as alternative solutions for the abovementioned problems [32
]. Mesoporosity can be induced by introducing intercrystalline mesopores into the nanoscale zeolite crystals or by creating a system of intracrystalline mesopores in the microporous channels (Figure 2
). The advantage of such hierarchical systems is that they can integrate the shape selectivity of the intracrystalline micropores and the efficacious mass transfer of the mesoporous system because of their increased diffusivity and decreased diffusion path length [47
]. Martens et al. [45
] studied the impact of hierarchical ZSM-22-based catalyst upon the hydroisomerization reaction pathways of the n
-nonadecane, and pristane model molecules. Both conventional and hierarchized zeolites had the same platinum content and similar dispersion behavior. The efficiency of hierarchization is demonstrated by the increase of maximum isomer yields of n
-decane hydroisomerization, reaching 82% versus the 67% of the conventional Pt/ZSM-22 catalyst. Similarly, in the case of n
-nonadecane, the conventional ZSM-22-based catalyst showed a relatively high maximum yield of 88%, which was further improved to 92% by hierarchization. Particularly, the formation of multibranched isomers was also enhanced. As mentioned above, the selectivity for multibranched isomers over this zeolite type was hindered; this selectivity was improved considerably by hierarchization. The maximum multibranch yields of 35% and 70% were observed at extremely high conversion levels in the cases of n
-decane and n
Furthermore, the greatest benefit of this hierarchical zeolite is its tendency to produce branched isomers having methyl groups near the center of the carbon chain; these are the most promising components to impart the desired low-temperature properties to vegetable oil-based insulating oils (Table 6
-decane, initially, both conventional and hierarchical ZSM-22 favor the formation of 2-methylnonane and hinder the formation of 4- and 5-methylnonanes. As the isomer yields approach their maximums, the positional distribution of methylnonanes reaches an equilibrium composition. The proportion of 2- over 5-methyl-nonane generation at 5% conversion is termed the refined constraint index CI°, reflecting the pore width [59
]. After hierarchization, the CI° drops from 14.5 to 8.0, which is still in the range of the 10-R zeolite (CI° > 2.2) [59
]. The efficient catalytic hydroisomerization observed for the hierarchized catalysts can be explained by the rearrangement of the spatial Brønsted acidity distribution. Namely, hierarchization reduces the number of acid sites in the micropores, thereby limiting the hydrocracking reaction. At the same time, it increases the number of acid sites at pore mouths, thereby favoring the isomerization reaction. This argument is also investigated in research by Tao et al. [32
], which focused on the preparation of hierarchical SAPO-11 zeolite by means of a dry-gel conversion method with 3-(trimethoxysilyl) propyl]octadecyldimethyl ammonium chloride (TPOAC) templating and its application for n
-dodecane hydroisomerization. Besides the integration of mesopores into microporous zeolite, acidity tuning was also carried out to improve the catalysis performance of hierarchical zeolite. It is mostly believed that the medium and strong acidity of Brønsted acid sites plays a crucial role in skeletal isomerization. Notably, although the number of Brønsted acid sites of hierarchical SAPO-11 is less than that of conventional SAPO-11, the hydroisomerization conversion over Pt/hierarchical SAPO-11 is moderately higher than that over its nonhierarchical counterpart. This is likely because of the greater availability of pore mouths in the case of the hierarchical SAPO-11 zeolite-based catalyst. Similarly, the uniform intercrystalline mesopores of the hierarchical structure and the increased number of medium-acidity sites present owing to water content control can enhance the diffusion of the multibranched isomer out of the micropores before cracking occurs, thereby leading to higher yields of multibranched isomers.