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Catalytic Upgrading of Light Naphtha to Gasoline Blending Components: A Mini Review Abdullah Aitani, Muhammed Naseem Akhtar, Sulaiman Saleh Fahad Al-Khattaf, Yaming Jin, Omer Refa Koseoglu, and Michael T Klein Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00704 • Publication Date (Web): 21 Apr 2019 Downloaded from http://pubs.acs.org on April 21, 2019

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Catalytic Upgrading of Light Naphtha to Gasoline Blending Components: A Mini Review Abdullah Aitani1*, Muhammad Naseem Akhtar1, Sulaiman Al-Khattaf1, Yaming Jin2*, Omer Koseoglo2, and Michael T. Klein1,3 1Center

for Refining & Petrochemicals, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia 2R&D

Center, Saudi Aramco, Dhahran 31311, Saudi Arabia

3Department

of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States

Email addresses of authors: 

Dr. Abdullah Aitani: [email protected]



Dr. Muhammad Naseem Akhtar: [email protected]



Sulaiman Al-Khattaf: [email protected]



Dr. Yaming Jin: [email protected]



Dr. Omer Koseoglo: [email protected]



Dr. Michael T. Klein: [email protected]

Corresponding authors: 

Abdullah M. Aitani, Center for Refining & Petrochemicals, P.O. Box 883, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia, E-mail: [email protected], Phone: +966-13-860-3007; ORCID ID: Orcid.org/0000-00015071-4034



Yaming Jin, R&D Center, Saudi Aramco, Dhahran 31311, Saudi Arabia, Email: [email protected], Phone: +966-13-872-5885

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ABSTRACT The upgrading of light naphtha (C5-C6 stream) to gasoline blending components has been the subject of intensive research at both academic and industrial laboratories. The combination of high volatility and low-octane number has made this stream surplus at many refineries worldwide. This review presents latest developments in selected catalytic upgrading processes and brief discussion on reaction mechanism and reactor models. A majority of the review falls within the development of catalysts for n-hexane isomerization to hydrocarbon isomers with high octane number. There are three types of isomerization catalysts that include Pt/Al2O3-Cl, Pt/SO4-ZrO2 and Pt/zeolite. Efforts are ongoing to improve catalyst performance for higher selectivity and catalyst life time. Very little work has been published on the conversion of n-pentane mainly due to its low activity and the limited options available for its transformation to gasoline blending components. Other approaches discussed in the review include dimerization and oligomerization of C5-C6 alkenes and methylative homologation. The review covers literature published during the period of 2000-2018.

Keywords: Light naphtha, alkanes, pentane, hexane, gasoline, isomerization, dimerization, oligomerization, zeolites, isomerate, octane number

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1. INTRODUCTION The transformation of light naphtha or C5-C6 stream, which originates from refinery and gas plants, into value-added gasoline blending components has been an ongoing challenge to researchers in academia and industry. Currently, the global demand for light naphtha is estimated at 378 million tons per year with an annual growth rate of 1.9 % up to 2021.1 The primary use for light naphtha is as feed for steam crackers (60 %) for the production of olefins (ethylene, propylene and butenes) and as a blending stock for gasoline production (30 %). However, the light naphtha stream has become undesirable gasoline blending component because of its low-octane number and high vapor pressure. This challenge has led refiners to seek novel approaches to upgrade this low-value stream into higher value products. The transformation of light naphtha has been hindered by inertness of carbon-carbon and carbon-hydrogen bonds, which results in elevated temperature and therefore unfavorable thermodynamics, low selectivity, yields, and high cost for commercial applications.2 As refiners continue to process lighter feeds such as shale oil and condenstates, a process that cost effectively converts excess C5-C6 components is highly desirable.3 The current global demand for gasoline is 26.1 million barrel per day (bpd) or about 26 % of global refined products demand.4 Table 1 presents the global gasoline demand by region between 2013 and 2017 showing an average annual growth rate of 2.3 %. The gasoline pool receives product streams such as isomerate, reformate, alkylate and FCC gasoline from different units in the refinery as well as the addition of renewable oxygenates. The composition of gasoline comprises different compounds of alkanes, isoalkanes, olefins, naphthenes and aromatics, known as (PIONA).5 The production of branched alkanes from light naphtha has become one of the important targets in refineries. Under clean-fuel regulations, refiners must reduce sulfur content in gasoline which will add more challenges to utilize available volumes of low-octane light naphtha in the gasoline pool. Table 2

presents the values of octane numbers (MON and RON) for light naphtha (C5 and C6) hydrocarbons.6

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In some studies, the scheme for n-hexane transformation was used as a model reaction for the catalytic upgrading of light naphtha.7 The required reaction pathways are towards dehydrogenation, isomerization and aromatization, while catalytic cracking is not needed. Other pathways include dimerization, oligomerization, homologation with methanol and etherification. Alternatively, when the refinery is integrated with chemicals production, the cracking of C5-C6 alkanes into lighter C2 and C3 alkenes would enable improved value capture by converting them into petrochemical feedstocks. The objective of this article is therefore to review latest developments in selected catalytic processes for the upgrade of light naphtha including isomerization, dimerization and methylative homologation. The discussion is divided according to the upgrading method and the type of catalyst used. 2. ISOMERIZATION OF C5-C6 ALKANES The C5-C6 alkanes isomerization to hydrocarbons with high degree of branching is an important refining process used to meet high-octane and clean fuel challenges. The main objective of isomerization is to maximize RON of C5-C6 alkanes (by as much as 25 points) along-with a maximum branching. For instance, the RON of n-hexane is 25 and that of npentane is 62, while the RON of 2,3-dimethyl butane (DMB) and i-C5 are 93 and 105, respectively. The three isomers of C5 alkane include n-pentane, isopentane (methylbutane) and neopentane (dimethylpropane). The five isomers of C6 alkane include n-hexane, 2,2dimethylbutane, 2,3-dimethylbutane, 3-methylpentane and 2-methylpentane. In isomerization, noble metals such as Pt atoms upgrade the alkane feed and small quantities of promoter metals atoms (Ir, Rh, Sn and Re) give activity for breaking C-H and C-C bonds.7 Bifunctional catalysts used in the isomerization process comprise supported platinum over solid acids, sulfated ZrO2, W oxide over ZrO2, oxycarbides of W and Mo, and reduced Mo oxide.8 Few reviews9-15 and book chapters16-18 discussed the isomerization of light naphtha in terms of catalysts, deactivation, reaction thermodynamics, kinetics and process developments. In the studies for upgrading light alkanes, the effects of catalyst composition and preparation (i.e., zeolite type, presence of promoters, and incorporation methods) on the upgrading reactions have been investigated.

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2.1. Reaction Engineering: Thermodynamics, Kinetics and Reactor Models. The isomerization of C5-C6 alkanes is an acid catalyzed reaction. The equilibrium has to be shifted to low-octane alkanes with the increase in process temperature. Therefore, temperature of isomerization needs to be lowered to increase the yield of dimethyl alkanes.10 The functions of bifunctional catalysts composed of metals and solid acids are normally understood in this way: the metal functions to promote dehydrogenation of alkanes to form alkenes which undergo protonation by protonic acid sites to form carbenium ions. The key catalyst parameter is the strength of acid sites, as stronger acidity allows operation at lower reaction temperatures which is beneficial from the thermodynamics point of view.19 Shakun et al.20 argued that the classification of various isomerization catalysts should be made based on their chemical composition and not based on the range of operating temperatures, as shown in Figure 1. A widely known classification distinguishes between low-temperature, mid-temperature and high-temperature isomerization catalysts. The first applies to highly chlorinated alumina catalysts; the second, to sulfated zirconia catalysts; the third, to zeoliteplatinum catalysts.20 Upgrading of C5-C6 alkanes may include the reactions of skeletal isomerization and alkylation of alkanes. The key steps of these reactions are activation of alkanes to form carbocations (carbenium ions), oligomerization, cracking and coke formation. Activation of alkanes occurs in different ways:17 i.

Alkanes protonation to form pentacoordinated carbocations (carbonium ions) and dehydrogenation or dealkylation;

ii.

Hydride abstraction from alkanes by Lewis acid sites; and

iii.

