Catalytic Hydroisomerization of Long-Chain Hydrocarbons for the Production of Fuels

catalysts

Review Catalytic Hydroisomerization of Long-Chain Hydrocarbons for the Production of Fuels

Päivi Mäki-Arvela *, Taimoor A. Kaka khel, Muhammad Azkaar, Simon Engblom and Dmitry Yu. Murzin

Johan Gadolin Process Chemistry Centre, Åbo Akademi University, 20500 Turku, Finland; taimoor.kakakhel@abo.fi (T.A.K.k.); muhammad.azkaar@abo.fi (M.A.); sengblom@abo.fi (S.E.); dmurzin@abo.fi (D.Y.M.) * Correspondence: pmakiarv@abo.fi; Tel.: +358-2215-4424

Received: 17 October 2018; Accepted: 6 November 2018; Published: 10 November 2018

Abstract: Hydroisomerization of long chain parafﬁns for the production of branched alkanes has recently been intensively studied due to a large availability of these compounds. The most interesting research topics have been the development of novel bifunctional catalysts to maximize the yield of isomers and to suppress the cracking reactions. Since both of these reactions are catalyzed by Brønsted acid sites, the optimum catalyst exhibits equal amounts of metal and acid sites, and it facilitates rapid mass transfer. Thus, several hierarchical and nano-shaped zeolites have been developed, in addition to composite catalysts containing both micro- and mesoporous phases. In addition to catalyst development, the effect of the reactant structure, optimal reaction conditions, catalyst stability and comparison of batch vs continuous operations have been made.

Keywords: hydroisomerization; reaction mechanism; zeolites; composite materials

1. Introduction Hydroconversion has played an important role in the petroleum industry for decades. It is a well-explored and recognized technology that has been used in crude oil refining. The term hydroconversion generally refers to two typical oil refinery processes, i.e., hydrocracking and hydroisomerization, carried out for different oil fractions in oil refinery. These processes are carried out in the presence of hydrogen, and therefore, known as hydroconversion. In this paper, the hydroisomerization of long-chain hydrocarbons is mainly discussed over different bi-functional catalysts. Transformations of normal to branched alkanes is carried out mainly by hydroconversion, i.e., hydrocracking and hydroisomerization. In hydrocracking, the feedstock is catalytically converted to lower carbon numbers. For example Mobil, using a ZSM-5 catalyst with strong acidity and a uniform pore size, commercialized selective catalytic cracking of long-chain hydrocarbons in the lube dewaxing (MLDW) process [1]. In hydroisomerization, the properties of the feedstock are improved by transforming normal hydrocarbons to branched ones having the same carbon number. Selective hydroisomerization is a highly desirable reaction in oil refineries, and it is used mainly in two processes; first, for the improvement of the octane number for the gasoline pool (C5–C6), and second, for dewaxing of long-chain hydrocarbons for improvement in their cetane number and cold flow properties. In both of these processes, linear hydrocarbons are transformed to branched alkanes exhibiting the same carbon number [2,3]. Selective isomerization of normal short-chain alkanes is easier to realize than for long-chain hydrocarbons, because of rapid cracking of the latter during hydroisomerization. Improving the selectivity of the bifunctional catalysts for the production of isomerized middle distillates is challenging [4], and several novel catalysts have been developed over

Catalysts 2018, 8, 534; doi:10.3390/catal8110534 www.mdpi.com/journal/catalysts Catalysts 2018, 8, 534 2 of 27 recent years. Several reviews on hydroisomerization of long-chain alkanes have been published a rather long time ago [2,5,6]. Recently, Guisnet [7] published a review on the hydroisomerization of alkanes over Pt acid zeolites. Furthermore, Yadav et al. [8] overviewed SAPO materials as catalysts in the hydroisomerization of alkanes and alkenes. Thus, in this work, the main emphasis is put on very recent developments, especially that of tailored catalysts for the hydroisomerization of long chain alkanes. Recently, long-chain hydrocarbons have attracted a signiﬁcant amount of interest because of their increased availability. These long-chain hydrocarbons (>C15) make up to 80 wt % of material known as wax, i.e., long straight-chained (normal) hydrocarbons [2]. Extensive research has been carried out to obtain value-added products from a low-grade long-chain waxy feedstock to branched products with improved fuel quality [2]. Such research has been mainly concentrated on catalyst development, in particular micro/mesoporous catalysts, to increase the mass transfer and to optimize the residence time of the molecules with the active sites. In this review paper, the main emphasis is on understanding the reaction pathways in transformations of long chain hydrocarbons on bifunctional catalysts. The role of different metals and supports will be elucidated especially for bifunctional catalysts. A detailed analysis of possible model compounds used to understand the behavior of long-chain hydrocarbons i.e., octane, n-decane, n-dodecane to n-hecadecane, and their activities, will be given. The review paper will also discuss the properties of fuels that are obtained by the hydroisomerization of long-chain hydrocarbons and the effect of carbon chain length on hydroisomerization activity. A detailed analysis of different catalysts, with their deactivation behavior in hydroisomerization of long-chain hydrocarbons in comparison with the straight chain will be made. The reactor and reactor conditions are discussed in depth for the hydroisomerization of long-chain hydrocarbons. The aim of this paper is to describe the most suitable catalysts that can be used in hydroisomerization of the long-chain hydrocarbons under optimized reaction conditions, providing the best isomerization selectivities with the lowest amount of by-products. In addition, batch and continuous operations will be compared, and the long-term performance of catalysts will be discussed.

2. Fuel Properties of Hydroisomerization of Long-Chain Paraffin Products The aim in hydroisomerization of long-chain alkanes is to transform the long-chain waxy alkanes to branched products. The long straight chain hydrocarbons on one side improve the thermal stability and viscosity of the fuel, having, however, adverse effects on other fuel properties such as freezing and pour points, and therefore, they should be removed to improve the fuel quality. The branched alkanes have superior cold flow properties, and they do not compromise other properties of the fuel [9]. The specification of the cold flow properties of fuels mainly depends on the geographical region where they are used, and they may vary from −3 ◦C from tropical regions to −35 ◦C for cold regions [10]. Hydroisomerization of long-chain alkanes is required to decrease the pour point, and to obtain a high viscosity index improving the cold flow properties of the oil [10]. For example, base oils with good lubrication properties contain large amounts of multibranched isoparaffins [11]. In hydroisomerization of long-chain alkanes several products are formed, such as light hydrocarbons (C1–C4), hydrocarbons in the gasoline range (C5–C8), jet fuels (C9–C14), and diesel (C15–C18). The pour point has been correlated with the product distribution in the hydroisomerization of hexadecane, and presented as a function of conversion (Figure1)[ 10]. It has been reported that the pour point increases with an increase in the paraffin molecular weight, and decreases with increasing branching; thus, (multi)branched products are desired in hydroisomerization of long-chain alkanes [12]. CatalystsCatalysts 20182018,, 88,, 534x FOR PEER REVIEW 3 of 2725 Catalysts 2018, 8, x FOR PEER REVIEW 3 of 25

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-10

Pour point ( point Pour -10

Pour point ( point Pour -20 -20

-30 -30 0 20 40 60 80 100 0 20 n-C1640 conversion60 (%) 80 100 n-C16 conversion (%)

Figure 1. Pour point as a function of hexadecane conversion: Conditions: temperature range of 260– Figure 1. Pour point as a function of hexadecane conversion: Conditions: temperature range of Figure 1. Pour point as a function of hexadecane conversion: Conditions: temperature range of 260−1– 320 °C and◦ 50–100 bar, respectively using weight hourly space velocity (WHSV) of 1.1–2.8 h−r 1 260–320 C and 50–100 bar, respectively using weight hourly space velocity (WHSV) of 1.1–2.8 hr −1 adapted320 °C andfrom 50 [10]–100. Reproduced bar, respectively with permission using weight from hourlyGomes spaceet al., Fuel velocity; published (WHSV by) ofElsevier, 1.1–2. 820 h1r7. adaptedadapted fromfrom [[10]10].. ReproducedReproduced withwith permissionpermission fromfrom GomesGomes etet al.,al., FuelFuel;; publishedpublished byby Elsevier,Elsevier,2017 2017.. 3. Reaction Mechanism in Hydroisomerization of Long-ChainLong-Chain ParaffinsParaffins 3. Reaction Mechanism in Hydroisomerization of Long-Chain Paraffins The reaction scheme for hydroisomerization hydroisomerization over long-chainlong-chain paraffinsparaffins of the the bifunctional bifunctional The reaction scheme for hydroisomerization over long-chain paraffins of the bifunctional catalysts was suggested suggested more more than than five five decades decades ago ago [9,10] [9,10. ]. The The role role of aof metal a metal is to is provide to provide the catalysts was suggested more than five decades ago [9,10]. The role of a metal is to provide the thehydrogenation/dehydrogenation hydrogenation/dehydrogenation function function,, while while the the acid acid sites sites are are responsible responsible for for skeletal skeletal hydrogenation/dehydrogenation function, while the acid sites are responsible for skeletal isomerization.isomerization. An An ideal ideal hydroisomerization hydroisomerization catalyst catalyst should should achiev achievee high high isomerization isomerization selectivity selectivity for isomerization. An ideal hydroisomerization catalyst should achieve high isomerization selectivity for forlong long-chain-chain hydrocarbons hydrocarbons,, resulting resulting in in a high a high liquid liquid yield yield,, avoiding avoiding at at the the same same time time any parallel long-chain hydrocarbons, resulting in a high liquid yield, avoiding at the same time any parallel reactions resultingresulting inin thethe formationformation ofof crackedcracked products.products. HydroisomerizationHydroisomerization occursoccurs consecutivelyconsecutively reactions resulting in the formation of cracked products. Hydroisomerization occurs consecutively (Figure(Figure2 2))[ [1313––15]15] when when n n-alkane-alkane is is first first dehydrogenated dehydrogenated on on a metal a metal site site forming forming an alkene an alkene,, and (Figure 2) [13–15] when n-alkane is first dehydrogenated on a metal site forming an alkene, and andthereafter thereafter a carbenium a carbenium ion, ion,which which in turn in turn is isomerized is isomerized onto onto an acidic an acidic site siteto an to isoolefin an isoolefin through through an thereafter a carbenium ion, which in turn is isomerized onto an acidic site to an isoolefin through an anisocarbocation isocarbocation,, finally finally forming forming bi- or bi- multibranched or multibranched alkanes alkanes (Figure (Figure 3) [13,16]3)[ 13., 16Alkylcarbenium]. Alkylcarbenium ions isocarbocation, finally forming bi- or multibranched alkanes (Figure 3) [13,16]. Alkylcarbenium ions ionscan also can alsofurther further undergo undergo 훽-scissionβ-scission reaction reaction [17] [, 17forming], forming cracking cracking products. products. can also further undergo 훽-scission reaction [17], forming cracking products.

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Selectivity (%)

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(moles C in product lump per mol C mol per lump C in product (moles 20 1.

(moles C in product lump per mol C mol per lump C in product (moles 1. 0 0 0 20 40 60 80 100 0 20 40 60 80 100 n-C15H34 conversion (%) n-C H conversion (%) 15 34 Figure 2. Selectivities of of 11.. monobranched, monobranched, 22. .dibranched dibranched,, and and 3.3 cracking. cracking products products as asa function a function of Figure 2. Selectivities of 1. monobranched,◦ 2. dibranched, and 3. cracking products as a function of ofhexadecane hexadecane conversion conversion at 310 at 310 °C underC under 30 bar 30 barover over different different Pd-SiO Pd-SiO2-Al22O-Al3 catalyst2O3 catalystss adapted adapted from from[18]hexadecane. Reproduced [18]. Reproduced conversion with permission with at 310 permission °C underfrom fromRegali 30 bar Regali et over al., et Catal.different al., Catal. Today Pd Today; -publishedSiO;2- publishedAl2O 3by catalyst Elsevier, by Elsevier,s adapted 2014.2014 fro. m [18]. Reproduced with permission from Regali et al., Catal. Today; published by Elsevier, 2014.

Catalysts 2018, 8, 534 4 of 27 Catalysts 2018, 8, x FOR PEER REVIEW 4 of 25

FigureFigure 3. Reaction 3. Reaction network network for for hydroisomerization hydroisomerization of of long long-chain-chain alkanes alkanes adapted adapted from from [19].[ 19The]. The numbersnumbers denote denote different different reaction reaction steps. steps. ReproducedReproduced with with perm permissionission from from Zhang Zhang et al., et Ind al.,. Ind.Eng. Eng. Chem.Chem Res..; R publishedes.; published by by American American Chemical Chemical Society,Society, 20162016..