Protonation of impurities such as alkenes and aromatics to form carbenium ions followed by hydride transfer from the reactant alkanes to the carbenium ions.. Products resulting from the above activation are the result of the reactions of carbenium

ions and olefins, the quantitative details of which being dictated by reaction conditions and catalyst properties. The details have been captured in the form of mathematical models of the reaction chemistry, the reactors in which these reactions take place, and the process schemes in which the reactors and possible separation units are placed. These are considered in turn. 5 ACS Paragon Plus Environment

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The isomerization of mixtures of hydrocarbons with high n-alkane and or cycloalkane concentration were conducted over Pt/SZr catalyst at 150-200 °C, 1.2-3 MPa, 1-4 h-1 LHSV and hydrogen/feed molar ratio of 1-2.21 Isomerization of pure cycloalkanes and cycloalkane rich light naphtha fraction was studied in a plug flow reactor to evaluate reaction rate and apparent activation energies. In another study, a mathematical model of an industrial n-C5/C6 alkanes isomerization unit was developed for different feed composition over chlorinated alumina catalyst.22 In the developed scheme, C7 and lower components were considered as pure hydrocarbons because their octane numbers and their reactivity are very much different. The composition of isomerate product measured using the developed model matched with the experimental data.22 Said et al.23 presented a reactor model using real data from process stream for the Hydrogen-Once-Through (HOT) process known as Penex which is licensed by Honeywell UOP. The authors used a network of nineteen components and twenty reactions for the reactor model which was validated using four months of industrial plant data. The developed model was used to optimize the operating conditions resulting in a potential fuel savings in the process. Koncsag et al.24 modeled the isomerization reactor as an ideal PFR in order to extract quantitative kinetics from an experimental program studying the isomerization of n-C5 and nC6 over a Pt/H-zeolite catalyst. Their model, which considered the details of the active center intermediates, provided rate laws and associated parameters that can be used for further prediction of isomerization reaction consequences. The kinetics of n-C6 isomerization over Pt sulfated supported catalysts using alumina and zirconia were determined by Volkova et al.25 using a plug flow reactor (PFR) model of the isomerization reactor. Emphasis was made on the density role for Lewis acid sites on the surface of the catalyst. Grozdanic et al.26 evaluated the isomerization of n-C6 using both one factor at a time (OFAT) and design of experiments (DOE) methods. Both revealed important dependencies of the isomerization results on process conditions. The Box-Behnken DOE model confirmed the OFAT relationships between process variables and responses. A kinetic model containing 15 lumps and 16 reactions was developed by Reza et al.27 The good agreement 6 ACS Paragon Plus Environment

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between model output and experimental data allowed examination of optimal operating conditions, such as space velocity and hydrogen to feed ratio. Toch et al.28 developed a kinetics model for n-hexane isomerization using Pt/MFI catalyst. Based on experimental results, it was observed that the composite activation energy was higher than that of protonated branching of cyclopropyl. The yield of cracking products such as propane was considered as a secondary reaction due to the increase in reaction temperature. The activation energy for protonated cyclopropyl branching toward 3methylpentane was higher than that yielding 2-methylpentane. The experimental molar ratio of 2-methylpentane/3-methylpentane dropped as the temperature was increased. The selective production of 2-methylpentane over MFI support was observed at low conversion. The results of C6 physisorption enthalpy corresponded well with published data.28 Chuzlov and colleagues29-32 have developed a rigorous mathematical basis for both extracting kinetic information from experiments and also using kinetics information in the simulation of isomerization processes. Chuzlov et al.29 summarized the catalytic reaction mechanism, the associated active centers, and the derived molecular transformations of the isomerization reactions over a Pt over sulfated zirconia catalyst in the context of a full set of material and energy balance equations. Placed in a flowsheet with separation units, the model allowed optimization of the process performance. In another study, Chuzlov et al.30 provided a simulation of light naphtha isomerization run in the scheme of maximum nparaffin conversion. The dependence of the produce RON and yield on inlet temperature was provided. Chuzlov and Molotov31 elaborated on the development and use of an isomerization model in a process context. This group also published further details on the mechanism and model.32 Mohamed et al.33 analyzed eight process optimization options and associated economics in terms of the quality of the isomerization product. Attractive alternatives included oncethrough isomerization and a scheme with a de-iso-pentanizer and de-hexanizer. Buitrago et al.34 simulated a process for the isomerization of n-pentane over Pt/SO42-/ZrO2 considering both untreated and flashed-recycled product streams. Their model, which contained both reaction and separation details, provided a basis for process optimization. The power of the neural network modeling approach was brought to bear in light naphtha isomerization by 7 ACS Paragon Plus Environment

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Sadighi et al.35 Their hybrid modeling approach combined a decay function for the catalyst and the recurrent-layer ANN to predict RON, RVP, gasoline flow rate and reactor temperatures. Ono8 extended this computational literature to include quantum chemical calculations related to alkane isomerization. This theoretical catalysis work considered the catalyst explicitly and acid sites role in the steps of isomerization. The reaction engineering basis for the use of isomerization catalysis in industrial processes is thus quite solid and, with proper experimental support, the basis for incorporating developments in catalyst technology should be straightforward. It is therefore cogent to turn attention toward the industrial catalysts for isomerization. 2.2. Industrial isomerization Catalysts. As mentioned earlier, there are three categories of industrial metal-acid isomerization catalysts for n-C5/C6 alkanes to a high octane gasoline blending components. Refiners have the choice between Pt-chlorinated alumina, sulfated zirconia (SZr) and zeolite catalysts according to the available process. While chlorinated catalysts are very sensitive to all types of contaminants in the alkane feed, zeolite catalysts can tolerate sulfur and water in the feed. The isomerization activity of chlorinated alumina is the highest followed by SZr.18,36,37 Table 3 presents the characteristics of the main industrial isomerization catalysts and isomerate product properties. Major C5C6 isomerization process licensors include Honeywell UOP, Axens and GTC. Light naphtha isomerization uses two major process schemes based on once-throughput and recycle feeds. Depending on the composition of the light naphtha, typical overall RON increments are 16 to 23 compared to the feed. In once-throughput, 84 RON is achieved compared with 90 RON in a recycle scheme. In many industrial units, specific isomerization catalysts types are required based on original process schemes or unit revamp work that recommend changing operating conditions and using different type of catalyst. High-RON isomerate is produced at high yield and relatively low operating cost from the isomerization of light naphtha. For example high RON isomerates can be produced using similar same type of catalysts when C5 or C6 components with low RON such as methyl-pentanes (74 RON) are separated from the reactor products and recycled back.16

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2.3. Pt/Zeolite. While the activity of the Pt/zeolite is low, resulting in a lower increase in RON (10 for Pt/MOR vs. 14 for Pt/chlorinated alumina), it outperforms the other isomerization catalysts in terms of sensitivity to contaminants such as water and small amounts of sulfur and regenerability. Pt/MOR has the maximum Brønsted acid sites strength providing the maximum isomerization activity at a SiO2/Al2O3 ratio of about 10. The dealumination of mordenite (MOR) zeolite decreases diffusional limitations and coking rate. The overall strength of Brønsted acid sites could be further increased by steaming and acid leaching.10,39 Bauer et al.18 reviewed the industrial isomerization over zeolite catalysts. Specific isomerization reaction mechanisms involve species of relatively similar size, so zeolites, with their precise morphologies, can be made into exceptional catalysts with high selectivity. The ability to adjust zeolite chemistry through innovative synthesis or postsynthesis treatments further enhances their versatility in isomerization applications. Ahari et al.40 investigated the influence of H2 partial pressure on light naphtha isomerization over Pt/MOR between 7.1 to 0.9 MPa, 260-270 C, and 1.5 h-1 LHSV. Conversion of n-pentane and n-hexane increased to 50 wt.% and 60 wt.%, respectively, showing the positive effect of pressure on decreasing catalyst deactivation. No effect was observed upon using Pt/MOR/MCM-41 for n-hexane isomerization.41 The reaction proceeded without obvious limitation in the diffusion of MOR porous catalyst system that was obtained by recrystallization in NaOH solution. Monteriro et al.42 showed that Pt/desilicated MOR catalysts had an improved selectivity to DMB isomers over MP isomers, which is due to preservation of MOR acidity and mesoporosity. Different concentrations of Pt/MOR were synthesized by ion exchange utilizing solutions of Pt complexes in aqueous form.43 All catalysts were more selective to 2-MP compared with 3-MP as well as to 2,3-DMB compared with 2,2-DMB. The ratio of MPs decreased as Pt content increased and the ratio of DMBs increased as Pt content increased. The best conversion and selectivity for 2,3-DMB were found at 1.10 wt.% Pt/MOR. Ni impregnated Pt/BEA (SiO2/Al2O3 =10) and Pt/MOR(12.5) were evaluated at 225-375 C and 1.3 h-1 LHSV.44 Nickel addition at 0.3 wt.% BEA and 0.1 wt.% MOR increased n-C6 conversion and selectivity to DMBs due to improved synergy between metal and acid and 9 ACS Paragon Plus Environment

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lower yield of cracked products. Ni impregnation increased the sustainability of the catalysts and favored the PCP intermediate mechanism. At 375 C, the isomerate obtained over BEA had better RON of 68 compared with that over MOR of 60 corresponding to n-hexane conversion of 75 wt.% and 65 wt.%, respectively. Bimetallic (Ni-Pt)/MOR and monometallic (Pt or Ni) catalysts and were evaluated in a fixed-bed reactor at 0.1 MPa, 250 C, H2/n-hexane molar ratio of 9.45 The TPR profiles suggested that the presence of Pt contributed to the reduction of Ni2+ cations in bimetallic catalysts. The bimetallic catalysts showed better activity for isomerization of n-hexane and improved stability upon increasing their Pt content. n-Pentane isomerization was conducted over several Pt supported MFI and BEA zeolite catalystss with different acidity and structure at 300 C, 0.1 MPa, 2.3 h-1 WHSV and H2/npentane molar ratio of 2.75.46 It was concluded that the zeolite structure is more important in isomerization reaction compared with acidity. The BEA catalyst was found to be more selective and active in the production of iso-C5 regardless of acidity. The mobility of the carbocation intermediate through the structure of BEA zeolite was much faster and interconnected. The effect of rare-earth elements on metal and acid sites in Pt/BEA catalyst was investigated for n-hexane isomerization.47 An increase in selectivity and activity into DMB isomers was found at very low content of Nd and La (< 0.4 wt.%). At higher content of rareearth elements and with reference to Yb doped sample, higher cracking selectivity and lower catalytic activity were observed due to the stereochemical limits within BEA zeolite. Ramos et al.48 evaluated the isomerization of three refinery naphthas containing various proportions of alkane, naphthenic and aromatic compounds over Pt/BEA (SiO2/Al2O3 = 12.5) agglomerated with bentonite. The ratio of isomers and linear alkanes (iC8/nC8, iC6/nC6 and iC7/nC7) increase in isomerate products compared with naphtha feed. In all cases, benzene was completed converted thereby increasing the production of naphthenic compounds. The authors concluded that after long-term tests and regardless of naphtha type used, the RON of the isomerate remained constant. Among various Pt/zeolites with different zeolites such as BEA with SiO2/Al2O3 ratio of 25, 75, 150), USY 60, MOR 20, 90, FER55 and MFI80, Pt/BEA25 gave the best results for n-hexane transformation with high 10 ACS Paragon Plus Environment

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selectivity to isomer products.49 The specific pore opening of the zeolites played an important role in the distribution of DMB and MP isomers. All zeolite samples showed low ratio of metal to acid sites which favored cracking due to additional reactions of DMB and MP isomers within acid sites. About 1 wt.% of Pt was introduced into ion-exchanged MCM-22 (with La, Nd and Yb) by impregnation, mechanical mixture or ion exchange with Pt/Al2O3.50 For the ionexchanged sample, TEM characterization revealed that Pt was found on the outer surface and inner micropores. However, for the impregnated catalyst, Pt was basically on the outer surface as large particles. For all catalysts, a fast initial drop in n-hexane conversion was observed. The presence of rare-earth elements influenced the catalyst hydrogenation function which led to a decrease in the selectivity to DMBs associated with an increase in light cracked products. Calero et al.51 concluded that MAZ zeolite yielded more DMBs and less MPs than either FAU or MOR based on the analysis of a several publications on n-hexane isomerization. It was suggested that FAU zeolite with high-effective hydrogenation function was the best suited for n-hexane isomerization compared with MAZ and MOR zeolites. Molecular simulations did not agree with the conventional view that the selectivity variation was specifically related to zeolite topology of MOR, MAZ or FAU zeolite. The results showed that FAU zeolite had the highest porosity as well as the highest activity and selectivity. A mixture of C5-C9 hydrocarbons (containing 30 vol.% n-alkanes) was isomerized over Pt supported on three types of zeolites comprising BEA25, MOR16 and nano MFI possessing different acidity and porosity.52 The catalytic tests were conducted at 240-320 C, 2 MPa, 2.0 h-1 WHSV and H2/HC = 4 mol/mol. Optimal balance between acid and metal sites within Pt/BEA as well as catalyst pore size contributed for the efficient isomerization and performance in enhancing RON. Benzene content was converted with no hydrogenolysis metal activity was reported to produce C1-C2. These results encouraged catalyst scale-up for further isomerization process development. Optimized balance of metal and acid functions in 0.6 wt.% Pt/BEA zeolite contributed to increasing RON to 80 from 44. Li and Iglesia53 investigated the isomerization of n-pentane on Pt cluster catalysts contained in Fe-MFI channels for the production of pentene isomers. The Pt based catalysts 11 ACS Paragon Plus Environment

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showed excellent isomerization catalytic activity and high stability with or without H2 addition. The selectivity to iso-pentene exceeded 60 wt.%. The Fe sites on the Pt clusters dehydrogenated n-pentane and the produced n-pentenes underwent skeletal rearrangements resulting in increased selectivity on Na-Fe-MFI weak acid sites after reducing exchanged Pt cations. MFI channels inhibited sintering and side reactions that which contribute to fast deactivation on Pt clusters. Bimetallic catalysts of Pt-Ni/USY and Pt-Ni/BEA consisting of nanosized (20-30 nm) crystallites were evaluated using a fixed-bed reactor at 0.1 MPa, 230 C and H2/n-hexane molar ratio of 9.0.54 Pt-Ni/BEA catalyst exhibited improved activity as compared with PtNi/H-USY due to the small BEA crystallites which enabled high Pt dispersion and fast diffusion of products and reactant. The high activity of BEA allowed the reaction to be conducted at low temperatures leading to very high isomerization selectivity of 100 wt.%. Among the bimetallic catalysts, best activity observed was with those containing 180 μmol metal/g catalyst of which 60 wt.% was Pt. Pt-Ni/USY catalysts having molar content of 130 μmol metal/g catalyst were tested at 0.1 MPa, 250 C and H2/n-hexane feed molar ratio of 9.0 in a fixed-bed reactor.55 The highest activity was obtained at low activation temperature of 400 C suggesting that the activity could be related to the acid/metal balance. The selectivity to DMBs was not affected by reduction method or temperature. Oliviera et al.56 evaluated the effects of temperature, TOS, and ratios of tetraethylammonium hydroxide/SiO2, H2O/SiO2, and SiO2/Al2O3 on the crystallization of Al-BEA. Pt-Ni/Al-rich BEA-5 exhibited increased selectivity and activity for the production of DMBs than those supported on a commercial BEA-9. The improvement of such properties was attributed to higher number of Brønsted acid sites resulting from the addition of Al atoms in BEA topology. At reduction temperatures above 400 C, the degree of Ni reduction did not take into consideration catalyst activity. It was concluded that the isomeration rate limiting step was alkane protonation on zeolite Brønsted acid sites. Li et al.57 concluded that the yield, conversion and selectivity for n-hexane isomerization over NaOH treated Pt/HZSM-22 and Pt/HZSM-48 were higher than untreated catalysts. The yield of n-hexane isomerization products over NaOH treated Pt/HZSM-22 was higher than 12 ACS Paragon Plus Environment

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NaOH treated Pt/HZSM-48 at relatively low temperature (< 300 °C), while at higher temperature (> 300 °C), NaOH treated Pt/HZSM-48 was superior to NaOH treated Pt/HZSM-22. However, after alkali treatment Pt/HZSM-48 produced more di-branched isomers than Pt/HZSM-22. 2.4. Pt/Mesoporous Materials. The effect of temperature (200-400 °C) on the performance of Pd and Pt (1 wt.% Pt or Pd) catalysts supported on mesoporous silica (HMS) impregnated with H3PW12O40 (HWP) was investigated for the selective n-pentane isomerization.58 The best activity was achieved at 350 °C. The Pt/HPW-HMS catalyst exhibited higher stability, conversion and i-C5 yield compared with the Pd based catalyst. The yields of minor catalytic reactions (< 6 wt.%) increased in the following order: disproportionation < dehydrogenation < cracking. Somorjai and co-workers studied the performance of Pt-Rh/meso silica for the transformation of n-hexane.7 The size and composition of bimetallic PtxRh1−x nanoparticles over mesoporous silica (MCF-17) support were changed to study their effects on n-hexane isomerization at H2/feed molar ratio of 5 and 360 C. The optimum catalyst surface composition for isomer production was Pt90Rh10 nanoparticles. The authors concluded that the size of bimetallic nanoparticles influenced selectivity to desired products by increasing isomerate yield and lowering the cracking reaction on larger-sized particles. Pt nanoparticles supported on Al modified MCF-17 enhanced activity and selectivity (> 90 wt.%) at 240-360 °C.59 The results were attributed to tandem effect between Pt metal and Al-MCF-17. In another study, colloidal Pt nanoparticles with controlled sizes of 1.7-5.5 nm were supported on Al2O3, SiO2, Ta2O5, TiO2, ZrO2 and Nb2O5 to study the effect of Pt size and type of support on the selectivity and activity of n-hexane isomerization.60 Among the various oxide supports, Ta2O5 and Nb2O5 showed the best selectivity, producing C6 isomers such as 2-MP and 3-MP, as desired isomerate at 360 °C and 1 MPa. The selectivity depended on the size of Pt nanoparticles with enhanced isomerate production at Pt size of 5.5 nm. The selectivity to C6 isomers was increased to 97 wt.% upon using the 5.5 nm Pt particles supported on Ta2O5 or Nb2O5. The product yields contained the lowest amount of benzene which is due to the strong interaction effect between support and metal as well as the size of Pt nanoparticle.