DifferentDifferent reaction reaction mechanisms mechanisms have have also also been proposed proposed for for the the hydroisomerization hydroisomerization of long of- long- chainchain paraffins, paraffins depending, depending on on the the catalyst catalyst acidityacidity [13,16] [13,16.]. For For example example,, for for Pt on Pt acidic on acidic Beta Betazeolite zeolite,, the kinetic results fit well with the reaction mechanism that was composed of both parallel, reversible, the kinetic results fit well with the reaction mechanism that was composed of both parallel, reversible, and irreversible steps [16]. With decreasing acidity, mono- and bibranched products were still and irreversibleproduced, reactin stepsg [16 further]. With in decreasingparallel to the acidity, cracking mono- products, and bibranched whereas over products a mildly were acidic still Pt- produced,Beta, reactingonly further a consecutive in parallel mechanism to the wascracking proposed products, to be operative whereas over[16]. These a mildly results acidic indicate Pt-Beta, that only a a consecutivesuitablemechanism balance between was proposedthe metal and to be acid operative functions [16 is]. required These results to provide indicate sufficient that a activity suitable and balance betweenhigh theselectivity. metal and acid functions is required to provide sufficient activity and high selectivity. In additionIn addition to the to the formation formation of of isomers, isomers, itit isis interestinginteresting to to consider consider from from the themechanistic mechanistic point point of viewof view how how cracking cracking products products are are formed formed over over bifunctionalbifunctional catalysts. catalysts. This This has has been been investigated investigated by by RegaliRegali et al. et [15 al.], [ who15], who confirmed confirmed that that there there is nois no correlation correlation between between the the hydroconversion hydroconversion activity and and the Brønsted acidity [15], indicating that hydrogenolysis forming cracking products is metal- the Brønsted acidity [15], indicating that hydrogenolysis forming cracking products is metal-catalyzed. catalyzed. This idea was additionally confirmed by selective poisoning of the acid sites, resulting in This idea was additionally confirmed by selective poisoning of the acid sites, resulting in high high hydrogenolysis activity, which originated from a charge transfer from the metal particles to the hydrogenolysisframework atoms activity,, and whichnot from originated the acid sites from [15] a. charge transfer from the metal particles to the frameworkRealkylation atoms, and of notcracked from products the acid can sites occur [15]. also during the hydroisomerization of long-chain Realkylationalkanes, such as of hexadecane cracked products [16]. It was can reported occur alsoby these during authors the that hydroisomerization e.g., isobutene can react of long-chain with alkanes,isopentene, such as isohexene hexadecane, and [isoheptene16]. It was and reported form C9 by–C12 these isoalkanes. authors Analogously that e.g., isobutene, isopentene can react reacted with isopentene,with isohexene isohexene, and and isoheptene isoheptene forming and form C11 C9–C12to C12 isoalkanes. isoalkanes. This Analogously, was also experimentallyisopentene reacted with isohexeneconfirmed via and testing isoheptene of the reactivity forming C11of linear to C12 n-hex isoalkanes.-1-ene and This 2-methylpent was also- experimentally1-ene using Pt-H- confirmedBeta- via testing26 as a ofcatalyst. the reactivity The results of showed linear that n-hex-1-ene the linear andalkene 2-methylpent-1-ene was unreactive, whereas using the Pt-H-Beta-26 alkylation of as a branched olefin occurred. catalyst. The results showed that the linear alkene was unreactive, whereas the alkylation of branched olefin4. occurred. Effect of the Alkane Carbon Chain Length 4. Effect ofIt theis generally Alkane consideredCarbon Chain that Lengthlong-chain hydrocarbons are more reactive than short-chain hydrocarbons, and their acidity requirements are inversely proportional to the chain length [10]. It is generally considered that long-chain hydrocarbons are more reactive than short-chain hydrocarbons, and their acidity requirements are inversely proportional to the chain length [10]. Hydroisomerization of a mixture of C15–C18 hydrocarbons over Ni-Mo-SAPO-11 was demonstrated at 350 ◦C under 30 bar by (Figure4)[ 20]. In their work, the isomerization selectivity increased with increasing carbon chain length from C12 to C16. However, at a lower reaction temperature, 310 ◦C, selectivity was very low for C8 to C15 hydrocarbons, increasing then exponentially up to C18. The highest isomer selectivity with C18 hydrocarbon was 57% over Ni-Mo-SAPO-11 catalyst [20]. Catalysts 2018, 8, x FOR PEER REVIEW 5 of 25

50 583 K 603 K 623 K Catalysts 2018, 8, 534 40 5 of 27 Catalysts 2018, 8, x FOR PEER REVIEW 5 of 25 30 50 583 K 20 603 K 40 623 K

Selectivity (%)

10 30

0 20 15 16 17 18 Selectivity (%) Length of carbon chains Figure 4. Effect of carbon10 chain length in the hydroisomerization using Ni-Mo-SAPO 11 as a catalyst

under 30 bar with H2/oil (C15–C18 alkanes) ratio of 800 with liquid hourly space velocity (LHSV) of −1 1 hr [20]. Reproduced with0 permission from Xing et al., Catal. Today; published by Elsevier, 2018. 15 16 17 18 Hydroisomerization of a mixture ofLength C15 of– carbonC18 hydrocarbonschains over Ni-Mo-SAPO-11 was demonstrated at 350 °C under 30 bar by (Figure 4) [20]. In their work, the isomerization selectivity FigureFigure 4. Effect 4. Effect of carbon of carbon chain chain length length in in the the hydroisomerizationhydroisomerization using using Ni Ni-Mo-SAPO-Mo-SAPO 11 as 11 a ascatalyst a catalyst increased with increasing carbon chain length from C12 to C16. However, at a lower reaction underunder 30 bar 30 withbar with H /oilH2/oil (C15–C18 (C15–C18 alkanes) alkanes) ratio of of 800 800 with with liquid liquid hourly hourly space space velocity velocity (LHSV (LHSV)) of of temperature, 310 °2C, selectivity was very low for C8 to C15 hydrocarbons, increasing then −11 hr−1 [20]. Reproduced with permission from Xing et al., CatalCatal.. Today Today; published by Elsevier, 2018. 1exponentially hr [20]. Reproduced up to C18. with The permissionhighest isomer from selectivity Xing et al., with C18 hydrocarbon; published was by 57% Elsevier, over Ni2018-Mo. - SAPO-11 catalyst [20]. A comparativeHydroisomerization study forof hydroisomerization a mixture of C15–C18 of C10hydrocarbons and C19 over alkanes Ni-Mo was-SAPO performed-11 was over A comparative study for hydroisomerization of C10 and C19 alkanes was performed over Pt-ZSM- Pt-ZSM-22,demonstrated giving at the 350 highest °C under isomer 30 bar yield by (Figure for C10 4) [20] at 230. In ◦theirC and work 84%, the conversion, isomerization whereas selectivity a higher increased22, giving the with highest increasing isomer carbon yield for chain C10 lengthat 230 °C from and C1284% conversion, to C16. However, whereas at a higher a lower yield reaction (89%) yield (89%) was obtained for C19 at 90% conversion and 240 ◦C[21]. As a conclusion, it can be stated temperature,was obtained 310for C19°C, at selectivity 90% conversion was very and low240 ° forC [21] C8. As to a C15 conclusion hydrocarbons,, it can be increasing stated that then the that theexponentiallyisomerization isomerization upof longto ofC18.-chain long-chain The hydrocarbons highest hydrocarbons isomer is selectivity promoted is promoted withat rather C18 hydrocarbonlow at rather reaction low wastemperatures, reaction 57% over temperatures, Niaround-Mo- ◦ ◦ aroundSAP230 230–240–O240-11 ° Ccatalyst overC over noble [20] noble. metal metal catalysts catalysts,, and slightly and slightlyabove 300 above °C with 300 transitionC with transitionmetal catalysts. metal catalysts. DueADu to comparativee a to lack a lack of comparativeofstudy comparative for hydroisomerization studiesstudies of of carbon carbon of C10 chain chain and length C19 length alkanes effect effectss wasin hydroisomerization performed in hydroisomerization over Pt- ZSMin the- in the recent22recent, giving years, years, the the thehighest highest highest isomer hydroisomerization hydroisomerization yield for C10 at 230 yields ° yieldsC and of hexadecane84% of hexadecane conversion, and dodecanewhereas and dodecane a werehigher compared wereyield compared(89%) by by collectingwascollecting obtained these these for values valuesC19 at obtained obtained90% conversion over different and 240 catalysts catalysts°C [21]. As under under a conclusion optimum optimum, it reaction can reaction be conditionsstated conditions that andthe and conversion levels, and plotted as a function of conversion (Figures 5 and 6). It can be observed from conversionisomerization levels, of and long plotted-chain hydrocarbons as a function is of promoted conversion at rather (Figures low5 rea andction6). Ittemperatures, can be observed around from these figures that the highest isomer yields for hexadecane were higher than for dodecane, being in these230 ﬁgures–240 ° thatC over the noble highest metal isomer catalysts yields, and for slightly hexadecane above 300 were °C with higher transition than for metal dodecane, catalysts. being in line line with the results of [21]. A detail discussion of these results using different catalysts is presented with the resultsDue to ofa lack [21]. of A comparative detail discussion studies ofof thesecarbon results chain length using effect differents in hydroisomerization catalysts is presented in the below. recentbelow. years, the highest hydroisomerization yields of hexadecane and dodecane were compared by collecting these values obtained over different catalysts under optimum reaction conditions and 100 conversion levels, and plotted as a function of conversion (Figures 5 and 6). It can be observed from these figures that the highest isomer yields for hexadecane were higher than for dodecane, being in 8. 10. line with the results of [21]80. A detail discussion of these results using different catalysts is presented below. 9. 7. 60 100 6.

5. 40 4. 8. 10. 80 3. 9. 7.

Yield of C16 isomers (%) isomers C16 Yield of 2. 20 60 1. 6.

0 5. 40 4. 20 3. 40 60 80 100 Conversion (%)

Yield of C16 isomers (%) isomers C16 Yield of 2. 20 Figure 5. The yield of C16 isomers1. as a function of conversion. Notation: 1. Pt-H-Beta, 30 bar, ◦ ◦ 220 C[22], 2. Pt-H-ZSM-23 (dual templated), 300 C, 40 bar, volumetric ratio of H2/feed 600 L/L [23], ◦ ◦ 3. Pt-H-ZSM-23, 270 C, 400 bar, volumetric ratio of H2/feed 600 L/L [19], 4. Pt-Al-Si, 310 C, 30 bar, 20 40 60 80◦ 100 −1 molar ratio of H2/feed 10 [15], 5. Pt-Al-H-Beta (Si/Al ratio 80), 290 C, 75 bar, WHSV of 1.3 hr [10], ◦ Conversion (%) ◦ 6. Pt-Al-MCM-41, 200 C, 20 bar, molar ratio of H2/feed pf 4 [24], 7. Pt-ZSM-23, 260 C, [13], 8. ◦ ◦ Ba-Pt-ZSM-12, 295 C, 60 bar [12], 9. Pt-H-Beta (Si/Al 26) 220 C, 30 bar, molar ratio of H2/feed 20 [16], ◦ 10. Pt-Beta-Al2O3, 220 C, 30 bar, molar ratio of H2/feed of 20 [9]. Catalysts 2018, 8, x FOR PEER REVIEW 6 of 25

Figure 5. The yield of C16 isomers as a function of conversion. Notation: 1. Pt-H-Beta, 30 bar, 220 °C

[22], 2. Pt-H-ZSM-23 (dual templated), 300 °C, 40 bar, volumetric ratio of H2/feed 600 L/L [23], 3. Pt- H-ZSM-23, 270 °C, 40 bar, volumetric ratio of H2/feed 600 L/L [19], 4. Pt-Al-Si, 310 °C, 30 bar, molar ratio of H2/feed 10 [15], 5. Pt-Al-H-Beta (Si/Al ratio 80), 290 °C, 75 bar, WHSV of 1.3 hr−1 [10], 6. Pt-Al-

MCM-41, 200 °C, 20 bar, molar ratio of H2/feed pf 4 [24], 7. Pt-ZSM-23, 260 °C, [13], 8. Ba-Pt-ZSM-12, Catalysts2952018 °C,, 860, 534 bar [12], 9. Pt-H-Beta (Si/Al 26) 220 °C, 30 bar, molar ratio of H2/feed 20 [16], 10. Pt-Beta6- of 27 Al2O3, 220 °C, 30 bar, molar ratio of H2/feed of 20 [9].

100

5. 4. 2. 60 1. 3.

Yield of isomer (%) isomer Yield of

0 0 10 20 30 40 50 60 70 80 90 Conversion (%) ◦ FigureFigure 6.6. YieldYield ofof isomersisomers asas aa functionfunction ofof n-dodecanen-dodecane conversion.conversion. Notation:Notation: 1.1. Pt-SAPO-11,Pt-SAPO-11, 260260 °C, ◦ −1 HH22/feed molar molar ratio ratio of of 25 25 [25] [25,], 2. 2. Ni Ni2P2-P-SAPO-11SAPO-11 [26] [26, 350], 350 °C, C,20 20bar, bar, WHSV WHSV 1 h 1r−1 hr, H2,H/feed2/feed molar molar ratio ◦ −1 ratioof 19, of 3. 19, Pt- 3.ZSM Pt-ZSM-22/ZSM-23,-22/ZSM-23, 320 °C, 320 80 C,bar, 80 LHSV bar, LHSV 1 hr−1 1, hrH2/feed,H2 volumetric/feed volumetric ratio of ratio 500 of[27] 500, 4. [27 Pt],- ◦ −1 4.ZSM Pt-ZSM-22-22 desilicated, desilicated, 280 °C, 280 20 bar,C, 20H2 bar,/feed H volumetric2/feed volumetric ratio of 600, ratio WHSV of 600, = WHSV2 hr−1 [28] = 2 5. hr Pt-Fe[28-ZF1,] 5. ◦ −1 Pt-Fe-ZF1,330 °C, 20 bar, 330 HC,2/feed 20 bar, volumetric H2/feed volumetricratio of 600, ratio LHSV of 2 600, hr−1 LHSV [29]. 2 hr [29]. 5. Catalyst Selection 5. Catalyst Selection Hydroisomerization of long-chain alkanes has been investigated for decades, but still there is a Hydroisomerization of long-chain alkanes has been investigated for decades, but still there is a demand to develop tailored materials to maximize the isomer yield. Hydroisomerization is typically demand to develop tailored materials to maximize the isomer yield. Hydroisomerization is typically performed over bifunctional catalysts comprising metals on acidic supports (Table1). These catalysts performed over bifunctional catalysts comprising metals on acidic supports (Table 1). These catalysts contain mostly noble metals, e.g., platinum and palladium, providing hydrogenation/dehydrogenation contain mostly noble metals, e.g., platinum and palladium, providing hydrogenation/dehydrogenation activity and such acidic supports, as ZSM-5, SAPO-11, ZSM-22, Y, and Beta-zeolites and composite activity and such acidic supports, as ZSM-5, SAPO-11, ZSM-22, Y, and Beta-zeolites and composite materials containing mesoporous MCM-41, MCM-48, and SBA-15 [17]. In addition to Pt and Pd, Ni materials containing mesoporous MCM-41, MCM-48, and SBA-15 [17]. In addition to Pt and Pd, Ni and Ni-Cu also have been used recently in hydroisomerization of long-chain alkanes [26,30]. The most and Ni-Cu also have been used recently in hydroisomerization of long-chain alkanes [26,30]. The important properties are the metal nature, amount, and dispersion, as well as the acid strength, location, most important properties are the metal nature, amount, and dispersion, as well as the acid strength, and concentration [31]. The catalyst texture is also of crucial importance, because isomerization occurs location, and concentration [31]. The catalyst texture is also of crucial importance, because in the micropore and the isomer should rapidly diffuse to the bulk phase prior to consecutive undesired isomerization occurs in the micropore and the isomer should rapidly diffuse to the bulk phase prior cracking reactions. to consecutive undesired cracking reactions. During the recent years, several tailored materials with tuned acidity and structural properties haveTable been 1. developed, The highest including yield of isomers shape-selective in hexadecane 10-membered and dodecane ring hydroisomerization zeolites (Table 2over)[ 19 different], they can facilitatecatalysts shape at optimum selectivity, reaction whereas conditions mesoporous and conversion materials levels. can enhance mass transfer. In this section, different types of supports, i.e., zeolites, hierarchical zeolites, and composite materials applied in Conversion Yield or Selectivity (%) hydroisomerizationFeed willConditions be discussed. A specialCatalyst emphasis will be put on the beneﬁts and limitationsRef. (%) of Isomer of the use of speciﬁc supports. 0.7 wt % Pt-H- 220 °C, 30 bar 95 80 [9] n- Beta/Al2O3 hexadecane 295 °C, 60 bar Ba-Pt-ZSM-12 90 80.3 [12] 280 °C, 60 bar Pt-SAPO-11-S 88 80 [14]

Catalysts 2018, 8, 534 7 of 27

Table 1. The highest yield of isomers in hexadecane and dodecane hydroisomerization over different catalysts at optimum reaction conditions and conversion levels.