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MFI, BEA, Al-modified MCF-17 and mesoporous-silica MCF-17 with or without Pt nanoparticles were investigated for n-hexane isomerization.61 The best catalyst was Pt loaded onto Al-MCF-17 with isomer selectivity exceeding 90 wt.% between 240 and 360 C. The high production of isomers was attributed to mild Brønsted acid and Lewis acid sites resulting from Al modification. The Pt/Al-MCF-17 catalyst was more efficient compared with other mesoporous zeolites which had strong Brønsted acid sites that contributed to undesired cracked products. Figure 2 shows that the optimum isomerization activity occurred over Al-MCF-17 with a SiO2/Al2O3 ratio of 8 and Pt loading of 0.5wt.%. Pinto et al.62 compared the performance of bifunctional and mon-functional catalysts comprising silicotungstic acid (H4SiW12O40) and/or Pt particles over mesostructured SBA15. In bifunctional catalysts, acidic and metallic functions were mixed either mechanically producing a bifunctional material (Pt/SBA-15, HSiW/SBA-15) or by double impregnation of and Pt and H4SiW12O40 leading to HSiW/Pt/SBA-15 (bifunctional catalyst). The monofunctional catalysts were synthesized by SBA-15 impregnation with either H4SiW12O40 or the Pt particle precursor, H2PtCl6. All the developed hybrid catalysts, except monofunctional Pt catalyst, were active for n-hexane isomerization. The two Pt bifunctional catalytic systems exhibited high selectivity and activity for the production of isomerate with no catalyst deactivation after 3 days of continuous operation. Hidalgo et al.64 studied the C5-C7 alkanes isomerization over Zr and Al-loaded SBA-15 catalysts (SiO2/ZrO2 = 30, SiO2/Al2O3 = 30) and 50 molar ratio of Pd (1, 2, 4 wt.%) and Pt (0.5, 1 wt.%). The results showed that Zr–SBA-15 catalysts were less active than Al–SBA15. The low-porosity SBA-15 loaded with Pd at 2 wt.% was the most active isomerixzation catalyst for n-heptane. Adding tungsten to the Pd-based catalyst raised catalytic activity for the reaction. For the case of Pt-based catalysts, the most active was 1 wt.%Pt supported on Al–SBA-15. 2.5. Pt/Sulfated Zirconia (SZr). Busca15 reviewed the role of acid catalysts in industrial isomerization including sulfated zirconia in light alkanes isomerization. The sulfation of metal oxides introduced strong Brønsted acidity and enhanced catalytic activity. The conversion of n-pentane increased associated with a decrease in selectivity due to increasing

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calcination temperature for SZr up to 500 °C.64 At temperatures above 350 C, the reduction of the sulfate group occurred along with H2S formation. The addition of Pt to SZr and WZr enhanced the isomerization activity of these two catalysts by 10 times more than the free-metal systems.65 The activity of Pt/SWZr catalysts for n-pentane and n-hexane isomerization decreased as follows (parenthesis = calcination temperature): Pt/SZr (625 C) > Pt/SWZr (625 C) > SWZr (800 C) = Pt/WZr (800 C). The presence of sulfate species in the Pt/WSZr(625 °C) and Pt/SZr (625 °C) catalysts was responsible for the strong acidic properties of these catalysts. The catalytic performance of conventional Pt/WZr and Pt/SZr was enhanced by combining the anions. Two catalyst formulations based on Pt/Nb-SZr and Pt/Ce-WZr were found as active and selective for isomerization of simulated commercial feed comprising n-pentane 20 wt.%, nhexane 60 wt.% and n-heptane 20 wt.%.66 Catalyst calcination temperature, metal content, nature of the starting precursor salt and promotor played important role in catalyst preparation and performance. However, the Pt/SZr catalyst had more isomerization activity compared with the Pt/WZr based system. Watanabe et al.67 investigated the function of Ru, Pt, Ni, Pd, W and Rh in SZr for light naphtha isomerization. While the addition of Pt maintained high catalytic activity because it prevented

coke

deposition,

Pd/SZr-Al2O3

promoted

desulfurization

and

skeletal

isomerization. Sulfur tolerance was improved by using a hybrid catalyst comprising Pt/SZrAl2O3 and Pd/Al2O3. In another study, Watanabe et al.68 discussed the isomerization of petrochemical raffinate feed containing moisture and heptane compounds over Pt/SZr catalyst. As shown in Figure 3, the isomerate product over Pt/SZr improved RON by 11 points, from 60 to 71, in the operation of an industrial isomerization unit at 140-300 C, 3 MPa, 1.5 h -1 LHSV and H2/HC ratio of 2.0. The modification of SZr by Pt and Pt-Re increased n-hexane conversion.69 While the impact of Re on Pt dispersion was favorable, it was not enough to improve catalytic efficiency. The selectivity and activity of Pt and Pt-Re/SZr catalysts depended on textural features, critical crystallite size, moderate acidity and finally on Pt dispersion. A commercial Pt/SZr catalyst was used to determine influence of H2/HC, WHSV and temperature on nhexane conversion.70 At a conversion of 70 wt.%, the optimum conditions were: 6.0 H2/HC 15 ACS Paragon Plus Environment

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ratio, 2 h-1 WHSV and 170 °C. Unlike DMBs, the yield of MPs depended on temperature between 130 to 170 °C. The highest yields of 3-MP and 2-MP were 23 wt.% and 41 wt.%, respectively, at 170 C. The highest yield of 2,3-DMB was 7 wt.% at 160 C that of 2,2DMB was 3 wt.% at 150 C. Pather et al.71 identified the kinetics for n-hexane isomerization over commercial Pt/SZr catalyst using a procedure of least squares minimization. The model was developed for bifunctional mechanism at 150-170 °C, 2 MPa and H2/HC ratio of 2:1. The results showed that the predicted yields compared well with the experimental data for 2-MP, 3-MP, 2,2DMB, and 2,3-DMB isomers. In another study, Smolikov et al.72 studied the role of Pt state in Pt/SZr/Al2O3 in n-hexane isomerization at 140-400 °C, 1.5 MPa, 2 h-1 LHSV and H2/HC ratio of 3. Characterization results showed that the reduced catalysts contained ionic forms of Pt capable of adsorbing up to three H2 atoms per each surface atom of Pt. It was speculated that the activity and stability of catalysts based on SZr was attributed to the involvement of ionic and metallic Pt in the activation of hydrogen for hydride transfer or modification of the surface acid sites. The formation of an ensemble of states of Pt with different functions ensured high levels of activity and stability for SZr based catalysts. Different sulfated mixed oxides (SZr-La) were studied to find the influence of the sulfate ion source and sol-gel Zr modification with La2O3. 73 The supports were prepared by varying La2O3 content between 2 to 10 wt.%. It was observed that the sulfate agent generated additional active catalysts for n-hexane isomerization and that La2O3 provided higher selectivity (above 80 wt.%) to C6 isomers. Guo et al.74 investigated using a luminescent mesoporous Zr oxy-hydroxyl acetateepersulfate complex in Pd/SZr catalyst for n-hexane isomerization at 0.1 MPa and 180 °C in a fixed-bed reactor. Maximum n-hexane conversion was 75 wt.% and selectivity to i-C6 was 95 wt.% (2,2-DMB at 15 wt.%). Catalyst activity was completely restored by calcination in air for 1.5 h at 450 °C. The high activity was due to the increased Brønsted acidity and the pore structure that comprised a mixture of mainly mesopores and low amount of macropores. 2.6. Pt/Tungstated Zirconia. Barrera et al.75 evaluated the performance of bimetallic and monometallic Pt (0.3 wt.%)-Pd (0.6, 1.0 wt.%) catalysts supported on Al2O3 (15, 0 wt.%), 16 ACS Paragon Plus Environment

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ZrO2 (70, 85 wt.%) and WOx (15 wt.%) which was prepared by sol-gel. The n-hexane isomerization activity of Pd/WZr catalysts was promoted by adding Pt to Pd/WZrAl catalyst and Al to WZr mixed oxide. The enhancement in Pd/WZrAl activity was attributed to a moderate acid strength which was correlated with availability of reduced-state WOx species (W0 or W4+ or) and W6+. Adding Pt to Pd/WZrAl catalyst did not improve the acidic characteristics. The results found that selectivity of n-hexane isomerization was towards MP and DMB isomers.75 The effect of the cetyl-trimethylammonium bromide (CTAB) on the performance of mesoporous Pt/Mn/WZr catalysts and their activity in n-hexane isomerization were investigated.76 Adding 1.0 wt.% manganese favored the formation of tetragonal metastable zirconia phase. The incorporation of CTAB/ZrO2 at different molar ratios resulted in mesoporous solids with narrow pore size distribution and high surface area. About 5 wt.% of tungsten formed WO3 crystalline phase and the remainder was dispersed forming polytungstate amorphous species on Zr surface. WOx sites were accessible to n-hexane feed resulting in higher selectivity and activity for DMB isomers. The activity of WOx/ZrO2 (WZrOH) catalyst was investigated for the isomerization of n-pentane in a fixed-bed down flow reactor at GHSV of 68 h-1 and 280 C.77 The WZrOH was prepared using model crystalline ZrO2 and amorphous ZrOx(OH)4-2x as support precursors. The activity was strongly influenced by the calcination temperature, tungsten oxide surface density and nature of the support. The increased in isomerization selectivity and activity was attributed to the induction period during the reaction which activated the clusters. A bimolecular mechanism was promoted by activated Zr-WOx sites and enhanced isomerization activity. In another study,78 1-pentene and propylene were co-fed to study their effect on n-pentane isomerization. In the case of 1-pentene, higher isomerization activity was obtained and the conversion of n-pentane increased by seven-fold due activated bimolecular pathway. However, in the case of co-feeding propylene, an opposite effect was noticed with main reaction product being isobutene indicating prevalence of bimolecular mechanism for disproportionation reaction. Pt supported on Pt/WZr doped with varying Fe contents were investigated in the isomerization of n-hexane using a high-throughput experiments (HTE) technique.79 The 17 ACS Paragon Plus Environment

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results showed that the presence of low Fe content (0.5-1.0 wt.%) increased conversion and the selectivity to 2,2-DMB isomer. The presence of Fe modified the structure of WOx on zirconia surface which produced active Brønsted acid sites. It was concluded that the incorporation of the surfactant improved Pt/WZr performance, although, the activity was increased by adding Fe at low concentration (< 1.0 wt.%). The results showed that W content could be optimized by adding Fe, since the catalysts with low W content (10 wt.%) promoted with 0.5 wt.% Fe showed high activity even better than catalysts with 15 wt.% W. Scheme 1 shows the effect of Fe promoter at low and high content. Pt/WZr catalysts were evaluated for optimum selectivity and activity using model nhexane mixtures and industrial C5-C6 feedstock at 250 C and 1.5 MPa.80 The Pt/W(24 wt.%)Zr with surface W coverage of 10 W per nm showed excellent performance for nhexane fractions compared with other catalysts with W in the range 15-24 wt.% for Al doped catalysts. The of n-hexane conversion to i-C6 was 45.3 wt.%. The performance of Pt/WZr catalyst for n-pentane isomerization was lower resulting in a drawback for using these catalysts for industrial isomerization of a feed containing n-pentane. Using n-hexane and a mixture of C5-C6 alkanes over Pt/WZr at 225 °C, RON increased by 20 and 17, respectively.81 Another advantage of Pt/W-Zr as compared with Pt/SZr is its high selectivity to isomerization/cracking. However, alkanes with low carbon number (C < 7) are difficult to crack and isomerization selectivity is almost 100 wt.% over Pt/SZr. Accordingly, the only advantage of Pt/WZr appears in the isomerization of alkanes with C > 6. 2.7. Other Isomerization Catalysts. Hancsok et al.82 conducted n-hexane isomerization in the presence of benzene between 0 to 4.6 wt.% over Pt/Al2O3/Cl catalysts (5-10 wt.% chlorine). The reaction over chlorinated catalyst at 110-190 °C produced higher yield (3-7 wt.%) and higher RON (2-4 points) of the free-benzene (< 0.01 wt.%) isomers compared to those produced over industrial catalysts active at 230-270 °C. Yadav and Sakthivel83 reviewed the use of various molecular sieves of silicoaluminophosphate (SAPO) as catalysts for isomerization of alkenes and alkanes. The review concentrated on the importance and role of SAPO in tuning the selectivity toward high-octane isomers. Sinha and Sivasanker84 synthesized two Pt supported SAPO-11 and SAPO-31 catalysts using non-aqueous (ethylene glycol) and aqueous media. The catalysts 18 ACS Paragon Plus Environment

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were evaluated for n-hexane isomerization. The catalysts prepared in ethylene glycol media had more acidity compared with those prepared in aqueous media due to preferential P5+ ions substitution by Si4+. The non-aqueous catalyst showed high n-hexane isomerization. SAPO-31 produced less DMB isomers than SAPO-11 due to its smaller pore size.84 Active sites for n-hexane isomerization were formed on the modified surface of MoO3.85 Isomerization reaction was conducted in a fixed-bed down-flow micro reactor at 300 C, 0.1-0.3 MPa and H2/n-hexane of 15. The MoO3 modification procedure comprised very low heating of 1 °C/min reaching 550 °C with a flow of H2 and CH4 at a ratio of 4:1. The catalyst activity and selectivity to DMBs were improved by catalyst exposure to air at room temperature. n-Hexane isomerization was conducted under H2 atmosphere in a micro-pulse reactor at 150-350 C over Pt/MoZr and MoO3-ZrO2 (MoZr) catalysts.86 The addition of Pt on MoZr changed the concentration of acid sites which was attributed to a lower ZrO2 crystallinity during the treatment.. 3. DIMERIZATION AND OLIGOMERIZATION Catalytic dimerization for upgrading light naphtha requires the dehydrogenation of nC5-C6 alkanes to alkenes and then their dimerization over solid acid catalysts. The growing interest in the dimerization of light alkenes for producing diesel and gasoline blending components is due to more restricted environmental regulations and higher demand for transportation fuels. Surplus of C5 alkenes resulting from its phase-out from gasoline has drawn attention to oligomerization. The dimerization of C5-C6 alkenes has the potential to increase refinery margins by converting these alkenes at low investment and through higher fuel sales. Oligomerization has received both academic and industrial attention since oligomers are clean blending components for gasoline and diesel.87 Sanati et al.88 and Corma et al.89 discussed the use of various catalysts for alkenes oligomerization. In contrast to the rich data available on long and short chain alkenes, published data on C5-C6 alkenes oligomerization is scarce. Most industrial oligomerization processes for light alkenes are operated above 230 °C to crack heavy oligomers and reduce the blockage by catalyst pores. The three major reactions occurring during alkenes conversion to fuel blending components comprise the following: (i) oligomerization of light alkenes, (ii) oligomerization of intermediate carbon-number alkenes, and (iii) cracking of 19 ACS Paragon Plus Environment

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alkenes with higher carbon number.90 The highly-exothermic oligomerization reaction is conducted at relatively low temperature and high pressure thereby shifting the reaction equilibrium to form heavy oligomers. Different reactor configurations such as moving-bed, fluidized-bed reactor, or fixed-bed reactor have been reported in alkenes oligomerization.91 Recently, Nicholas92 reviewed reaction mechanisms and catalysts used in the C2-C4 alkenes oligomerization to fuels and chemicals. The review included metal catalysts, acidic catalysts, aluminum alkyls, Ni and Zr catalytic complexes.92 As shown in Table 4, oligomerization of olefins is conducted over several types of catalysts such as aluminosilicates, zeolites, metal supported aluminosilicates, solid phosphoric acid (SPA), ion exchange resins and pillared clays.93 SPA is one of the earliest oligomerization catalysts (developed by Honeywell UOP) that is still used especially in producing high-octane gasoline components. The structure of oligomer products is affected by the porosity of the zeolite. The catalyst is usually optimized taking into consideration the type of alkene to be oligomerized. The addition of a mesoporous catalyst system is needed in the processing higher carbon number alkenes.89 As the porosity of the zeolite catalyst is increased, the degree of oligomers branching is also increased.90 It has been reported that amorphous silica–alumina (ASA) yield more branched oligomer products having good gasoline octane properties. 3.1. Zeolite Catalysts. Oligomerization over medium pore MFI zeolites showed that the degree and type of oligomer branching are affected by the porous catalyst structure which produces oligomers having less branching.93 MFI showed excellent activity and stability in C3-C6 alkenes oligomerization in several processes such as the Mobil-Olefins-to-Gasolineand-Distillate process (MOGD). The selectivity and life of zeolites used in alkenes oligomerization depend strongly aspects related to SiO2/Al2O3 ratio, crystal size, and deactivation of catalyst surface sites. While MFI has been widely reported for the oligomerization of C3-C4 alkenes, zeolites with large pores and high SiO2/Al2O3 ratio were used in the oligomerization of internal and terminal long-chain alkenes into synthetic lubricant base stocks. Corma et al.89 discussed the importance of intra-crystalline zeolite diffusion paths for oligomer products as a function of density of Brønsted acid sites, crystal size and crystallites 20 ACS Paragon Plus Environment