Yield or Conversion Feed Conditions Catalyst Selectivity (%) Ref. (%) of Isomer 0.7 wt % 220 ◦C, 30 bar 95 80 [9] Pt-H-Beta/Al2O3 295 ◦C, 60 bar Ba-Pt-ZSM-12 90 80.3 [12] 280 ◦C, 60 bar Pt-SAPO-11-S 88 80 [14] 296 ◦C, H /feed = 1000, 2 0.5 Pt-ZSM-22 70 49 [32] WHSV = 1 hr−1 310 ◦C, 30 bar, H /feed 2 Pt-Al-Si 53 35 [15] n-hexadecane molar ratio 10 290 ◦C, 75 bar, WHSV = 35.5 Pt-Al-Beta (80) 36.5 [10] 1.3 hr−1 (monosubstituted) 270 ◦C, 40 bar, H /feed 2 Pt-ZSM-23 32 29 [19] volumetric ratio 600 300 ◦C, 40 bar, H /feed Pt-ZSM-23 dual 2 29 20 [23] volumetric ratio 600 templated 220 ◦C, 30 bar Pt-Beta 10 8 [22] ◦ C15–C18 310 C, 30 bar, LHSV = XC15,iC18,i = SC15,iC18,i = −1 Ni-Mo-SAPO-11 [20] alkanes 1 hr ,H2/oil = 800 51–74% 58–65% 300 ◦C, 1 bar, WHSV = 0.5 wt % −1 57 S = 68% [33] 2 hr ,H2/feed = 15 Pt-SAPO-11 307 ◦C, 20 bar, WHSV = 1 wt % 92 84 [34] 2 hr−1 Pt-SAPO-11 280 ◦C, 1.5 hr−1, 0.5 wt % 94 70 [25] H2/n-dodecane = 25 Pt-Meso-SAPO-11 dodecane 320 ◦C, 80 bar Pt-ZSM-22/ZSM-23 85 68.3 [27] 300 ◦C, H /feed = 600, 2 Pt-ZSM-22/MCM-41-H 91.3 66 [35] LHSV = 2 hr−1, 20 bar 300 ◦C Pt-ZSM-22-BUN 96 49 [36] 350 ◦C, 20 bar, WHSV = 3 wt % −1 70 40 [26] 3 hr ,H2/n-C12 = 19 NixPy-SAPO-11

Table 2. Cavity sizes and topology of zeolites exhibiting shape selectivity in hydroisomerization of long-chain alkanes. MR denotes membered ring channels in zeolites.

Zeolite Cavity Size (Å) Topology Ref. ZSM-22 5.7 × 4.6 TON (10 MR) [19] ZSM-23 5.2 × 4.5 MTT (10 MR) [19] ZSM-35 3.5 × 4.8, 4.2 × 5.4 FER (8–10 MR) [19] ZSM-48 5.6 × 5.3 MRE (10 MR) [19] SAPO-11 3.9 × 6.3 (10 MR) [34]

5.1. Zeolites in Hydroisomerization of Long-Chain Parafﬁns

5.1.1. Structure of Zeolites Different types of zeolites have been used in hydroisomerization including both 8, 10, and 12 membered structures [22]. The following zeolites possessing a one-dimensional structure, i.e., ZSM-22, Catalysts 2018, 8, 534 8 of 27

ZSM-23, ZSM-48 [19], and SAPO-11 exhibit shape selectivity (Table2), whereas 12 MR zeolites are devoid of shape selectivity. ZSM-35 has a ferrierite (FER) topology and contains 8–10 MR (Table2). Shape selectivity is very characteristic for 10 MR zeolites, when a long-chain alkane cannot diffuseCatalysts 2018 into, 8 the, x FOR pores PEER of, REVIEW for example, Pt-H-ZSM-22 [22]. In this case, the substrate is reacting at8 of the 25 pore entrance giving higher selectivity to monobranched products. This phenomenon is called pore mouth selectivity. Furthermore,Furthermore, thethe zeolitezeolite structurestructure hashas aa major effect on the product distribution in hydroisomerization ofof long-chain long-chain parafﬁns; paraffins for; for example, example an, an inverse inverse correlation correlation has has been been shown shown for thefor formationthe formation of an of ethyl-branched an ethyl-branched C8 isomer C8 isomer vs monobranched vs monobranched C9 in decaneC9 in decane isomerization. isomerization. The highest The selectivityhighest selectivity to monobranched to monobranched products products was with was 10 with MR 10 zeolites, MR zeolites exhibiting, exhibiting better better performance performance than 12than MR. 12 TheMR. lowest The lowest selectivity selectivity was obtained was obtained for 14 for MR 14 zeolites MR zeolites [21]. [21]. A comparativecomparative work on shape selectivity from 1010 MRMR zeoliteszeolites (ZSM-22,(ZSM-22, ZSM-23,ZSM-23, ZSM-35,ZSM-35, andand ZSM-48)ZSM-48) was performed in [[19]19] usingusing hexadecanehexadecane hydroisomerization as a model reaction, showing thatthat thethe channelchannel systemsystem hashas aa largelarge effect effect on on the the product product distribution distribution (Figure (Figure7). 7). Over Over Pt-ZSM-35 Pt-ZSM-35 with with a twoa two dimensional dimensional channel channel system, system, a high conversion a high conversion was obtained was without obtained formation without of multibranched formation of products,multibranched indicating products a pore, indi mouthcating mechanism.a pore mouth Pt-SAPO-11 mechanism. has Pt- alsoSAPO displayed-11 has also high displayed selectivity high to skeletalselectivity isomers to skeletal in n-dodecane isomers hydroisomerization in n-dodecane hydroisomerization (Table1)[ 33], due (Table to its pore 1) [33] dimensions, due to (Table its pore2). Platinumdimensions was (Table loaded 2). into Platinum this catalyst was via loaded the incipient into this wetness catalyst method, via the resulting incipient in a wetness highly reducible method, Ptresulting located in at a the highly external reducible surface Pt of located SAPO-11. at th Thise external catalyst surface exhibited of SAPO also high-11. long-termThis catalyst stability exhibited (see Sectionalso high8). long-term stability (see Section 8).

100 ZSM-23 ZSM-35 ZSM-48 80 ZSM-22

Selectivity of i-C16 (wt.%) i-C16 Selectivity of 20

0 260 265 270 275 280 285 290 295 300 305 o Temperature ( C) Figure 7.7. The selectivity to isohexadecane as a function of reaction temperature under 40 bar over different Pt-ZSMPt-ZSM catalystscatalysts [[19]19].. Reproduced with permission fromfrom Zhang et al.,al., Ind.Ind. Eng.Eng. Chem.Chem. R Res.es.; publishedpublished byby AmericanAmerican ChemicalChemical Society,Society,2013 2013..

Siliceous Pt Pt-H-ZSM-22-H-ZSM-22 is is an an excellent excellent catalyst catalyst for for hydroisomerization hydroisomerization of n of-dodecane n-dodecane,, giving giving 96% ◦ 96%selectivity selectivity to isomers, to isomers, however, however, only at only 29% at conversion 29% conversion at 290 at°C 290 underC under40 bar 40using bar H using2/feed H volume2/feed volumeratio of 750 ratio [37 of]. 750High [37 selectivity]. High selectivity was obtained was at obtained relatively at low relatively conversion, low conversion, over siliceous over Pt- siliceousH-ZSM- Pt-H-ZSM-2222 is due to its islow due acid to itssite low density, acid sitethe presence density, the of acidic presence silanol of acidic groups, silanol and an groups, overall and good an balance overall goodbetween balance the metal between and the acidic metal functions. and acidic It was functions. also stated It was in also [37] stated that Pt in particles [37] that in Pt this particles catalyst in thisare catalystlocated are on locatedthe zeolite on the external zeolite surface. external Note surface. that Note ZSM that-22 ZSM-22 exhibits exhibits shape selectivity, shape selectivity, which which is not ispresent not present in zeolites in zeolites with a with similar a similar pore size, pore but size, containing but containing a three a three-dimensional-dimensional pore porestructure structure;; e.g., e.g.,ZSM ZSM-5-5 and MCM and MCM-22-22 [38]. [38]. A novel zeolite, IZM-2,IZM-2, with both 10 MR and 12 MR rings rings,, facilitated formation of multibranched isomers,isomers, giving an analogous analogous product product distribution distribution as as 10 10-MR-MR zeolites. zeolites. The The yield yield of of dibranched dibranched C10 C10 is isshown shown as as a a function function of of monobranched monobranched C10 C10 in in n n-decane-decane hydroisomerization hydroisomerization ( (FigureFigure 88)[) [3399].]. This This catalyst was prepared in the presence of ethanol as a solvent, leading to formation of intergrowth zeolite nanocrystals mostly containing two types of pores. Such structure in addition to surface area of 201 m2/g and a large external surface can explain high isomer selectivity [39].

Catalysts 2018, 8, 534 9 of 27 catalyst was prepared in the presence of ethanol as a solvent, leading to formation of intergrowth zeolite nanocrystals mostly containing two types of pores. Such structure in addition to surface area of 201 m2/g and a large external surface can explain high isomer selectivity [39]. Catalysts 2018, 8, x FOR PEER REVIEW 9 of 25

ZSM-12 12-MR 40 Offretite (wt.%) L 10 Beta, USY 30 IZM-2 ZSM-12 SAPO-5 ZSM-48 20 SAPO-11

ZSM-11 10 ZSM-22 di-branched C di-branched 10-MR ZSM-5

0 50 55 60 65 70 75 80 85 90 95 100 mono-branched C (wt.%) 10 FigureFigure 8. TheThe yield yield of ofdi-branched di-branched C10 C10 as a as function a function of monobranched of monobranched C10 in C10 n-decane in n-decane hydro- ◦ hydro-isomerizationisomerization at 5% at isomerization 5% isomerization yield.yield. Condit Conditions:ions: 4.5 bar, 4.5 300bar, ° 300C, HC,2/n H-C102/n-C10 molar molar ratio ratio of 214, of 214,WHSV WHSV of 0.37 of h 0.37r−1 [3 hr9−].1 Reproduced[39]. Reproduced with permission with permission from Mota from et Motaal., J. etCatal al.,.; J.published Catal.; published by Elsevier by, Elsevier,2013. 2013.

OnOn the the contrary, contrary, 12 12 MR MR zeolites, zeolites, such such as Beta as andBeta USY and (USY USY denotes(USY denotes ultra stable ultra Y stablezeolite) Y produce zeolite) mono-,produce di, mono and multibranched-, di, and multibranched compounds, compounds as well as, crackingas well as products cracking (Figure products9)[ (21Figure]. One 9) Pt-H-Beta [21]. One zeolitePt-H-Beta catalyst zeolite with catalyst optimized with optimized acidity (Pt-H-Beta acidity (Pt with-H-Beta Si/Al with ratio Si/Al of 26) ratio was of giving 26) was a 76%giving yield a 76% of ◦ isomersyield of atisomers 94% conversion at 94% conversion and 220 andC under220 °C 30under bar with30 bar the with molar the feedmolar ratio feed of ratio H2/feed of H2/feed of 20 of [16 20]. However,[16]. However, over over Pt-H-Beta Pt-H-Beta with with Si/Al Si/Al molar molar ratio ratio of of 15 15 the the yield yield of of isomerization isomerization products products inin thethe hydroisomerizationhydroisomerization of of hexadecane hexadecane exhibited exhibited a maximum, a maximum, 44 wt 44 %wt at % 60% at conversion,60% conversion, thereafter, thereafter more, crackingmore cracking products produ werects formedwere formed [22]. This[22]. result This result was explained was explained by ability by ability of reactant of reactant to diffuse to diffuse into largeinto zeolite large zeolite pores and pores further and cracking further reactions cracking of reactions the formed of products. the formed The products.formed monobranched The formed productsmonobranched exhibited products high cracking exhibited rate. high An cracking enhanced rate. formation An enhanced of C1, formation C2, and C14 of C1, compound C2, and wasC14 confirmedcompound to was occur confirmed via hydrogenolysis to occur via on hydrogenolysis Pt sites present inon the Pt Pt-H-Betasites present catalyst in the [22 Pt].- OneH-Beta conclusion catalyst of[22] that. One study conclusion was that of acidity that study of Pt-H-Beta was that is acidity too high, of Pt leading-H-Beta to is the too undesired high, leading cracking to the products. undesired crackingZeolite products. nanosheets have shown high selectivity to branched isomers, when isomerization occurs intensively also on the external catalyst surface [40]. When increasing the zeolite sheet thickness, the isomer yield decreased from 50% with 2 nm thick sheets, to 22% when using 300 nm sheets [40]. It was, however, interesting120 that no correlation was found between acidity and selectivity to isoheptane in heptane hydroisomerization when using thin zeolite nanosheets SiO2-Al (22 nm)O3 [40]. It was concluded in [40] that due to diffusion limitations,100 bulk zeolites are not efficient USY catalysts for the hydroisomerization of long-chain paraffins. ZSM-22 (H) 80

ratio 40

0 0 20 40 60 80 100

Cracking yield (%)

Catalysts 2018, 8, x FOR PEER REVIEW 9 of 25

ZSM-12 12-MR 40 Offretite (wt.%) L 10 Beta, USY 30 IZM-2 ZSM-12 SAPO-5 ZSM-48 20 SAPO-11

ZSM-11 10 ZSM-22 di-branched C di-branched 10-MR ZSM-5

0 50 55 60 65 70 75 80 85 90 95 100 mono-branched C (wt.%) 10 Figure 8. The yield of di-branched C10 as a function of monobranched C10 in n-decane hydroisomerization at 5% isomerization yield. Conditions: 4.5 bar, 300 °C, H2/n-C10 molar ratio of 214, WHSV of 0.37 hr−1 [39]. Reproduced with permission from Mota et al., J. Catal.; published by Elsevier, 2013.