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mesopores. To achieve better conversion and lower deactivation, the authors suggested having additional mesoporosity and additional Brønsted acid sites. Mesoporous MFI, which is prepared by post-synthesis desilication treatment, showed improvement in 1-pentene conversion and catalyst life. In a recent study, Corma and co-workers94 showed that using BEA zeolite with crystal size between 10 to 15 nm and optimum number and strength of Brønsted acid sites led to high stable and active catalysts with good selectivity to oligomer diesel products. The conversion of 1-pentene increased 80 wt.% after 6 h time-on-stream using two nanosized BEA zeolites (10-15 nm and SiO2/Al2O3 of 15) with short diffusion path length in the small crystallites as well low Brønsted acid site density.94 The effect of WHSV in the oligomerization of alkenes within light cracked naphtha (LCN) was studied over commercial MFI with a SiO2/Al2O3 ratio of 26.90 All experiments were conducted at 6 MPa and 240 C which are similar to industrial oligomerization conditions. At WHSV of 0.5 h-1 and 80 h TOS, conversion of LCN alkenes was 80 wt.% and the yield of gasoil fraction was 71 wt.%. At higher WHSV (> 1.0 h-1), the yield of gasoline fraction increased to more than 32 wt.%. Wang et al.95 investigated 1-hexene oligomerization using MFI zeolite at super- and sub-critical conditions in a fixed-bed down flow reactor at 1-7 MPa and 220-250 °C. 1-Hexene conversion ranged between 83 to 99 wt.% as a function of operating parameters. Increasing temperature from 220 to 250 °C resulted in higher oligomerization selectivity similar to increasing from 1 to 7 MPa. The amount of deposited coke on the catalyst increased from dropped from about 19 wt.% at 235 °C and 1 MPa in sub-critical region to about 10 wt.% at 235 °C and 4 MPa in super-critical region. Using two layers of different MFI zeolites, Kriván et al.96 found favorable process parameters for the oligomerization of light FCC alkenes to gasoline, jet and diesel components at 240 °C, 5 MPa and 2.0 h-1 LHSV. At these conditions, conversion was about 38 wt.%, and share of C12+ hydrocarbons in the oligomer product was 8.0 wt.%. The yield of liquid product reduced greatly because operating at higher temperature. Four amine-free MFI zeolites were synthesized and dealuminated by acid treatment under different conditions.97 The catalysts were studied for 1-hexene oligomerization at 4.5 MPa, 245 C and 0.5 h-1 WHSV. Increasing SiO2/Al2O3 molar ratio, the conversion of 1-hexene decreased

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and a maximum yield of jet fuel components was obtained at a SiO2/Al2O3 ratio of 102. MFI with lower SiO2/Al2O3 ratio had more by-products and more carbon deposition. 1-Hexene oligomerization was investigated at 5 MPa and 200 °C in a fixed-bed down flow reactor over USY zeolite.98 Liquid, super-critical and vapor states of the reactants in the reactor were achieved using octane, propane, dodecane and pentane as solvents. At 12 h time-on-stream, conversion was 85 wt.% in octane compared with 60 wt.% in dodecane. The solvent chain length affected catalyst selectivity, yield of C6 cracked products alkanes and catalyst deactivation. In another study, the authors compared the performance of various catalysts such as silica-alumina, USY, BEA, MCM-41, MCM-22, silica-magnesia and alumina in the oligomerization of 1-hexene at 80-300 C and 5 MPa.99 The activity of MCM-41 and silica-alumina was very similar to that of USY, MCM-22 and BEA zeolites. In the micropores of BEA and USY zeolites, hexenes underwent dimerization-cracking reactions yielding products such as iso-hexenes and cracked products. Silica–alumina cogels, MCM-22 and MCM-41 were the most selective catalysts for 1-hexene oligomerization since the cracking reactions were suppressed. The dimerization of pentenes over BEA, MFI and MOR zeolites was conducted at moderate conditions to minimize undesirable cracking reactions.100 The dimerized product was recycled to the gasoline pool after C12+ fraction separation for use as diesel blending components. It was found that MOR with a SiO2/Al2O3 ratio of 90 gave higher conversion of iso-pentene at 95 wt.% and pentene conversion of 25 to 45 wt.% at conditions that produced 92–95 wt.% dimers. The optimum dimerization reaction was obtained with a at liquid state independent of reaction temperature. Constant conversion was maintained at slow increase in temperature with catalyst cycling times of 600 h between each regeneration. The presence of dienes in the feed did not cause poisoning of the catalyst as compared to undesirable nitrogen and sulfur compounds in the feed. Moon et al.101 conducted 1-hexene oligomerization at 3.5 MPa and 200 °C over H- and NiH-forms of BEA and MFI zeolites with similar Ni content and SiO2/Al2O3 ratio. H-zeolites of MFI and BEA having 10 nm sheet-like crystals and 5nm mesoporosity were more active in converting 1-hexene to oligomer products with carbon number above 10.

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The zeolite framework has a strong role in determining the selectivity of catalytic dimerization. For instance, zeolites with medium-to-large pores have higher activity to dimerization and cracking reactions. Bertrand-Drira et al.102 studied pentene oligomerization over desilicated MOR at different NaOH concentrations. The mesoporous MOR was highly selective and stable for producing higher oligomers (C15-C30). The primary reactions over MFI, FAU and BEA were dimerization, isomerization, and cracking of dimers.103 Other zeolites such as FER and MOR showed various selectivities compared with other zeolites. While MOR fully converted C10 alkenes, FER showed high selectivity and activity for dimerization, with minimal formation of cracked products. Nano-FER zeolite which was synthesized using two organic structure directing agents (OSDA) showed enhanced accessibility of 1-pentene zeolite active sites.104 The improved product diffusion resulted in higher selectivity and conversion at low catalyst deactivation rate for the dimerization of 1pentene into higher oligomers. 3.2.

Mesoporous

Aluminosilicates.

The

catalytic

performance

of

various

aluminosilcates (Al-SBA-15, Al-MCM-41, Al-MTS), micro and nano-MFI zeolites was evaluated for 1-hexene oligomerization at 5 MPa, 200 °C in n-octane solvent.105 The conversions of all catalysts were above 75 wt.% except for micro MFI (8.4 wt.%) due to fast deactivation and low external surface area of 5 m2/g. Total selectivity to higher oligomer products was about 95 wt.% and the best yields of C9-C12 dimers (47 wt.%), C13-C18 trimers (33 wt.%) and C19-C30 heavy oligomers (33 wt.%) were produced over Al-SBA-15, nano-MFI and Al-MTS, respectively. In another study, Escola et al.106 concluded that that the templated sol-gel MTS catalyst showed high oligomerization conversions (70-80 wt.%) with corresponding selectivities to dimers at 30-50 wt.% at SiO2/Al2O3 ratio between 5 and 100 (Figure 4). The effect of nitrogen and sulfur poisoning on 1-hexene oligomerization over mesostructured aluminosilicates (Al-MTS, Al-MCM-41) and nano MFI was studied at 5 MPa, 200 °C in n-octane solvent.107 The presence of 250 ppm nitrogen and 7000 ppm sulfur in the 1-hexene feed resulted in a sharp drop in conversion (from 90 to 27 wt.%) for nano MFI associated with a large increase in the selectivity to C7-C8 light oligomers. However, neither Al-MCM-41 nor Al-MTS catalysts were influenced by S and N poisons due to their meso-porosity, medium acid strength distribution, and high surface area. The 23 ACS Paragon Plus Environment

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oligomerization of FCC olefinic naphtha using Al-MTS at similar operating conditions resulted in 58 wt.% conversion and 90 wt.% selectivity to higher oligomers (32 wt.% of them C13-C18). Modified SBA-15 with B, Al, Cr and Fe was prepared using various methods and tested at 125 °C for 1-hexene oligomerization.108 The highest conversion of 30 wt.% was achieved over Al-SBA-15 with SiO2/Al2O3 at 30. SBA-15 modified with Cr showed 21 wt.% conversion while having low content of heteroatom at a Si/Cr ratio of 240. The major products were dimers using modified SBA-15 catalysts with Al, Fe and Cr (selectivity above 40 wt.%) while SBA-15 modified with B yielded less than 10 wt.% dimers and the remaining heavy oligomers (above 60 wt.%). These results were attributed to mesoscopic ordering of SBA-15 modified with Fe and B and to the differences in the nature of acid sites for SBA-15 modified with Cr and Fe. 3.3. Clays and Solid Phosphoric Acid. Al-pillared saponite (APS) and Al-pillared montmorillonite (APM) showed different activities for 1-pentene oligomerization.109 The activity correlated with structural characteristics and acidity results of the catalysts. At 200 C and 1.5 MPa, 1-pentene conversion was 63.0 wt.% over APS compared with 12.3 wt.% over APM catalyst. The selectivity to gasoline range products was higher over APS due to its weaker acid sites and was very resistant to the regeneration treatments. The influence of free acid composition of SPA catalysts on activity and selectivity for the dimerization of 1-hexene was investigated.110 The rate of 1-hexene dimerization at a reaction temperature of 200 °C in batch reactor was used to compare the selectivity and activity of various SPA catalysts.