On the contrary, 12 MR zeolites, such as Beta and USY (USY denotes ultra stable Y zeolite) produce mono-, di, and multibranched compounds, as well as cracking products (Figure 9) [21]. One Pt-H-Beta zeolite catalyst with optimized acidity (Pt-H-Beta with Si/Al ratio of 26) was giving a 76% yield of isomers at 94% conversion and 220 °C under 30 bar with the molar feed ratio of H2/feed of 20 [16]. However, over Pt-H-Beta with Si/Al molar ratio of 15 the yield of isomerization products in the hydroisomerization of hexadecane exhibited a maximum, 44 wt % at 60% conversion, thereafter, more cracking products were formed [22]. This result was explained by ability of reactant to diffuse into large zeolite pores and further cracking reactions of the formed products. The formed monobranched products exhibited high cracking rate. An enhanced formation of C1, C2, and C14 compound was confirmed to occur via hydrogenolysis on Pt sites present in the Pt-H-Beta catalyst [22]. One conclusion of that study was that acidity of Pt-H-Beta is too high, leading to the undesired Catalysts 2018, 8, 534 10 of 27 cracking products.

120

SiO2-Al2O3 100 USY ZSM-22 (H)

ratio 40

0 0 20 40 60 80 100

Cracking yield (%) Figure 9. The ratio between mono- to dibranched isomers as a function of cracking in pristane hydroisomerization over Pt-supported silica alumina, USY, and hierarchical H-ZSM-22 catalysts [21]. Reproduced with permission from Martens et al., Catal. Today; published by Elsevier, 2013. 5.1.2. Effect of Acidity and Metal Dispersion in Bifunctional Metal Supported Zeolites Two main parameters determining activity and selectivity in hydroisomerization of long chain alkanes are acidity and metal dispersion. It has been reported that the selectivity of bifunctional catalysts in isomerization is expected to depend on the balance between the metal and acidic functions [41]. Furthermore, typically acidity of zeolites is too high for highly selective hydroisomerization, and several methods have been used to suppress it. A correlation has been found between the metal and acid site concentration ratios and the formation of different products, such as isomers and cracking products [9]. It has been stated that an optimum hydroisomerization catalyst exhibits this ratio that is close to one, i.e., the acid catalyzed reaction becomes rate limiting, and the number of acid sites between two metal sites is low enough. When either metal or acid functions are too dominant, more hydrogenolysis on metal sites or cracking on acid sites occurs, respectively. It has been stated that the maximum yield of isomers with an ideal catalyst can be obtained when cracking and isomerization are rate-determining steps, (de)/hydrogenation steps are in quasi-equilibrium, and the transport of molecules between different sites is very rapid [41]. This was also confirmed in hexadecane hydroisomerization over 1 wt % Pt-Beta-(26), for which the isomer yield was independent on reaction pressure and temperature, increasing with increasing conversion level [16]. Zeolites with different geometrical constrains and an optimum acidity exhibit the following order of the isomer yield: Pt-H-ZSM-22 > Pt-H-USY > Pt-H-ZSM-5, showing clearly that Pt-H-ZSM-22 exhibits shape selectivity, which is absent in large pore zeolites. The three dimensional zeolite is the least selective [4]. On the other hand, the yield of isodecane over Pt-H-ZSM-5 was only 19% at an 83% conversion. It was concluded that Pt-ZSM-22 facilitated pore-mouth catalysis being more selective towards isodecane. The challenge in acidity optimization for hydroisomerization of long-chain alkanes is linked to occurrence of both hydroisomerization and hydrocracking reactions on Brønsted acid sites. After isomerization, the reaction can also proceed with the cracking reaction. More acidic catalysts are prone to excessive cracking resulting in the yield loss, while less acidic catalysts do not catalyze the desired reaction, or allow only weak isomerization, resulting in the formation of low octane number long-branched products with low volatility. The acidity of the catalyst determines the extent of isomerization/cracking reactions, with a decrease in acidity decreasing cracking [4,31,42]. The acidity of zeolites can be tuned via changing the silica-to-alumina ratio. In addition, the acidity of zeolites can be suppressed by the addition of basic elements into the structure; e.g., Ba [12], Mg [13], Fe [29], and coating zeolite with alumina [38]. The location of aluminum and its acidity is also crucial, namely if it is present on external and internal zeolite surfaces [36]. Catalysts 2018, 8, 534 11 of 27

Table 3. Different method used for tailoring the catalyst acidity and structure.

Catalyst Modification Method Catalyst Change in Properties Effect in Hydroisomerization Ref. lower yield of isomers than over higher isomer yield than in Pt-HY without acid Pt-HY-A [24] dealumination: steamed, acid leached Pt-Al-MCM-41 leaching increased amount of acid sites on Pt-ZSM-22 monobranched isomers [43] the external surface Lewis acid sites formed, 70% isomer yield at 94% conversion in n-dodecane dealumination with HCl treatment Pt-SAPO-11 [25] mesopores on crystal surface hydroisomerization total acid sites decreased partially isomer 87.3% selectivity, 76.6% monobranched desilication: alkaline treatment Pt-ZSM-22 due to blocking of micropores [28] products, 80% conversion n-dodecane with Al in situ alkali treatment in the presence lower Brønsted acidity than in the 72.3% selectivity to monobranched products at a ZSM-2/MCM-41 [35] of cetyl triammonium bromide corresponding desilicated catalyst 91% conversion level short channels, lower surface area, 76% isomerization selectivity at 50% conversion in desilication ZSM-22 nanosheets lower acidity, high external [44] n-decane hydroisomerization surface, thermal stability lower amount of Brønsted acid desilication, NaOH treatment Pt-SAPO-11 sites, rearranging acid site high isomerization selectivity [28] location removal of external Al, which is more monobranched and dibranched in desilication, acid washing Pt-ZSM-22 nanorods [21] blocking the pore mouth comparison to linear and tribranched Acidity on internal surface, 84% selectivity to isomers at 92% conversion in desilication Pt-SAPO-11 [34] mesoporosity, faster diffusion dodecane hydroisomerization, faster diffusion ethanol as a cosolvent in 38% selectivity to monobranched isomers at 90% Pt-ZSM-22 nanobundles Al location on the external surface [36] hydrothermal zeolite synthesis conversion in n-dodecane hydroisomerization dual templating method for the high external surface area, low 38 wt % 5-methylpentadecane formed in Pt-ZSM-23 [23] synthesis of zeolites microporous surface area n-hexadecane hydroisomerization decreased acidity, increased Lewis higher isomer selectivity in hexadecane Mg modification Pt-ZSM-23 [13] acidity, higher Pt dispersion hydroisomerization 80% yield to multibranched products in lower Brønsted acidity, high Pt Ba-modification of zeolite Ba-ZSM-12 hydroisomerization of hexadecane at 90% [12] dispersion conversion 72% selectivity to monobranched C12 products in Fe-substitution Pt-H-ZSM-22 lower acidity, Fe/Al ratio 1 hydroisomerization of dodecane at 91% [29] conversion Catalysts 2018, 8, 534 12 of 27

Table 3. Cont.

Catalyst Modiﬁcation Method Catalyst Change in Properties Effect in Hydroisomerization Ref. 66% selectivity to isomers at 96% conversion in solvent free synthesis Pt-SAPO-s lower Pt dispersion hydroisomerization of hexadecane, 50% selectivity [14] to multibranched products 49 wt % multibranched isomers and 21 wt % enhanced acidity, large amounts alumination of MCM-48 Pt-Al-MCM-48 monobranched in hydroisomerization of [24] of Lewis acid sites hexadecane at 85% conversion mesoporous shell around the Y-MCM-41 composite Pt-Y/MCM-41 yield of C10 isomers 30.6% at 34% conversion [19] zeolite phase, rapid diffusion alumina coated composite catalyst, less agglomeration 80% isomer yield at 92% conversion in hexadecane aluminated H-Beta zeolite α-Al O -Pt-H-Beta [38] 2 3 compared to parent zeolite, hydroisomerization isolated nanocrystals monosubstituted isomers 24 wt % at 36% composite γ-Al O -Pt-H-Beta γ-Al O -Pt-H-Beta (Si/Al = 80) low acidity [10] 2 3 2 3 conversion Catalysts 2018, 8, 534 13 of 27

One example of using different Si/Al ratios in Pt-H-Beta catalysts was demonstrated in hexadecane hydroisomerization at 220 ◦C under 30 bar hydrogen [16]. In that work, three Si/Al ratios (11, 16, and 26) were used in Pt-H-Beta catalysts. The isomer yield decreased at 70% conversion from 52% over 1 wt % Pt-H-Beta (hydroisomerization) to 32% for 1 wt % Pt-H-Beta-(11) in the hydroisomerization of hexadecane at 220 ◦C under 30 bar hydrogen. The best results were obtained for 1.9 wt % Pt-H-Beta (26), giving 72% yield of isomers at 90% conversion [16]. Modification of zeolites with basic (Ba2+, Mg2+, Na+) and acidic (Fe2+) elements, aiming at a decrease/modification of acidity and a higher metal dispersion has been investigated in hydroisomerization of long-chain alkanes [12,13,22,29]. Ba-modification of ZSM-12 favored hydroisomerization over cracking in hexadecane hydroisomerization [12], due to an increase of Pt dispersion, and a lower Brønsted acidity in comparison with the parent zeolite. The highest lumped yield of isomers, i.e., mono- and multibranched products (weight ratio) was obtained over 0.43 wt % Pt-25 wt % Ba-ZSM-12 at 295 ◦C under 60 bar [12]. It was stated that the role of Ba is to stabilize Pt dispersion and tailor the catalyst acidity. The main products with 0.43 wt % Pt-25 wt % Ba-ZSM-12 catalyst were multibranched isomers. Another example is Mg modified Pt-H-ZSM-23 catalyst being a medium pore size MTT type zeolite. When using trace amounts of Mg 55% yield of isomers at 65% ◦ −1 conversion level at 260 C under 30 bar with H2/HD ratio of 6 and WHSV of 1 hr [13] was obtained. This performance was explained by the fact that because the radius of Mg2+ cation is smaller than the pore diameter of ZSM-23, it can also enter the zeolite pores. A slight decrease of BET surface area was observed when Mg2+ was added into Pt-ZSM-23. Simultaneously, Mg2+ increased the Lewis acidity and decreased the Brønsted acidity [13]. Furthermore, sodium modified Pt-Beta zeolite was highly selective to multibranched isomers in hexadecane hydroisomerization at 220 ◦C under 30 bar, in comparison with Pt-H-Beta [22]. Isomorphous iron substitution decreases the catalyst acidity, and thus iron-modified Pt-H-ZSM-22 exhibits higher selectivity towards formation of dibranched isomers, compared to the parent catalyst [29]. This result was explained by the presence of Si-OH-Fe bridge hydroxyl, whereas the unmodified zeolite contains Si-OH-Al [29].

5.2. Composite Materials and Hierarchical Zeolites in Hydroisomerization of Long-Chain Alkanes Composite materials and hierarchical zeolites have been prepared in order to increase the mass transfer inside zeolites. Several methods exist for preparation of hierarchical zeolites, such as dual templating [25,36], desilication via alkali treatment [35], dealumination via acid treatment [25], and synthesis of composite materials such as intergrowth micro/mesoporous ZSM-22/ZSM-23 (Table3)[ 27]. Agglomeration of zeolites can be suppressed via their alumination, which was performed in [9,10]. Several composites, including mesoporous materials, have been developed and tested in hydroisomerization, such as Pt-Y-MCM-41 [45], ZSM-5/SBA-15 [46], and aluminated MCM-48 [24]. Solvent-free synthesis method was used for SAPO-11 in order to enhance its acidity, and simultaneously, Pt metal dispersion in comparison with a conventional SAPO-11 [14]. This method promotes the formation of many Si species in the SAPO region, enhancing its acid site density and the amount of Brønsted acid sites. The amount of mesoporous was lower than in hydrothermally prepared SAPO-11. This material was present in hexadecane isomerization at a relatively low reaction temperature, 290 ◦C, with an 80% yield of isomerization products under 60 bar with WHSV of 1 hr−1 and H2/feed molar ratio of 15, whereas the hydrothermally synthesized Pt-SAPO-11 gave only a 62% maximum isomer yield at 355 ◦C. More multibranched isomers were formed over Pt-SAPO-11 prepared by the solvent free method, due to a relatively low Pt dispersion [14]. The reaction mechanism was proposed to be of the lock-and-key type mechanism over this catalyst, i.e., the two ends of an alkane penetrated into different pore mouths, favoring the formation of the multibranched products. Catalysts 2018, 8, 534 14 of 27

Desilication of zeolites by using an alkali enhances the molecular diffusion inside zeolites [24]. Alkali treatment is also more selective for removing Si, and realumination occurs on the zeolite external surface (Figure 10). Alkali-treated Pt-ZSM-22 exhibited lower amounts of Brønsted acid sites with Catalysts 2018, 8, x FOR PEER REVIEW 14 of 25 increasing NaOH concentration. Over this alkali-leached Pt-SAPO-11 catalyst, a 70% yield of isomers wasisomers obtained was obtained at 80% conversion at 80% conversion in the hydroisomerization in the hydroisomerization of n-dodecane of n-dodecane at 280 ◦C underat 280 20°C barunder using 20 −1 −1 thebar volumetricusing the volumetric ratio of H2/feed ratio ofof 600H2/feed and aof WHSV 600 and of 2a hr WHSV[28 ].of Alkali 2 hr treatment [28]. Alkali can treatment rearrange can the acidrearrange distribution, the acid and distribution at the same, and time at it the increases same time the Brønsted it increases acid the sites Brønsted in zeolites acid [28 sites]. For in example zeolites hierarchical[28]. For example zeolite hierarchical ZSM-22 contained zeolite ZSM less- acid22 contained sites inside less the acid zeolite sites framework,inside the zeolite and an framework increased, amountand an increased of acid sites amou onnt its of externalacid sites surface on its external [21], when surface acidity [21] was, when determined acidity was using determined pyridine using and 2,6-lutidine,pyridine and respectively, 2,6-lutidine, as respectively, probe molecules. as probe molecules.

Figure 10.10. AA schematicschematic picturepicture ofof desilicationdesilication ofof MFIMFI zeolitezeolite [[47]47].. ReproducedReproduced withwith permissionpermission fromfrom Groen etet al.,al., J.J. Mater.Mater. Chem.Chem.;; publishedpublished byby RoyalRoyal SocietySociety ofof Chemistry,Chemistry,2006 2006. .