At low acid strength, linear hexenes were directly

oligomerized while at high acid strength, a two-step reaction route was favored where hexenes were isomerized before dimerization. The analysis of oligomer product using proton NMR did not show any significant change in the branching upon changing the dimerization pathway as a function of catalyst acid strength. 110 The oligomerization of 1-hexene was investigated over amorphous silica-alumina, solid phosphoric acid, sulfated zirconia, MFI, MCM-41, omega and Y zeolite in a fixed-bed reactor.111 The effect of adding Ni and Cr was also investigated. Oligomerization below 200 °C, where cracking is limited, yielded mainly dimers. The reaction network showed high 24 ACS Paragon Plus Environment

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skeletal isomerization, double bond isomerization and no cracking. It was concluded that acidic catalysts with more than 10 nm average pore size showed improved catalyst lifetime since heavy oligomer products were not readily trapped inside the catalyst. Magnetic NiSO4/γ-Al2O3 containing 7.0 wt.% nickel and NiSO4 monolayer dispersion was investigated for oligomerization of LCN alkenes in both and magnetically stabilized bed (MSB) and fixed-bed reactors.112 The optimum temperature for the MSB reactor was decreased to 170C and the space velocity was between 2.0-6.0 h-1. The diesel range blending components which were produced in the MSB reactor for 100 h showed good lowtemperature flow property and higher cetane number compared with fixed-bed reactor. 3.4. Ion Exchange Resins. Much of the recent work on light alkenes oligomerization was conducted using various resins including Purolite, Amberlyst, Nafion and other resins. Antunes et al.113 reviewed the effect of the resin type, structural and physical properties, additives used such as alcohols and operating conditions on the selectivity and conversion of n-hexene oligomerization. Five types of ion exchange resins such as Amberlyst 35, Amberlyst 70, Amberlyst 15, Purolite CT-276 and Purolite CT-252 and four zeolites (FAU-30, BEA-25, MOR-20 and FAU-6) were compared for the dimerization and trimerization of isoamylene.114 FAU-30 and Amberlyst 15 resin were found most suitable for trimerization yielding trimers (45-50 wt.%) at 100 C in a stirred tank batch reactor. BEA-25 and MOR-20 were found to be the best zeolites to improve dimerization and reduce cracking reaction. The important physical properties that influenced product selectivity were the specific surface area, acid strength and acid capacity for resins and microporous zeolites. In another study,115 various ion exchange resins were screened to select the best catalyst for dimerization of isoamylenes. The reaction was conducted at 70 C to promote dimerization and at 110 C to evaluate side reactions such as copolymerization and cracking. The results showed no significant differences in the performance of the catalysts, however, at 110 C the resins showed better selectivity for dimerization. The best active catalysts for the dimerization of pentene were found to be the sulfonated resins with high-acid capacity and high degree of cross-linking.

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The kinetics of isoamylenes dimerization over Amberlyst-15 was studied in a batch reactor at 50-120 C in n-hexane.116 Reaction data were collected without any internal or external mass transfer effects. The obtained apparent activation energy was 37 kJ/mol. In another study, the oligomerization of 1-hexene was performed over macroporous resins using batch reactor at 77-117 C and 2 MPa.117 At 100 C and 6 h TOS, the conversion of 1hexene was 100 wt.% and selectivity to dimers was 56 wt.%, trimers 0.8 and 43.2 wt.% to isomerization. The apparent activation energy of the dimerization reaction was 64-68 kJ mol-1 based on a homogeneous kinetic model. The oligomerization of FCC naphtha containing 34.2 wt.% C5-C6 alkenes was studied over three ion-exchange resins.118 The highest alkene conversion was 90.9 wt.% and the yield of oligomer products was 29.0 wt.% using an ion-exchange resin having the high surface and low acidity compared with other resins. The favorable process parameters were at 120 to 130 °C, 3 MPa, 1.0 to 2.0 h-1. In another study, the authors used two resins with a layered bed at various operating conditions.119 Alkene conversion was 92.4 wt.% and yield of heavy oligomers (C12+) was 26.2 wt.% at 130 °C, 3 MPa and 1.0 h-1. Another advantage of using the layered bed was better temperature control of the oligomerization reaction. 4. OTHER UPGRADING APPROACHES 4.1. Methylation of Alkanes. The selective C-C bond formation between methanol/DME and light alkane co-feeds is an attractive option for the upgrading of light naphtha. Liu and coworkers used a pulse reaction system to elucidate the reaction mechanism of n-hexane coupling with methanol122 or ethanol123 over MFI(16) at 200-350 °C. Intermediate products (methoxy or ethoxy groups) from the conversion of methanol or ethanol activated n-hexane conversion via a bimolecular hydride transfer. A similar reaction pathway was proposed by Yu et al.2 for the coupling of propane and labeled methanol (13CH3OH) over MFI zeolite using NMR and GC-MS. Scheme 2 presents the reaction for producing value-added gasoline components from the activation of C-H bond via hydride transfer and the formation of methoxy groups. It was concluded that methanol is an excellent methylating agent for the transformation of alkanes at low temperature (< 200 C).2 Triptane or 2,2,3-TMB (heptane isomer) has attracted several research studies because of its high-octane number and high value fuel component. Bercaw et al.124 demonstrated the 26 ACS Paragon Plus Environment

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upgrade of isopentane by methylative homologation to high-value gasoline blending components such as triptane. The authors investigated adding a hydride transfer co-catalyst such as adamantine to enhance the isomerization of alkanes in the presence of methanol and dimethyl ether. The alkanes conversion provides stoichiometric balance, whereas the conversion of methanol and dimethyl ether to alkanes is low in H2 and needs rejection of additional carbon in the form of arenes. This leads to a lower yield of desired high-octane products. It was proposed using other catalysts with molecules that donate hydrogen such as H3PO3 or H3PO2 over homogeneous catalysts or cation modified heterogeneous catalysts to activate it.124 In another study, Simonetti et al.125 found that adamantane was a potential H2 donor that increased isomerization and light alkanes chain length through selective addition of methyl species such as dimethyl ether or methanol. The co-homologation of alkanes with dimethyl ether avoided rejecting carbon in the form of arenes to meet H2 balance in the upgrading to branched alkanes. The rates of alkanes methylation over InI3 and BEA were enhanced by adding catalytic amounts of adamantine, as shown in the equimolar reaction of a mixture of methanol and iso-pentane over InI3. In this reaction, the methylation of dimethyl butane proceeded without arenes formation.126 Different methylation agents such as dimethyl ether, dimethylcarbonate and methanol were investigated in the methylation of 2,3-dimethyl-2-butene to yield a triptene and triptane known as triptyls. Dimethylcarbonate was found as the best methylation agent compared to dimethyl ether and methanol producing triptyls at a high yield.127 A Methaforming process for upgrading light naphtha into high-octane gasoline blendstock was reported by the New Gas Technologies Synthesis (NGTS) company.128 Pilot plant tests on light naphtha (containing about 30 wt.% C5 and 30 wt.% C6) co-fed with methanol achieved about 60 wt.% reduction in n-C5/C6 alkanes at 89 RON and 96% yield of Methaformate. About 44 wt.% of the methanol converted to Methaformate and the remaining 56 wt.% was water. Unlike conventional reforming, Methaforming process can tolerate sulfur content up to 500 ppm. Methaforming process converts 83 wt.% of n-alkanes, retains 89 wt.% of iso-alkanes, increases aromatics by 36 wt.% and dehydrogenates 30 wt.% of naphthenes. 27 ACS Paragon Plus Environment

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4.2. Alkane/Alkene Coupling Labinger and co-workers

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129,130

used tandem Ta/Ir

homogenous catalysts for n-heptane/1-hexene coupling at 100 to 150 °C. This approach involves the dehydrogenation of n-heptane by a pincer-ligated Ir complex and 1-hexene dimerization by a catalyst comprising Cp*TaCl2 (alkene) where Cp* = C5Me5. The dual catalyst works with up to 30/60 turnovers (Ta/Ir) yielding 40 wt.% C13/C14 products. Scheme 3 shows an ideal dual catalyst system on which the alkene is dimerized to a higher alkene and the other catalyst converts the alkane to 1-alkene. The dimerization catalyst produces alkenes that are inert with respect to the transfer of hydrogen. In order to complete the cycle, it was recommended to combine the dehydrogenation and dimerization catalysts such as nickel-exchanged zincosilicate that yields mainly linear and branched alkenes. 5. SUMMARY AND OUTLOOK This mini review provides an overview on the utilization of heterogeneous catalysts (mainly modified zeolites) for the conversion of low-value C5-C6 stream to gasoline blending components. The majority of the studies falls within the development of catalysts for the isomerization of mainly n-hexane to high-RON isomers. Very little work has been published on the conversion of n-pentane mainly due to the limited options available for its transformation. The isomerization of n-hexane continues to play a major role in fulfilling the demand for octane that result from changing regulations and the increasing market share of premium gasoline grades. There are three types of isomerization catalysts which include Pt/Al2O3-Cl, Pt/SO4-ZrO2 and Pt/zeolite (mordenite). Pt/Al2O3-Cl is the most active and operated at the lowest temperature which is favorable for branched alkenes for equilibrium reasons. Pt/zeolite is least active, but is tolerant to the impurities such as H2O and sulfur compounds. Pt/SO4-ZrO2 catalyst shows activity of alkane skeletal isomerization in between Pt/Al2O3-Cl and Pt/zeolite, and is tolerant to H2O and sulfur compounds. In addition to the industrial catalysts, WOx-ZrO2 has been extensively studied for skeletal isomerization of alkanes, though it has not been commercialized. For this catalyst, the presence of Pt and hydrogen is required for a stable activity otherwise activity decays rather rapidly. The current and future trend in the design of isomerization catalysts for n-C5/C6 alkanes is to develop catalysts that operate at low temperatures (< 200C) and tolerate sulfur and water in the feed. The ideal catalyst is the one that combines high activity of chlorinated 28 ACS Paragon Plus Environment

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alumina with the excellent stability of zeolite catalysts. Of the recent publications that attracts attention in light naphtha isomerization is the research on the use of Al-modified meso silica as support for Pt nanoparticles which enhanced activity and selectivity (> 90 wt.%) at 240-360 °C. Other options discussed in this review included the dimerization and oligomerization of C5-C6 alkenes to higher oligomers in the gasoline range having lower RVP and higher RON. Dimerization is best suited for the upgrade of C5/C6 alkenes to dimers within the gasoline range components. The key to making a major discovery in this area is to determine the optimum zeolite and metal types to carry out the dimerization of C5/C6 into dimers boiling in the naphtha range and preventing the formation of larger oligomers which can lead to catalyst instability and undesirable byproducts. In addition to finding the right combination of zeolite structures and metal function, discovery of the appropriate method of combining these two functions and establishing the optimum process conditions will be the main pathways to the solution. The co-upgrading of light alkanes with another feed such as methanol over zeolites is emerging as an alternative route to the longstanding challenge of alkane conversion. Alternatively, when the refinery is integrated with chemicals production, the cracking of nC5/C6 alkanes into lighter C2 and C3 alkenes would enable improved value capture by converting them into basic petrochemical feedstocks. One possible role of methanol may be the production of carbocations (carbenium ions) which are more easily formed from methanol than directly from alkanes. The carbocations can activate alkanes by hydride transfer to produce the carbocations originating from the alkanes. ACKNOWLEDGEMENTS The authors acknowledge the support provided by Saudi Aramco in funding the work at KFUPM through project CRP02269. M.T.K. acknowledges collaboration with and support of colleagues via the Saudi Aramco Chair Program at KFUPM. The support provided by KFUPM in publishing this paper is highly appreciated.

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Figure 1. Thermodynamic and kinetic limitations on n-hexane isomerization for different catalysts: (1) SbF5-HF; TaF5-HF; (2) Pt/Al2O3-Cl; (3) SI-2; (4) Pt/ZrO2-SO4; (5) Pt/ZrO2WO3; (6) Pt/zeolite; (7) thermodynamic balance of 2,2-dimethylbutane (DMB).20

Figure 2. (a) The effect of platinum nanoparticle loadings on the selectivity to C6 isomer and (b) n-hexane conversion activity for Al-MCF-17.61

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Figure 3. Effect of isomerization catalyst type (Pt/SZr and Pt/zeolite), (a) relationship between reaction temperature and isomerization activity, (b) relationship between RON and yield.68

Figure 4. Conversion and selectivity of 1-hexene oligomerization over Al-MTS catalysts having different SiO2/Al2O3 ratio at 200 °C, 5 MPa, WHSV 0.40 h-1 and 240 min TOS.106

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Scheme 1. (a) Promoting effect of Fe at low content through the WOx modification, and (b) Iron oxides and WO3 crystals segregation at high Fe content.79

Scheme 2. Activation of C-H bond on acidic zeolites initiated by hydride transfer to surface methoxy intermediate.2

Scheme 3. Tandem catalytic method for alkane/alkene transformatio.130

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Table 1. Global demand for gasoline by region4 Gasoline demand, million bpd 2013 2017 9.81 10.21 2.58 2.77 1.38 1.42 1.80 1.81 1.53 1.74 0.97 1.07 5.81 7.04 23.86 26.05

Region North America Latin America E. Europe W. Europe Middle East Africa Asia and Pacific Total World

Annual growth rate: 2013-2017, % 1.03 1.84 0.73 0.14 3.43 2.58 5.20 2.30

Table 2. Octane numbers of light naphtha (C5-C6) hydrocarbons6 Component Paraffins i-Paraffins

Naphthenes Aromatics Olefins

Oxygenates

C5 Hydrocarbons Compound RON n-pentane (n-C5) 62 i-pentane (i-C5) 92 neopentane -----cyclopentane 100 ----n-pentenes 90 i-pentenes 103 cyclopentene 93 tertiary amyl 115 methyl ether

MON 63 90 80 --85 --77 82 70 98

C6 Hydrocarbons Compound RON n-hexane (n-C6) 25 2,2-dimethylbutane (DMB) 92 2,3-dimethylbutane (DMB) 106 2-methylpentane (MP) 76 3-methylpentane (MP) 76 Methylcyclopentane (MCP) 91 cyclohexane 83 benzene 103 n-hexenes 90 i-hexenes 100 C6 cyclic olefins 95 ---

MON 26 93 94 74 74 80 77 105 80 83 80 --

Table 3. Selected characteristics of industrial C5-C6 isomerization catalysts and isomerate properties.14-18,38,39 Catalyst Activity Sensitivity to Contaminants Temperature, C Isomerate RON iC5/C5, wt.% Product yield, wt.% Deactivation Regenerable Catalyst suppliers UOP Albemarle/Axens GTC/Neftekhim Clariant

Pt/Chlorinated Al2O3 High High S, N removal, Cl make-up 110-140 91-93 65-78 99.5-99.8 Chlorine loss No

Pt/Sulfated Zirconia Medium Medium to low Less sensitive to water 180-210 88-89 45-75 95-96 Coking Yes

Pt/MOR Low Low, tolerates water and small amount of S 200-300 86-88 58-68 95-96 Coking Yes

Penex I-82, I-84, I-122 ATIS-2L ---

Par-Isom PI-242, PI-244 -Isomalk-2 Hysopar SA

Hysomer HS-10 Ipsorb IP-632 Ismalk-1 Hysopar 5000

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Table 4.Summary of catalysts used in C5-C6 alkenes dimerization/oligomerization. Catalyst

Feed

Reactor setup

MFI (Si/Al: 26)

Cracked naphtha

Desilicated MFI

1-pentene

MFI(40)

1-hexene

MCM-41

1-hexene n-octane C5 alkenes

Up-flow fixed-bed reactor Down-flow fixed bed Down flow fixed bed Down flow reactor Bench reactor

Reaction conditions 240 C, 6 MPa, WHSV 1.5 h-1

Conv. wt.% 90.0

Dimers select. % 47.1% yield

Reference

200 C, 4 MPa, contact time 0.08 h 235 C, 7 MPa

100

75.0

Corma et al.93

97.0

Wang et al.95

200 C, 5 MPa

82.0

60.0 (yield) 77.0

60

44.2

80.0

47.0

Schmidt et al.100 Escola et al.106

30.0

40.0

Bellussi et al.90

Pater et al.98

1-hexene n-octane 1-hexene

Micro-reactor Batch reactor

75 C, 2.8 MPa, LHSV 1.8 h-1 200 C, 5 MPa, WHSV 0.4 h-1 125 C

Al pillared montmorillonite Al pillared saponite Solid Phosphoric Acid Sulfated Zirconia Purolite CT-276 resin Purolite CT-275 resin Layered bed of two resins Amberlyst 35

C5 alkenes

300 C, 1.5 MPa, WHSV 3 h-1 200 C, 5 MPa, WHSV 2 h-1 200 C, 1.9 MPa

34.6

63.1

46.0

54.5

1-hexene

Fixed-bed reactor Fixed-bed reactor Batch reactor

88.0

--

1-hexene

Micro-reactor

75.0

78.0

C5 alkenes

Batch reactor

100 C, 0.8 MPa, LHSV 1.2 h-1 70 C, 2 MPa

95

70.0

C6 alkenes

Batch reactor

77-119 C, 2 MPa

100

56.0

FCC naphtha C5 alkenes

120 C, 3 MPa, LHSV 1.0 h-1 60 C, LHSV 4 h-1

85.0

95.0

--

93.0

Amberlyst 15 resin Purolite CT-275 resin

C5 alkenes methanol C5 alkenes methanol

Fixed-bed reactor Fixed-bed reactor Batch reactor

80 C, 1.9 MPa

59.0

88.2

Marchionna et al.120 Cruz et al.121

Batch reactor

80 C, 1.9 MPa

60.0

89.0

Cruz et al.121

MOR (40) Al-MTS (100) Al-SBA-15(30)

C5 alkenes

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van Grieken et al.108 Casagrande et al.109 Casagrande et al.109 Schwarzer et al.110 de Clerk111 Granollers et al.115 Cadenas et al.117 Krivan et al.119