A comparative comparative study study of of ZSM ZSM-22-22 desilication desilication vs in vs situ in alkaline situ alkaline treatment treatment in the presence in the presence of cetyl oftriammonium cetyl triammonium bromide bromide(CTAB) for (CTAB) preparation for preparation of ZSM-22/MCM of ZSM-22/MCM-41-41 composite material composite was materialmade in was[35]. madeIn the indesilicated [35]. In theZSM desilicated-22, no destruction ZSM-22, of no zeolite destruction was visible, of zeolite while was in the visible, presence while of inalkali the presenceand CTAB, of alkalithe formation and CTAB, of MCM the formation-41 phase of was MCM-41 observed phase in was29Si magic observed angle in spinning29Si magic nuclear angle spinningmagnetic nuclearresonance magnetic spectroscopy resonance (29Si MAS spectroscopy NMR). This (29 Sistu MASdy show NMR).ed that This a lower study Brønsted showed ac thatidity a lowerdetermined Brønsted by acidity pyridine determined Fourier-transform by pyridine infrared Fourier-transform spectroscopy infrared (FTIR) spectroscopywas obtained (FTIR) for ZSM was- obtained22/MCM- for41, ZSM-22/MCM-41,in comparison with in the comparison desilicated with material. the desilicated Furthermore, material. this desilicated Furthermore, material this desilicatedexhibited materiala slightly exhibited higher a slightlyamount higher of external amount Brønstedof external acid Brønsted sites acid, as sites,determined as determined by 2,6 by- 2,6-dimethylpyridine-FTIRdimethylpyridine-FTIR [35 []35. Especially]. Especially with with an anoptimum optimum amount amount of NaOH of NaOH added added in inthe the synthesis synthesis of ofmicro/mesoporous micro/mesoporous hierarchical hierarchical zeolite, zeolite, ZSM ZSM-22/MCM-41,-22/MCM-41, high high selectivity selectivity to to monobranched monobranched isomer waswas obtainedobtained inin dodecanedodecane hydroisomerizationhydroisomerization [[3535].]. Another option to enhance the aluminum presence on the external zeolite (ZSM-22) surface, is to use ethanol as a co-solvent in hydrothermal zeolite synthesis [36], leading to the formation of ZSM- 22 nano-bundles. These particles exhibited, according to 27Al 2D 3Q MAS NMR, more aluminum that was located on the T1, T2, and T3 sites than a conventional ZSM-22. This result was interpreted by the oriented fusion of nanorods, as promoted by the presence of ethanol and formation of new micropores. This Pt-ZSM-22 nanorod catalyst was tested in n-dodecane hydroisomerization, giving 52% selectivity at 96% conversion at 300 °C under 20 bar using LHSV of 2 hr−1 and H2/feed volumetric

Catalysts 2018, 8, 534 15 of 27

Another option to enhance the aluminum presence on the external zeolite (ZSM-22) surface, is to use ethanol as a co-solvent in hydrothermal zeolite synthesis [36], leading to the formation of ZSM-22 nano-bundles. These particles exhibited, according to 27Al 2D 3Q MAS NMR, more aluminum that was located on the T1, T2, and T3 sites than a conventional ZSM-22. This result was interpreted by the oriented fusion of nanorods, as promoted by the presence of ethanol and formation of new micropores. This Pt-ZSM-22 nanorod catalyst was tested in n-dodecane hydroisomerization, ◦ −1 giving 52% selectivity at 96% conversion at 300 C under 20 bar using LHSV of 2 hr and H2/feed volumetric ratio of 600. Another method for producing nanorods is to separate them during desilication [48] leading to formation of mesopores between the nanorods. This type of catalyst loaded with Pt was successfully used in hydroisomerization of decane, nonadecane, and pristane (2,6,10,14-tetramethylpentadecane) [21]. The results showed that the hierarchical Pt-ZSM-22 exhibited a much higher ratio of mono- and dibranched isomers to cracked products in comparison with Pt-USY and Pt on alumosilicate catalysts (Figure9)[ 21]. It was further concluded that catalyst washing after desilication with a mild acid is essential to remove the Al species blocking the pore mouths. Dealumination via an acid treatment is a method to prepare hierarchical zeolites containing mesopores. It is typically performed by stirring zeolite in the acid solution at a low temperature; e.g., 40 ◦C, followed by washing, drying and calcination at a high temperature (550 ◦C) [25]. Dealumination of SAPO-11 was investigated using either HCl or citric acid treatment, and facilitating the formation of mesopores [25]. The HCl-treated catalyst exhibited mesopores mainly on the crystal surface, whereas citric acid treatment formed mesopores both inside and outside the crystals. With HCl treatment, Lewis acid sites were formed. The latter catalyst, treated with citric acid and containing Pt as an active metal, was more selective in the hydroisomerization of n-dodecane at 280 ◦C under 1 bar giving the isomer yield of 74%. This catalyst was prepared with an optimum amount of citric acid, as high amounts of citric acid resulted in a lower crystallinity. It is also noteworthy that the ratio between mono- to multibranched isomers was elevated using citric acid-modified Pt-SAPO-11 as a catalyst compared to commercial SAPO-11, especially at high reaction temperatures [25]. Since the dealuminated hierarchical zeolites, such as ZSM-22, can have Al on the external surface of the zeolite, blocking the micropores, it has to be washed away. This catalyst exhibited a higher selectivity to isomerization products [21]. Dealumination of zeolites is beneficial to enhance isomer yields in hydroisomerization of long-chain alkanes [24]. An acid-leached treated Pt-HY with Si/Al ratio of 24.8 containing both micro- and mesopores has also been investigated in hexadecane hydroisomerization, giving the isomer yield of 53% at 67% conversion [24] which was higher than the corresponding values for Pt-HY (Si/Al = 28.4), namely 42% at a 62% conversion. The Pt-HY catalyst intracrystalline diffusion limitations were revealed in the Arrhenius plots, whereas for acid-leached catalyst, noted as Pt-HY-A, this was absent and the activation energy for hydroisomerization of n-hexadecane was 140 kJ/mol [24]. For hierarchical zeolites exhibiting both meso- and micropores, such as dealuminated HY-30 with 30 nm mesopores, an efficient mass transfer was obtained while the reaction occurs in micropores enhancing selectivity towards isomers [49]. It is also important to note that an acid-leached catalyst (Pt-HY-A) gives a bell shaped cracking products distribution, whereas microporous Pt-HY facilitates the formation of shorter hydrocarbons (Figure 11)[24]. When the Si/Al ratio was changed in Pt-HY zeolites applied for hexadecane hydroisomerization, a bell-shaped product distribution was obtained for Pt-HY with Si/Al of 5, whereas a flatter shape for the cracking product distribution was obtained for Pt-HY (Si/Al 100) [17]. Mesoporosity in zeolites can be created by using structure-directing agents [34]. Carbon black has been used for this purpose to create mesoporosity in Pt-SAPO-11 [34]. This catalyst exhibited high hydroisomerization selectivity due to diminished external acidity, as determined by the adsorption of 2,6-ditertbutylpyridine. Catalysts 2018, 8, x FOR PEER REVIEW 15 of 25

ratio of 600. Another method for producing nanorods is to separate them during desilication [48] leading to formation of mesopores between the nanorods. This type of catalyst loaded with Pt was successfully used in hydroisomerization of decane, nonadecane, and pristane (2,6,10,14- tetramethylpentadecane) [21]. The results showed that the hierarchical Pt-ZSM-22 exhibited a much higher ratio of mono- and dibranched isomers to cracked products in comparison with Pt-USY and Pt on alumosilicate catalysts (Figure 9) [21]. It was further concluded that catalyst washing after desilication with a mild acid is essential to remove the Al species blocking the pore mouths. Dealumination via an acid treatment is a method to prepare hierarchical zeolites containing mesopores. It is typically performed by stirring zeolite in the acid solution at a low temperature; e.g., 40 °C, followed by washing, drying and calcination at a high temperature (550 °C) [25]. Dealumination of SAPO-11 was investigated using either HCl or citric acid treatment, and facilitating the formation of mesopores [25]. The HCl-treated catalyst exhibited mesopores mainly on the crystal surface, whereas citric acid treatment formed mesopores both inside and outside the crystals. With HCl treatment, Lewis acid sites were formed. The latter catalyst, treated with citric acid and containing Pt as an active metal, was more selective in the hydroisomerization of n-dodecane at 280 °C under 1 bar giving the isomer yield of 74%. This catalyst was prepared with an optimum amount of citric acid, as high amounts of citric acid resulted in a lower crystallinity. It is also noteworthy that the ratio between mono- to multibranched isomers was elevated using citric acid-modified Pt-SAPO- 11 as a catalyst compared to commercial SAPO-11, especially at high reaction temperatures [25]. Since the dealuminated hierarchical zeolites, such as ZSM-22, can have Al on the external surface of the zeolite, blocking the micropores, it has to be washed away. This catalyst exhibited a higher selectivity to isomerization products [21]. Dealumination of zeolites is beneficial to enhance isomer yields in hydroisomerization of long- chain alkanes [24]. An acid-leached treated Pt-HY with Si/Al ratio of 24.8 containing both micro- and mesopores has also been investigated in hexadecane hydroisomerization, giving the isomer yield of 53% at 67% conversion [24] which was higher than the corresponding values for Pt-HY (Si/Al = 28.4), namely 42% at a 62% conversion. The Pt-HY catalyst intracrystalline diffusion limitations were revealed in the Arrhenius plots, whereas for acid-leached catalyst, noted as Pt-HY-A, this was absent and the activation energy for hydroisomerization of n-hexadecane was 140 kJ/mol [24]. For

Catalystshierarchical2018, 8 ,ze 534olites exhibiting both meso- and micropores, such as dealuminated HY-30 with 1630 of nm 27 mesopores, an efficient mass transfer was obtained while the reaction occurs in micropores enhancing selectivity towards isomers [49]. It is also important to note that an acid-leached catalyst (Pt-HY-A) givesFurthermore, a bell shaped in [34 cracking] an effort products was made distribution to quantify, whereas the accessibility microporous of the Pt catalyst-HY facilitates pores; thus the adsorptionformation studies of shorter were hydrocarbons made with 2,5-dimethylhexane (Figure 11) [24]. Whenfor SAPO-11 the Si/Al catalysts ratio prepared was changed using in different Pt-HY templates.zeolites applied Thereafter, for hexadecane the diffusion hydro constantisomerization, of this molecule a bell-shaped (D) was product calculated distribution and divided was obtained by the calculatedfor Pt-HY diffusionwith Si/Al length of 5, whereas (L) of this a materialflatter shape using for the the formula: cracking D/L product2. This distribution value was correlated was obtained with thefor isomerPt-HY (Si/Al yield revealing100) [17]. a linear correlation between diffusion characteristics and this yield [34].

Mol % Mol 4

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Carbon number Figure 11. The distribution of cracked products in hexadecane hydroisomerization over Pt-HY-30 (diamond), Pt-Al-MCM-48 (cirle, and Pt-HY-acid leached at 45% yield of cracking products [24]. Reproduced with permission from Kenmogne et al., J. Catal.; published by Elsevier, 2015.

A dual templating method was used to tune the zeolite properties, such as acidity and external surface area. This has been successfully demonstrated in [23], in which a dual templated Pt-ZSM-23 with SiO2/Al2O3 ratio of 155 exhibited a lower microporous surface area than the material for which only isopropyl amine was used as a template. In the dual template Pt-ZSM-23 zeolite, pyrrolidine and isoprolylamine were used as templates, resulted in a rapid crystallization [50]. The isomer yield in hydroisomerization of hexadecane was 20% at a 22% conversion (Figure5)[ 23]. Notably, with this 2 dual templated catalyst, the external surface area was high, 9 m /gcat, and the amount of strong acidic sites was low, promoting high isomer formation and inhibiting cracking. Mesoporous materials exhibit excellent properties for mass transfer possessing at the same time low acidity. It has, however, been demonstrated that Pt-Al/SBA-15 has suitable acidity, predominantly Lewis acidity, and a pore structure favoring the formation of isomers [46]. In decane isomerization ◦ −1 at 325 C under 40 bar using a H2/feed molar ratio 9 and a WHSV of 5.2 hr over Pt-Al-SBA-15, 68% selectivity towards C6 and C7 hydrocarbons was obtained at a 62% conversion [46]. It was hypothesized that for hexadecane hydroisomerization, with this mesoporous support, the product selectivity was determined only by the pore size, which was determining the optimum adsorption strength. In the case of microporous zeolites, the adsorption strength of molecules was too high, being detrimental for product selectivity towards isomers [24]. One example of a mesoporous composite catalyst is aluminated Pt-MCM-48 exhibiting a narrow pore size distribution with 3.2 nm pores. The acidity of MCM-48 has been enhanced by alumination using Al(O-sec-C4H9)3 in suspension with MCM-48 [24]. It was stated that the surface acidity of the resulting Al-MCM-48 material resembles that of zeolites with the surface containing large amounts of Lewis acid sites. This material exhibited a higher acid density than directly in situ aluminated MCM-48. For Pt-modiﬁed catalyst [24], the yield of isomers was 70% at 82% conversion in hexadecane transformation at 240 ◦C under 20 bar using H2/feed molar ratio of 4. Noteworthy is also the preferential cracking during hydroconversion of hexadecane in the center of a reactant favoring the formation of C8 products (Figure 11) when using the mesoporous catalyst [24]. Catalysts 2018, 8, 534 17 of 27

A catalyst made from a commercial Y zeolite dispersed with CTAB; followed by synthesis of MCM-41, resulted in the formation of a composite material exhibiting both faujasite and MCM-41 phases [45]. This catalyst exhibited much lower acidity than the corresponding Pt-Y, but on the other hand, its metal dispersion was 1.8 fold that of Pt-Y. The composite Pt-Y-MCM-41 catalyst afforded 91% selectivity to isomers, while Pt-Y gave only 76% selectivity at a 30% conversion rate in hexadecane isomerization at 270 ◦C under 30 bar [19]. It was concluded that the microporous-mesoporous composite with a mesoporous shell structure is superior, due to a rapid diffusion of the formed isomers, while Pt-Y has interior zeolite crystals in which secondary reactions occur. One example of a micro-mesoporous composite material that is not efﬁcient in hydroisomerization was Pt-ZSM-5/SBA-15, which mainly favored cracking in n-decane hydroisomerization, further emphasizing the effect of Brønsted acidity in this reaction [46]. A composite catalyst comprising α-Al O particles coated with Pt-H-Beta (Si/Al ratio of 11) Catalysts 2018, 8, x FOR PEER REVIEW 2 3 17 of 25 having zeolite loading of 13 wt % was investigated as a catalyst in hexadecane hydroisomerization ◦ at220 220 °C C under under 30 30 bar bar using using a aH H2/feed2/feed molar molar ratio ratio of of 20 20 [9] [9].. The The formed formed catalyst catalyst exhibited lower agglomeration of of the the catalyst catalyst particles particles than than the the parent parent Pt- Pt-H-BetaH-Beta zeolite zeolite,, facilitating facilitating high high activity activity and andselectivity selectivity towards towards isomers. isomers. The Pt The particle Pt particle size in size this in catalyst this catalyst was 2.6 was nm. 2.6 T nm.he total The isomer total isomer yield yieldwas 80 was wt 80 % wtat %a 95% at a 95%conversion conversion level, level, while while the parent the parent Pt-H Pt-H-Beta-Beta gave gave a 28 a wt 28 wt% isomer % isomer yield yield at at78% 78% conversion conversion (Table (Table 1,1 ,Figure Figure 12).12). The The ratio ratio between between mono mono-- to bibranched bibranched isomers isomers was was 0.2, 0.2, indicating thatthat thethe yieldyield of of dibranched dibranched isomers isomers was was high high at at high high conversion. conversion. It was It was concluded concluded that that the αthe-Al 훼2O-Al3-Pt-H-Beta-112O3-Pt-H-Beta composite-11 composite catalyst catalyst with isolatedwith isolated nanocrystals nanocrystals suppressed suppressed the secondary the secondary reactions ofreactions the formed of the isomers, formed due isomers to a short, due residence to a short time residence of the isomers time inside of the the isomers zeolite inside framework. the zeolite As a comparison,framework. As a composite a comparison catalyst, a composite Pt-γ-Al2O catalyst3–H-Beta Pt (with-훾-Al2 aO high3–H- Si/AlBeta (with ratio a of high 150) Si/Al was investigatedratio of 150) inwas the investigated hydroisomerization in the hydroisomerization of hexadecane [10 of]. hexadecane When the Al[10]2O. 3When-to-zeolite the Al ratio2O3-to was-zeolite varied, ratio it was observedvaried, it thatwas theobserved rate was that proportional the rate was to proportional the amount ofto Brønstedthe amount acid of sites,Brønsted while acid distribution sites, while of thedistribution isomerization of the and isomerization cracking products and cracking was independent products was on independent the zeolite content. on the Azeolite yield content. of isomers A (35.5yield wtof isomers %) was obtained(35.5 wt %) at awas 36.5 obtained wt % conversion at a 36.5 rate,wt % withconversion Pt-Al2O rate,3-H-Beta with (80%Pt-Al zeolite)2O3-H-Beta at 75 (80 bar% andzeolite) 290 at◦C 75 using bar and WHSV 290 of°C 1.3 using hr− WHSV1. of 1.3 hr−1.

100 MB CP 80 LL

Isomerization yiled (wt.%) yiled Isomerization 20

0 0 20 40 60 80 100 120 Conversion (%) Figure 12. 12. TheThe yield yield of of isomerization isomerization products products as as a a function of conversion in hexadecane hydroisomerization over over Pt-Beta, Pt-Beta, low low loading loading Al 2 AlO32O-H-Beta3-H-Beta (LL) (LL) and and high high loading loading Al2O Al3-H-Beta2O3-H-Beta [38]. Reproduced[38]. Reproduced with with permission permission from from Batalha Batalha et al.,et al.Microporous, Microporous Mesoporous Mesoporous Mater. Mater;.; publishedpublished by Elsevier,Elsevier, 20132013..

Intergrowth of ZSM-22/ZSM-23, loading of Pt by impregnation resulted in the formation of micro-mesoporous catalyst with such specific properties as lower acidity, needle-shaped particles, which were agglomerating, forming spherical 10 μm particles [27]. Although this catalyst had the lowest activity, it exhibited the highest selectivity towards the hydroisomerization of dodecane, with a 68% yield of isododecane at 320 °C, in comparison with the corresponding Pt-H-ZSM-22 and Pt-H- ZSM-23 catalysts. Pt-loaded zeolite nanosheets with the zeolite framework type MFI structure exhibited a much higher yield-to-isomer products in n-decane hydroisomerization than the parent Pt-ZSM-5. This catalyst exhibited a lower micropore volume, lower acidity, higher external surface area, and high crystallinity, compared to Pt-ZSM-5, yielding an 80% conversion in dodecane isomerization with a 42% yield of isomers, whereas the corresponding yield with Pt-ZSM-5 was 32% [44]. It was concluded that this type of nanosheet Pt-ZSM-5 catalyst exhibited short channels, suppressing the formation of ethyl- and propyl-branching among the products, in line with the transition state shape selectivity concept.

5.3. Effect of Metal

Catalysts 2018, 8, 534 18 of 27

Intergrowth of ZSM-22/ZSM-23, loading of Pt by impregnation resulted in the formation of micro-mesoporous catalyst with such speciﬁc properties as lower acidity, needle-shaped particles, which were agglomerating, forming spherical 10 µm particles [27]. Although this catalyst had the lowest activity, it exhibited the highest selectivity towards the hydroisomerization of dodecane, with a 68% yield of isododecane at 320 ◦C, in comparison with the corresponding Pt-H-ZSM-22 and Pt-H-ZSM-23 catalysts. Pt-loaded zeolite nanosheets with the zeolite framework type MFI structure exhibited a much higher yield-to-isomer products in n-decane hydroisomerization than the parent Pt-ZSM-5. This catalyst exhibited a lower micropore volume, lower acidity, higher external surface area, and high crystallinity, compared to Pt-ZSM-5, yielding an 80% conversion in dodecane isomerization with a 42% yield of isomers, whereas the corresponding yield with Pt-ZSM-5 was 32% [44]. It was concluded that this type of nanosheet Pt-ZSM-5 catalyst exhibited short channels, suppressing the formation of ethyl- and propyl-branching among the products, in line with the transition state shape selectivity concept.

5.3. Effect of Metal The majority of studies on hydroisomerization of long-chain alkanes has been made with Pt-supported catalysts, since this noble metal facilitates hydrogenation/dehydrogenation reaction at lower temperatures compared to transition metal catalysts. Moreover, a lower reaction temperature is beneﬁcial for maximizing the selectivity towards isomers. Pt catalysts used in hydroisomerization have been supported on H-Beta [9], HY [17], MCM-41 [45], MCM-48 [24], ZSM-12 [12], ZSM-23 [19,23,27], ZSM-22 [19,21,22,27,28,32,35,36], ZSM-35 [19], ZSM-48 [19], intergrown ZSM-22/ZSM-23 [27], IZM-2 [39], sulphated ZrO2 [50], γ-Al2O3 [51], Pt-γ-Al2O3-H-Beta [10], SAPO-11 [14,25,33,34], MFI nanosheet [44]. There are only a few publications applying other metals than Pt, such as Pd supported on SiO2-Al2O3 [18], Fe-ZSM-22 [29], Ni2P-SAPO-11 [26], Ni-SAPO-11 [26], Ni-Mo-SAPO-11 [20], Ni-Cu-SAPO-11 [30]. Loading of a noble metal (Pt and Pd) in hydroisomerization catalysts is typically very low. The drawback in using noble metals is their high price; therefore, several efforts have recently been made to perform hydroisomerization over cheap transition metals, such as Ni [30] and Ni2P-SAPO-11 [26]. It has, however, been observed that Ni-SAPO-11 gave selectivity towards hydroisomerization of octane lower than a bimetallic Ni-Cu-SAPO-11, which allowed lower hydrogenolysis activity. Ni2P-SAPO-11 catalyst was investigated in hydroisomerization of n-dodecane [26]. The results showed that the optimum metal loading was 3 wt % and the highest isododecane yield was 65% obtained at 350 ◦C under 20 bar using a WHSV of 3 hr−1 at 73% conversion [26], which is a promising result in hydroisomerization of long-chain alkanes in comparison to noble metal catalysts.

5.4. Effect of Metal Dispersion and Loading High metal dispersion facilitates selective hydroisomerization [32]. Several factors determine the metal dispersion, such as the metal loading, precursor type and the catalyst preparation method [32,33]. In addition, a very important parameter is the metal location, i.e., whether the metal is on the pore mouth, on the internal or external surface of the catalyst. The metal location can also be tuned with a proper selection of the catalyst preparation method [52]. Catalysts 2018, 8, 534 19 of 27

The effect of metal loading and the catalyst preparation method was investigated in a comparative work, using different metal loading methods for SAPO-11 [33]. It was conﬁrmed that the reducibility of Pt in Pt-SAPO-11 prepared by the incipient wetness method was very good, because Pt was mainly located on the outer surface of SAPO-11. On the other hand, when ion exchange and atomic layer epitaxy methods were used for catalyst preparation, Pt was mainly located inside of SAPO-11, making it difﬁcult to reduce. Out of these three Pt-SAPO catalysts, the best behavior was shown be Pt-SAPO-11 prepared by the incipient wetness method [33]. For Pt sites located outside SAPO-11 close to the pore mouth, when the catalyst was prepared by the incipient wetness method, the yield of 58% to multibranched isomers was obtained at an 88% conversion rate in the hydroisomerization of ◦ −1 n-dodecane at 290 C under 80 bar, using a LHSV of 2 hydroisomerization hr , and a H2/feed molar ratio of 22.6 [52]. The catalyst precursor can have a substantial effect on the metal dispersion; for example, (NH4)2PtCl4 could produce Pt particles on ZSM-22 surface that were smaller than Pt(NH3)4Cl2 as a precursor [32]. It was also concluded [32] that Pt-ZSM-22 catalysts prepared from anionic precursors exhibited smaller particle sizes than those prepared from cationic precursors (PtCl4, Pt(NH3)4Cl2), promoting selective hydroisomerization of n-hexadecane [32].

6. Effect of Reaction Conditions in the Hydroisomerization of Long-Chain Paraffins The effect of reaction conditions is compared in this review for continuous operation, since majority of the recent studies have been made in fixed bed reactors. The effect of reaction temperature has been very intensively investigated, while variations in the contact time have received less attention, and only a few studies have report data with varying WHSV [19,23,26]. Temperature and the contact time are the most important parameters in the hydroisomerization of long-chain paraffins, due to their high reactivity towards undesired cracking products. In addition, other parameters such as pressure and the ratio between hydrogen to feed are discussed below.

6.1. Effect of Temperature and Pressure The optimum temperature and pressure for the hydroisomerization of hexadecane over Pt-supported bifunctional catalysts are typically in the range of 220–310 ◦C and 20–60 bar, respectively [9,12,14,15,19,33] in the ﬁxed bed reactor (Table1). However, much higher pressures are typically required in the batch reactor due to different H2/feed ratios. It is also known that a high hydrogen pressure inhibit catalyst deactivation [53]. Activation energy has been determined for the isomerization of hexadecane, being ca. 140 kJ/mol, which is a typical value for alkane hydroconversion in the absence of mass transport limitations [24]. In dodecane hydroisomerization over Pt-supported bifunctional catalysts, the optimum temperatures and pressures are 260–330 ◦C and 20 bar, respectively [14,28,29], although the reaction has also been performed at an ambient pressure at 330 ◦C[25]. When using a non-noble metal catalyst, ◦ Ni2P-SAPO-11, a high temperature of 350 C and 20 bar gave the highest isomer yield (Figure 13b). The reaction order for hydrogen in the hydroisomerization of hexadecane was equal to −0.42, whereas when the catalyst was poisoned by pyridine, the reaction order with respect to hydrogen was zero [15]. Analogous results were obtained in the work of [33], in which both the activity and the selectivity for the formation of isomers decreased with increasing hydrogen partial pressure and increasing conversion over Pt-ZSM-23. Catalysts 2018, 8, x FOR PEER REVIEW 19 of 25

6.1. Effect of Temperature and Pressure The optimum temperature and pressure for the hydroisomerization of hexadecane over Pt- supported bifunctional catalysts are typically in the range of 220–310 °C and 20–60 bar, respectively [9,12,14,15,19,33] in the fixed bed reactor (Table 1). However, much higher pressures are typically required in the batch reactor due to different H2/feed ratios. It is also known that a high hydrogen pressure inhibit catalyst deactivation [53]. Activation energy has been determined for the isomerization of hexadecane, being ca. 140 kJ/mol, which is a typical value for alkane hydroconversion in the absence of mass transport limitations [24]. In dodecane hydroisomerization over Pt-supported bifunctional catalysts, the optimum temperatures and pressures are 260–330 °C and 20 bar, respectively [14,28,29], although the reaction has also been performed at an ambient pressure at 330 °C [25]. When using a non-noble metal catalyst, CatalystsNi2P-SAPO2018, 8-,11 534, a high temperature of 350 °C and 20 bar gave the highest isomer yield (Figure 2013b). of 27

(a) 100

40 conversion

iso-C12 selectivity (%)

iso-C12 yield (%) mono/multi-branched iso-C 20 12 molar ratio

Conversion, selectivty, yield and ratio and yield selectivty, Conversion,

0 0 1 2 3 4 H pressure/MPa 2 (b) 100

n-C conversion (%) 40 12

iso-C12 selectivity (%) iso-C12 yield (%) mono-multi-branched iso-C12 20 molar ratio

Conversion, selectivty, yield and ratio and yield selectivty, Conversion,

0 325 350 375 400 o Temperature ( C) FigureFigure 13. 13.Conversion, Conversion, selectivity, selectivity, yield yield of isomer, of isomer and, the and ratio the between ratio between mono/multibranched mono/multibranched isomers inisomers dodecane in dodecane hydroisomerization hydroisomerization over Nix Povery-SAPO-11 NixPy-SAPO as a function-11 as a function of (a) hydrogen of (a) hydrogen pressure pressure and (b) Hand2/feed (b) H molar2/feed ratio molar [26 ratio]. Reproduced [26]. Reproduced with permission with permission from Tian from et al., TianFuel et Process. al., Fuel Technol. Process; published. Technol.; bypublished Elsevier, by2014 Elsevier,. 2014.

6.2. Effect of H2/Feed Molar Ratio

The effect of the molar ratio H2/feed has been investigated in several publications [12,14,15,22,24,26,33.] This ratio for hexadecane hydroisomerization is typically varied, from 4 to 20 [9,12,14–16,22,24,33], while for a mixture of C15–C18, this ratio was 30 [17]. An optimum molar ratio of H2/feed of 10 was found, giving a stable time-on-stream conversion in the hydroisomerization of hexadecane over Pt on a sulphated zirconia catalyst at 225 ◦C under 20 bar, with a WHSV of 18.4 hr−1 [50]. It was also stated that a high hydrogen pressure is required, since long-chain paraffins give rise to form heavier coke precursors compared to short chain feedstock molecules, for which the optimum ratio of H2/feedstock is 6. Catalysts 2018, 8, x FOR PEER REVIEW 20 of 25

The reaction order for hydrogen in the hydroisomerization of hexadecane was equal to −0.42, whereas when the catalyst was poisoned by pyridine, the reaction order with respect to hydrogen was zero [15]. Analogous results were obtained in the work of [33], in which both the activity and the selectivity for the formation of isomers decreased with increasing hydrogen partial pressure and increasing conversion over Pt-ZSM-23.

6.2. Effect of H2/Feed Molar Ratio

The effect of the molar ratio H2/feed has been investigated in several publications [12,14,15,22,24,26,33]. This ratio for hexadecane hydroisomerization is typically varied, from 4 to 20 [9,12,14–16,22,24,33], while for a mixture of C15–C18, this ratio was 30 [17]. An optimum molar ratio of H2/feed of 10 was found, giving a stable time-on-stream conversion in the hydroisomerization of hexadecane over Pt on a sulphated zirconia catalyst at 225 °C under 20 bar, with a WHSV of 18.4 hr−1 [50]. It was also stated that a high hydrogen pressure is required, since long-chain paraffins give rise Catalysts 2018, 8, 534 21 of 27 to form heavier coke precursors compared to short chain feedstock molecules, for which the optimum ratio of H2/feedstock is 6. InIn the hydroisomerization hydroisomerization of of n n-dodecane,-dodecane, the the H2 H/feed2/feed ratio ratio was wasvarying varying,, in the in range therange of 15– of25 15–25[25,26,[3325,,4626],,33 while,46], for while n-decane for n-decane,, the H2/feed the Hratio2/feed was ratio9 over was Pt-Al 9- overSBA- Pt-Al-SBA-1515 catalyst at 250 catalyst–350 ° atC, ◦ −1 −1 250–350using a WHSVC, using of 5.1a WHSV hr [46] of. 5.1This hr illustrates[46]. Thisthat the illustrates H2/feed that ratio the is Hslightly2/feed lower ratio for is slightly shorter loweralkanes. for The shorter optimum alkanes. H2/feed The optimum molar ratio H2 /feedwas, however, molar ratio 18 was,over however,N2P/SAPO 18-11 over (Figure N2P/SAPO-11 14) [26] in (Figurethe hydroisomerization 14)[26] in the hydroisomerization of dodecane, showing of dodecane,that the catalyst showing type that also thehas catalystan effect type on the also optimum has an effectreaction on theconditions optimum. reaction conditions.

100

40 n-C12 conversion (%) iso C12 selectivity (%) iso C12 yield (%) mono/multibranched iso C12 20 molar ratio

Conversion, selectivty, yield and ratio and yield selectivty, Conversion,

0 10 15 20 25 30 35 40 45 50 H /n-dodecane molar ratio (mol/mol) 2 FigureFigure 14.14.Conversion, Conversion, selectivity,selectivity, yieldyield ofof isomer, isomer and, and ratio ratio between between mono/multibranched mono/multibranched isomersisomers

inin dodecane dodecane hydroisomerization hydroisomerization over over Ni NixxPPyy-SAPO-11-SAPO-11 as a a function function of of H H22/feed/feed molar ratio [[26]26].. ReproducedReproduced withwith permissionpermission fromfrom TianTian et et al., al.,Fuel Fuel Process. Process. Technol. Technol;. published; published by by Elsevier, Elsevier,2014 2014. .

6.3.6.3. EffectEffect ofof WHSVWHSV inin ContinuousContinuous OperationOperation HydroisomerizationHydroisomerization ofof long-chainlong-chain alkanesalkanes isis aa complexcomplex reaction,reaction, composedcomposed ofof consecutiveconsecutive andand parallelparallel reaction reaction routes. routes. For an ideal catalystcatalyst with optimized optimized properties, properties, thethe reaction reaction is is mainly mainly consecutiveconsecutive (Figure (Figure2)[ 2)18 [18]], and, and thus, thus typically, typically relatively relatively short short residence residence times and times low and WHSV low (LHSV) WHSV −1 are(LHSV) required are required [19,23,26 ].[19,23,26] The optimum. The optimum WHSV are WHSV typically are quitetypically short, quite being short, 1.5 hr beingfor 1.5 n-dodecane hr−1 for n- hydroisomerization at 350 ◦C under 20 bar over Ni P-SAPO-11 [26]. At longer WHSV, isomers can dodecane hydroisomerization at 350 °C under 20 2 bar over Ni2P-SAPO-11 [26]. At longer WHSV, ◦ reactisomers further can to react cracking further products. to cracking In hexadecane, products. In hydroisomerization hexadecane, hydroisomerization over Pt-ZSM-23/35 over at Pt 300-ZSMC- under23/35 at 40 300 bar ° theC under optimum 40 bar contact the optimum time was contact ca. 0.7 time s [19 ].was ca. 0.7 s [19]. ◦ ForFor C15–C18C15–C18 alkanesalkanes overover Pt-HYPt-HY (Si/Al(Si/Al ratio of 100) at 310 °C under 31 bar,bar, thethe effecteffect ofof LHSVLHSV waswas clearlyclearly demonstrateddemonstrated (Figure(Figure 15 15),), namelynamely thatthat forfor highhigh conversion,conversion, thethe selectivityselectivity toto isomersisomers waswas low and vice versa; i.e., at a LHSV of 1.0 hr−1 conversion, and selectivity were 90% and 47%, while an increase of LHSV to 4.0 hr−1 resulted in a conversion of 52%, and a selectivity of 60% [17]. Catalysts 2018, 8, x FOR PEER REVIEW 21 of 25 low and vice versa; i.e., at a LHSV of 1.0 hr−1 conversion, and selectivity were 90% and 47%, while an Catalystsincrease2018 of, LHSV8, 534 to 4.0 hr−1 resulted in a conversion of 52%, and a selectivity of 60% [17]. 22 of 27

(a) 100

80 conversion iso-diesel (iso-C15-18) jet-fuel (C9-14) gasoline (C5-8) 60 light gases (C1-4)

Conversion (%), Selectivity (wt.%) (%), Conversion

0 0 20 40 60 80 100 120 140 160

TOS (h) (b) 100

conversion iso-diesel (iso-C15-18) 80 jet-fuel (C9-14) gasoline (C5-8) light gases (C1-4)

Conversion (%), Selectivity (wt.%) (%), Conversion

0 0 20 40 60 80 100 120 140 160 TOS (h)

−1 −1 ◦ FigureFigure 15.15. TheThe effecteffect ofof LHSV: LHSV: of of ( a()a 1.0) 1.0 hr hr−1and and ( b(b)) 4.0 4.0 hr hr−1,, atat 310310 °CC underunder 3131 bar bar using using H H2/BDH2/BDH molarmolar ratio of of 30 30 in in the the hydroisomerization hydroisomerization of ofC15 C15–C18–C18 alkanes alkanes over over Pt-HY Pt-HY-100-100 (a Si/Al (a Si/Al ratio ratioof 100), of 100),conversion conversion and selectivity and selectivity as a asfunction a function of time of- time-on-streamon-stream adapted adapted from from[17]. [17Reproduced]. Reproduced with withpermission permission from fromHengsawad, Hengsawad, et al., etInd al.,. EnInd.g. Chem Eng.. R Chem.es.; pu Res.blished; published by American by American Chemical Chemical Society, Society,2018. 2018. 7. Effect of Reactor Selection in Hydroisomerization of Long-Chain Paraffins 7. Effect of Reactor Selection in Hydroisomerization of Long-Chain Paraffins Comparison of Continuous and Batch Reactors Comparison of Continuous and Batch Reactors Typically, hydroisomerization of long-chain alkanes has been performed in fixed bed Typically, hydroisomerization of long-chain alkanes has been performed in fixed bed reactors reactors [9,12,15,18,19,25], with few exceptions when batch reactors were used [1,45]. The reason for [9,12,15,18,19,25], with few exceptions when batch reactors were used [1,45]. The reason for this is this is that a higher hydrogen-to-reactant ratio in a continuous mode, facilitating a higher conversion in that a higher hydrogen-to-reactant ratio in a continuous mode, facilitating a higher conversion in comparison with a batch mode. comparison with a batch mode. Hydroisomerization of hexadecane was performed in batch reactors with bifunctional catalysts containing 0.5 wt % platinum with ZSM-5, ZSM-22, SAPO-11, Al-MCM-41, H-Y, or H-β as supports [1]. The same reaction was used with decane as a substrate over a Pt-Al-MCM-41, Pt-Y, or Pt-Y-MCM-41 Catalysts 2018, 8, 534 23 of 27 composite, and a mechanical mixture of Pt-Y-MCM-41 [45]. A high conversion of n-hexadecane was achieved in a batch reactor at very harsh reaction conditions, i.e., at 350 ◦C under 103 bar over Pt-ZSM-22 catalyst in 2 hr [1]. On the other hand, only a 38% of conversion of hexadecane was obtained over Pt-SAPO-11 in a batch reactor under these harsh conditions after 14 hr. At the same time, 96% conversion of n-hexadecane was obtained at 60 bar, 325 ◦C and WHSV of 1 hr−1 using the molar ratio of H2/n-hexadecane of 15 over Pt-SAPO [14], clearly illustrating that a batch operation requires higher temperatures and pressures to achieve comparative performance with the fixed bed operation. The highest selectivity to isomers was obtained at 350 ◦C under 103 bar in a batch reactor at conversion levels between 37–45% over 0.5 wt % platinum supported on ZSM-5, ZSM-22, SAPO-11, Al-MCM-41, H-Y, and H-Beta [1]. Pt-Al-MCM-41 displayed the best hydroisomerization as well as the highest selectivity to iso-hexadecanes (89 wt. %) at a 44% conversion. These results can be explained by the mild acidity and a metal dispersion of Pt-MCM-41, which is higher than in other catalysts. Pt-SAPO-11 also showed comparative conversion, but exhibited a lower selectivity towards the branched products than Pt-H-MCM-41. It is important to mention that no alkenes were found in products, which is due to a high hydrogen pressure, allowing for the hydrogenation of all alkenes to respective alkanes [1]. The residence time is of crucial importance for the selective hydroisomerization of long-chain alkanes in continuous operation. It is beneficial to shorten the residence time in order to diminish the diffusion path of alkenes from one metal site to another and at the same time decrease the number of acid sites the alkene is in contact with [16,23]. For example in hydroisomerization of hexadecane a high selectivity to isomer was obtained with a mildly acidic dual template, Pt-ZSM-23, using contact times in the range of 1–2.5 s at 300 ◦C under 40 bar when conversion varied in the range of 24–60% [23]. This result indicates clearly that in a continuous reactor, a high yield of isomers can be obtained only with tailored catalysts (Figures5 and6)[ 9,12,38], whereas with e.g., conventional Pt-supported zeolites, short contact times are preferred [22]. On the other hand, a shortened diffusion path can also be obtained by modifying the catalyst structure e.g., via dealumination, desilication, using nano-structured zeolites and applying composite catalysts (see Section5).

8. Catalyst Deactivation and Stability in Hydroisomerization of Long-Chain Parafﬁns Catalyst stability is very important from an industrial viewpoint. Several highly stable catalysts have been identiﬁed in hydroisomerization, including Pt-HY [17], Pt-SAPO-11 [33], Pt-ZSM-22 [17], and Pt-SAPO-11 [19]. Long term stability (ca. 200 hr) of siliceous Pt-ZSM-22 was demonstrated in the ◦ hydroisomerization of hexadecane at 343 C under 40 bar with H2/feed volumetric ratio of 750 using LHSV of 1.2 hr−1 [37]. Analogously Pt-HY(Si/Al ratio of 100) exhibited a very high stability in hydroisomerization of biohydrogenated diesel composed of C15- to C18 alkanes at 310 ◦C using 31 bar and a molar ratio of H2/feed of 30 [17]. This catalyst exhibited a relatively low acidity and a high Pt dispersion (56%) [17]. A stable performance was also obtained for Pt-SAPO-11 in dodecane hydroisomerization at 320 ◦C under atmospheric pressure. In addition, 10 MR ring zeolites can have high stability and resist coke formation in hydroisomerization [19]. Analogously, 0.5 wt % Pt-SAPO-11 was a stable catalyst in hydroisomerization of n-dodecane at 300 ◦C under 1 bar using WHSV of 2 hr−1 for 120 min [33]. This catalyst was prepared by an incipient wetness method and reducibility of Pt was high for this catalyst, as Pt was locating onto the external surface of the catalyst (Section5). An overall conclusion is that deactivation is not an issue when cracking is not too extensive.

9. Conclusions The hydroisomerization of long-chain paraffins for the production of isomers with better fuel properties has recently been intensively investigated. Main efforts have been devoted to developing active and selective bifunctional catalysts, such as different 10 MR zeolites, giving shape selectivity, hierarchical zeolites, and composite materials. The main drawback in zeolites is that their acidity Catalysts 2018, 8, 534 24 of 27 is too high, whereas mesoporous catalyst generally have too low an acidity. The acidity of zeolites has been decreased via coating the zeolite with alumina, or by adding basic or acidic elements to the zeolite. This modification can decrease acidity and enhance metal dispersion at the same time. On the other hand, the acidity of mesoporous materials has been increased via alumination, or by making zeolite-mesoporous composite catalysts. Hierarchical zeolites with optimized acidity and high metal dispersion also facilitate a high mass transfer, promoting a high selectivity to the formation of isomers. These materials have been produced via desilication and dealumination, using either alkaline or acidic treatments, respectively. The hierarchical zeolites typically contain both micro- and mesopores, and lower amounts of acid sites, compared to parent zeolites. The texture of desilicated alkali-treated zeolites is changed so that they contain short channels and a high external surface area. Furthermore, the acid sites can also locate on the external surface. Another method to promote mass transfer in zeolites is to use metal-modified nanoshaped zeolites, i.e., nanosheets and nanobundles also facilitating a rapid reaction and a short residence time for the formed alkene to react further to cracking products.

Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

References

1. Park, K.; Ihm, S. Comparison of Pt/zeolite catalysts for n-hexadecane hydroisomerization. Appl. Catal. A Gen. 2000, 203, 201–209. [CrossRef] 2. Deldari, H. Suitable catalysts for hydroisomerization of long-chain normal parafﬁns. Appl. Catal. A Gen. 2005, 293, 1–10. [CrossRef] 3. Mériaudeau, P.; Tuan, V.; Nghiem, V.; Lai, S.; Hung, L.; Naccache, C. SAPO-11, SAPO-31, and SAPO-41 Molecular sieves: Synthesis, characterization, and catalytic properties in octane hydroisomerization. J. Catal. 1997, 169, 55–66. [CrossRef] 4. Martens, J.; Parton, R.; Uytterhoeven, L.; Jacobs, P.; Froment, G. Selective conversion of decane into branched isomers: A comparison of platinum/ZSM-22, platinum/ZSM-5 and platinum/USY zeolite catalysts. Appl. Catal. 1991, 76, 95–116. [CrossRef] 5. Akhmedov, V.M.; Al-Khowaiter, S.H. Recent advances and future aspects in the selective hydroisomerization of high n-alkanes. Catal. Rev. 2007, 49, 33–139. [CrossRef] 6. Bouchy, C.; Hastoy, G.; Guillon, E.; Martens, J.A. Fischer-Tropsch waxes upgrading via hydrocracking and selective hydroisomerization. Oil Gas Sci. Technol. 2009, 64, 91–112. [CrossRef] 7. Guisnet, M. “Ideal” bifunctional catalysis over Pt-acid zeolite. Catal. Today 2013, 218–219, 123–134. [CrossRef] 8. Yadav, R.; Sakthivel, A. Silicoaluminophosphate molecular sieves as potential catalysts for hydroisomerization of alkanes and alkenes. Appl. Catal A Gen. 2014, 481, 143–160. [CrossRef] 9. Batalha, N.; Pimnard, L.; Bouchy, C.; Guilolon, E.; Guisnet, M. n-Hexadecane hydroisomerization over Pt-HBEA catalysts. Quantiﬁcation and effect of the intimacy between metal and protonic sites. J. Catal. 2013, 307, 122–131. [CrossRef] 10. Gomes, L.C.; de Oliveiro Rosas, D.; Chistone, R.C.; Zotin, F.M.Z.; de Araujo, L.R.R.; Zotin, J.L. Hydroisomerization of n-hexadecane using Pt/alumina-Beta zeolite catalysts for producing renewable diesel with low pour point. Fuel 2017, 209, 521–528. [CrossRef] 11. Polzmann, G.; Baladincz, J.; Hanchok, J. Investigation of producing modern base oils. Hung. J. Ind. Chem. 2008, 36, 107–112. 12. Parmar, S.; Pant, K.K.; John, M.; Kumar, K.; Pai, S.M.; Gupta, M.; Newalkar, B.L. Hydroisomerization of n-hexadecane over Brønsted acid site tailored Pt/ZSM-12. J. Porous Mater. 2014, 21, 849–857. [CrossRef] 13. Lee, S.-W.; Ihm, S.-K. Characteristics of magnesium-promoted Pt/ZSM-23 catalyst for the hydroisomerization of hexadecane. Ind. Eng. Chem. Res. 2013, 52, 15359–15365. [CrossRef] 14. Du, Y.; Feng, B.; Jiang, Y.; Yuan, L.; Huang, K.; Li, J. Solvent-free synthesis and n-hexadecane hydroisomerization performance of SAPO-11 catalyst. Eur. J. Inorg. Chem. 2018, 2599–2606. [CrossRef] Catalysts 2018, 8, 534 25 of 27

15. Regali, F.; Liotta, L.F.; Venezia, A.M.; Boutonnet, M.; Järås, S. Hydroconversion of n-hexadecane on Pt/silica catalysts: Effect of metal loading and support acidity on bifunctional and hydrogenolytic activity. Appl. Catal. A Gen. 2014, 469, 328–339. [CrossRef] 16. Batalha, N.; Astafan, J.; Dos Reis, C.; Pouilloux, Y.; Bouchy, C.; Guillon, E.; Pinard, L. Hydroisomerization of n-hexadecane over bifunctional Pt-HBEA catalysts. Inﬂuence of Si/Al ratio on activity selectivity. React. Kinet. Mech. Catal. 2015, 114, 661–673. [CrossRef] 17. Hengsawad, T.; Srimingkwanchai, C.; Butnark, S.; Resasco, D.E.; Jongpatiwut, S. Effect of metal-acid balance on hydroprocessed renewable jet fuel synthesis form hydrocracking and hydroisomerization of biohydrogenated diesel over Pt-supported catalysts. Ind. Eng. Chem. Res. 2018, 57, 1429–1440. [CrossRef] 18. Regali, F.; Liotta, L.F.; Venezia, A.M.; Montes, V.; Boutonnet, M.; Järås, S. Effect of metal loading on activity, selectivty and deactivation behavior of Pd/silica-alumina catalysts in the hydroconversion of n-hexadecane. Catal. Today 2014, 223, 87–96. [CrossRef] 19. Zhang, M.; Chen, Y.; Zhang, Q.; Tsang, C.-W.; Liang, C. Shape selectivity in hydroisomerization of hexadecane over Pt supported on 10-ring zeolites: ZSM-22, ZSM-23, ZSM-35, and ZSM-48. Ind. Eng. Chem. Res. 2016, 55, 6069–6078. [CrossRef] 20. Xing, G.; Liu, S.; Guan, Q.; Li, W. Investigation on hydroisomerization and hydrocracking of C15-C18 n-alkanes utilizing a hollow tubular Ni-Mo/SAPO-11 catalyst with high selectivity of jet fuel. Catal. Today 2018.[CrossRef] 21. Martens, J.A.; Verboekend, D.; Thomas, K.; Vanbutsele, G.; Pérez-Ramirez, J.; Gilson, J.P. Hydroisomerization and hydrocracking of linear and multibranched long model alkanes on hierarchical Pt/ZSM-22 zeolite. Catal. Today 2013, 219, 135–142. [CrossRef] 22. Merabti, R.; Pinardi, L.; Lemberton, J.L.; Magnoux, P.; Barama, A.; Moljord, K. Effect of Na exchange of a HBEA zeolite on the activity and the selectivity of a bifunctional Pt-HBEA catalyst for n-hexadecane hydroisomerization. Comparison with a Pt-HZSM-22 catalyst. React. Kinet. Mech. Cat. 2010, 100, 1–9. [CrossRef] 23. Chen, Y.; Li, C.; Chen, X.; Liu, Y.; Liang, C. Synthesis of ZSM-23 zeolite with dual structure directing agents for hydroisomerization of n-hexadecane. Microporous Mesoporous Mater. 2018, 268, 216–224. [CrossRef] 24. Kenmogne, R.; Finiels, A.; Cammarano, C.; Hulea, V.; Fajula, F. Hydroconversion of n-hexadecane over bifunctional microporous and mesoporous catalysts. Inﬂuence of pore architecture on selectivity. J. Catal. 2015, 329, 348–354. [CrossRef] 25. Song, H.; Liu, Z.; Xing, W.; Ma, Z.; Yan, Z.; Zhao, L.; Zhang, Z.; Gao, X. Preparation of hierarchical SAPO-11 molecular sieve and its application for n-dodecane isomerization. Appl. Petrochem. Res. 2014, 4, 401–407. [CrossRef] 26. Tian, S.; Chen, J. Hydroisomerization of n-dodecane on a new kind of bifunctional catalyst: Nickel phosphide supported on SAPO-11 molecular sieve. Fuel Process. Technol. 2014, 122, 120–128. [CrossRef] 27. Chi, K.; Zhao, Z.; Tian, Z.; Hu, S.; Yan, L.; Li, T.; Wang, B.; Meng, X.; Gao, S.; et al. Hydroisomerization performance of platinum supported on ZSM-22/ZSM-23 growth zeolite catalyst. Pet. Sci. 2013, 10, 242–250. [CrossRef] 28. Wu, X.; Qiu, M.; Chen, X.; Yu, G.; Yu, X.; Yang, C.; Sun, J.; Liu, Z.; Sun, Y. Enhanced n-dodecane hydroisomerization performance by tailoring acid sites on bifunctional Pt/ZSM-22 via alkaline treatment. New J. Chem. 2018, 42, 111–117. [CrossRef] 29. Liu, S.; Ren, J.; Zhu, S.; Zhang, H.; Lv, E.; Xu, J.; Li, Y.-W. Synthesis and charascterization of the Fe-substituted ZSM-22 zeolite catalyst with high n-dodecane isomerization performance. J. Catal. 2015, 330, 485–496. [CrossRef] 30. Yang, Z.; Liu, Y.; Liu, D.; Meng, X.; Liu, C. Hydroisomerization of n-octane over bimetallic Ni-Cu/SAPO-11 catalysts. Appl. Catal. A Gen. 2017, 543, 274–282. [CrossRef] 31. Soualah, A.; Lemberton, J.; Pinard, L.; Chater, M.; Magnoux, P.; Moljord, K. Hydroisomerization of long-chain n-alkanes on bifunctional Pt/zeolite catalysts: Effect of the zeolite structure on the product selectivity and on the reaction mechanism. Appl. Catal. A Gen. 2008, 336, 23–28. [CrossRef] 32. Wang, Y.; Tao, Z.; Wu, B.; Xu, J.; Huo, C.; Li, K.; Chen, H.; Yang, Y.; Li, Y. Effect of metal precursors on the performance of Pt/ZSM-22 catalysts for n-hexadecane hydroisomerization. J. Catal. 2015, 322, 1–13. [CrossRef] Catalysts 2018, 8, 534 26 of 27

33. Lee, E.; Yun, S.; Park, Y.-K.; Jeong, S.-Y.; Han, J.; Jeon, J.-K. Selective hydroisomreization of n-dodecane over platinum supported on SAPO-11. J. Ind. Eng. Chem. 2014, 20, 775–780. [CrossRef] 34. Kim, M.Y.; Lee, K.; Choi, M. Cooperative effects of secondary mesoporosity and acid site location in Pt/SAPO-11 on n-dodecane hydroisomerization selectivity. J. Catal. 2014, 319, 232–238. [CrossRef] 35. Liu, S.Y.; Ren, J.; Zhang, H.K.; Lv, J.L.; Yang, Y.; Li, Y.W. Synthesis, characterization and isomerization performance of micro/mesoporous materials based on H-ZSM-22 zeolite. J. Catal. 2015, 335, 11–23.36. [CrossRef] 36. Chen, Y.; Liu, S.; Wang, H.; Ning, Q.; Zhang, H.; Yun, Y.; Ren, J.; Li, Y.-W. Synthesis and characterization of bundle-shaped ZSM-22 zeolite via the oriented fusion of nanorods and its enhanced isomerization performance. J. Catal. 2018, 361, 177–185. [CrossRef] 37. Niu, P.; Xi, H.; Lin, M.; Wang, Q.; Jia, L.; Hou, B.; Li, D. High selectivity fro n-dodecane hydroisomerization over highly siliceous ZSM-22 with low Pt loading. Catal. Sci. Technol. 2018.[CrossRef] 38. Batalha, N.; Morisset, S.; Pinard, L.; Maupin, I.; Lemberton, J.L.; Lemos, F.; Pouilloux, Y. BEA zeolite nanocrystals dispersed over alumina for n-hexadecane hydroisomerization. Microporous Mesoporous Mater. 2013, 166, 161–166. [CrossRef] 39. Mota, F.M.; Bouchy, C.; Guillon, E.; Fécant, A.; Bats, N.; Martens, J.A. IZM-2: A promising new zeolite for the selective hydroisomerization of long-chain n-alkanes. J. Catal. 2013, 301, 20–29. [CrossRef] 40. Kim, J.; Kim, W.; Seo, Y.; Kim, J.-C.; Ryoo, R. n-heptane isomerization over Pt/MFI zeolite nanosheets: Effects of zeolite crystal thickness and platinum location. J. Catal. 2013, 301, 187–197. [CrossRef] 41. Choudhary, J.R.; Hayasaka, K.; Thybaut, J.W.; Narasimhan, C.S.L.; Denayer, J.F.; Martens, J.A.; Marin, G.B. Pt/H-ZSM-22 hydroisomerization catalysts optimization guided by Single-Event Microkinetic modeling. J. Catal. 2012, 290, 165–176. [CrossRef] 42. Chao, K.; Wu, H.; Leu, L. Hydroisomerization of light normal parafﬁns over series of platinum-loaded mordenite and beta catalysts. Appl. Catal. A Gen. 1996, 143, 223–243. [CrossRef] 43. Claude, M.C.; Martens, J.A. Monomethyl-branching of long n-Alkanes in the range from decane to tetracosane on Pt/H-ZSM-22 bifunctional catalyst. J. Catal. 2000, 190, 39–48. [CrossRef] 44. Verheyen, E.; Changbum, J.; Kurttepeli, M.; Vanbutsele, G.; Gobechiya, E.; Koranyi, T.I.; Bals, S.; Van tendeloo, G.; Ryoo, R.; Kirxscchock, C.E.A.; et al. Molecular shape-selectivity on MFI zeolite nanosheets in n-decane isomerization and hydrocracking. J. Catal. 2013, 300, 70–80. [CrossRef] 45. Zhang, Y.; Liu, D.; Lou, B.; Yu, R.; Men, Z.; Li, M.; Li, Z. Hydroisomerization of n-decane over micro/mesoporous Pt-containing bifunctional catalysts: Effects of the MCM-41 incorporation with Y zeolite. Fuel 2018, 226, 204–212. [CrossRef] 46. Huyen, P.T.; Nam, L.T.H.; Vinh, T.Q.; Martinez, C.; Parvulescu, V.I. ZSM-5/SBA-15 versus Al-SBA-15 as supports for the hydrocracking/hydroisomerization of alkanes. Catal. Today 2018, 308, 121–127. [CrossRef] 47. Groen, J.C.; Peffer, L.A.A.; Mouilijn, J.A.; Pérez-Ramirez, J. Desilication: On the controlled generation of mesoporosity in MFI zeolites. J. Mater. Chem. 2006, 16, 2121–2131. [CrossRef] 48. Martens, J.A.; Verboekend, D.; Thomas, K.; Vanbutsele, G.; Gilson, J.-P.; Perez-Ramirez, J. Hydroisomerization of emerging renewable hydrocarbons using hierarchical Pt/H-ZSM-22 catalyst. ChemSusChem 2013, 6, 421–425. [CrossRef][PubMed] 49. Keogh, R.A.; Sparks, D.; Hu, J.; Wender, I.; Tiernye, J.W.; Wang, W.; Davis, B.H. Hydroisomerization and hydrocracking of n-hexadecane over a platinum promoted sulfated zirconia catalyst. Energy Fuels 1994, 8, 755–762. [CrossRef] 50. Busto, M.; Vera, C.R.; Grau, J.M. Optimal processing conditions for the isomerization-cracking of long-chain 2− n-parafﬁns to high octane isomerizate gasoline over Pt/SO4 -ZrO2 catalysts. Fuel Process. Technol. 2011, 92, 1675–1684. [CrossRef] 51. Dhar, A.; Vekariya, R.L.; Sharma, P. Kinetics and mechanistic study of n-alkane hydroisomerization reaction on Pt-loaded γ-alumina catalyst. Petroleum 2017, 3, 489–495. [CrossRef] 52. Lv, G.; Wang, C.; Chi, K.; Liu, H.; Wang, P.; Ma, H.; Qu, W.; Tian, Z. Effects of Pt site distributions on the catalytic performance of Pt/SAPO-11 for n-dodecane hydroisomerization. Catal. Today 2018, 316, 43–50. [CrossRef] Catalysts 2018, 8, 534 27 of 27

2 53. Comelli, R.A.; Finelli, Z.R.; Vaudagna, S.R.; Figoli, N.S. Hydroisomerization of n-hexane on Pt/SO4 -/ZrO2: Effect of total and hydrogen partial pressure. Catal. Lett. 1997, 45, 227–231. [CrossRef]

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