catalysts

Article Selective Trimerization of α-Olefins with Immobilized Chromium Catalyst for Lubricant Base Oils

Jun Won Baek, Young Bin Hyun, Hyun Ju Lee, Jong Chul Lee, Sung Moon Bae, Yeong Hyun Seo, Dong Geun Lee and Bun Yeoul Lee *

Department of Molecular Science and Technology, Ajou University, 206 Worldcup-ro, Yeongtong-gu, Suwon 16499, Korea; [email protected] (J.W.B.); [email protected] (Y.B.H.); [email protected] (H.J.L.); [email protected] (J.C.L.); [email protected] (S.M.B.); [email protected] (Y.H.S.); [email protected] (D.G.L.) * Correspondence: [email protected]; Tel.: +82-31-219-2519; Fax: +82-31-219-1592

 Received: 5 August 2020; Accepted: 20 August 2020; Published: 1 September 2020 

Abstract: The demand for poly(α-olefin)s (PAOs), which are high-performance group IV lubricant base oils, is increasingly high. PAOs are generally produced via the cationic oligomerization of 1-, wherein skeleton rearrangement inevitably occurs in the products. Hence, a transition-metal-based catalytic process that avoids rearrangement would be a valuable alternative for cationic oligomerization. In particular, transition-metal-catalyzed selective trimerization of α-olefins has the potential for success. In this study, (N,N0,N”-tridodecyltriazacyclohexane)CrCl3 complex was reacted with MAO-silica (MAO, methylaluminoxane) for the preparation of a supported catalyst, which exhibited superior performance in selective α-olefin trimerization compared to that of the corresponding homogeneous catalyst, enabling the preparation of α-olefin trimers at ~200 g scale. Following , the prepared 1-decene trimer (C30H62) exhibited better lubricant properties than those of commercial-grade PAO-4 (kinematic viscosity at 40 ◦C, 15.1 vs. 17.4 cSt; kinematic viscosity at 100 ◦C, 3.9 vs. 3.9 cSt; viscosity index, 161 vs. 123). Moreover, it was shown that 1-/1-dodecene mixed co-trimers (i.e., a mixture of C24H50,C28H58,C32H66, and C36H74), generated by the selective supported Cr catalyst, exhibited outstanding lubricant properties analogous to those observed for the 1-decene trimer (C30H62).

Keywords: selective α-olefin trimerization; lubricant base oil; supported catalyst; chromium

1. Introduction The materials obtained by oligomerization of linear α-olefins (LAOs, i.e., 1-) are referred to as poly(α-olefin)s (PAOs) and they are extensively utilized as high-performance group IV lubricant base oils [1–7]. PAOs are routinely produced via the oligomerization of 1-decene using cationic initiators (e.g., BF3–ROH), followed by a hydrogenation process [8–12]. These PAOs display outstanding lubricant properties, relative to those of Group I through III base oils, such as excellent viscosity indices, low pour points, and high oxidative stabilities. In the cationic oligomerization of 1-decene, a wide range of n-mers (i.e., dimer, trimer, tetramer, pentamer, etc.) are generated. The trimer is the major product of the cationic oligomerization process, and is fractionated by distillation; the trimer fraction containing a small amount of tetramer and pentamer is referred to as PAO-4, and is used in many industrial and automotive lubricant applications such as gear oils, compressor oils, engine oils, hydraulic fluids, and greases. In the cationic oligomerization of 1-decene, carbocation rearrangement is concomitant, leading to severe skeleton rearrangement, as well as the formation of tetra-substituted olefins, which are resistant

Catalysts 2020, 10, 990; doi:10.3390/catal10090990 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 990 2 of 12 to hydrogenation [13,14]. In particular, numerous methyl groups are generated at random positions along the carbon chain, leading to deterioration in lubricant properties. PAOs can also be prepared using transition-metal-based catalysts (e.g., metallocene catalysts), whereby n-mers are generated via sequential 1,2-insertion and β-elimination processes [15–20]. Thus, skeleton rearrangement, occurring in cationic oligomerizations, can be avoided. A portion of PAOs are currently produced via metallocene catalysis of 1-decene [21,22]. However, in this case, a wide range of n-mers require fractionation via an energy-intensive distillation process. Catalysts which are selective for trimers have also been discovered, however, their performance is inadequate for commercialization, their turnover number (TON) is low (<6000), and they necessitate low-temperature conditions (<20 ◦C) [23–27]. Moreover, lubricant properties evaluated for 1-decene trimer samples prepared on a laboratory scale were reportedly not superior to those of PAO-4 fractionated in the cationic oligomerization process, and while their lubricant properties were inferior to those of the product fractionated in the metallocene oligomerization process, the two carbon skeletons were comparable [1,24]. While the demand for PAOs is increasingly high, their market expansion is curtailed by the limited supply of 1-decene. 1-Decene is produced via oligomerization of , whereby, in addition to the formation of 1-decene, a wide range of LAOs (Schulz-Flory distribution) are generated [28]. Each α-olefin is fractionated by distillation for appropriate applications. 1-, 1-, and 1-octene are used as co-monomers in the polyolefin industry; 1-decene is used mainly for the production of PAOs; 1-dodecene, 1-tetradecene, and 1-hexadecene are functionalized for detergents; 1-tetradecene, 1-hexadecene, and 1-octadecene are isomerized into biodegradable offshore drilling fluids; 1-octadecene and heavier LAOs (C20–24) are used as lubricant oil additives and functional fluids. With outstanding lubricant properties, PAOs produced from 1-decene are sold at a high price, while other α-olefin oligomers (e.g., 1-octene oligomers and 1-dodecene oligomers) exhibit inferior properties as lubricant base oils. It is important to consider whether a courier selectively generated from a mixed 1-octene/1-dodecene feed would exhibit lubricating properties comparable to those of PAOs prepared solely from 1-decene. 1-Dodecene and 1-octene may not be restricted by a limited supply–demand predicament, unlike 1-decene. In the early 2000s, a catalytic system for the selective generation of 1-octene was discovered [29–32]. Recently, we also discovered a highly efficient and economical catalytic system for 1-octene production [33–35]. It is anticipated that, in the near future, 1-octene will be produced cleanly, without the generation of a wide range of other LAOs in the process.

2. Results and Discussion

2.1. Catalyst Screening As the titanium-based selective ethylene trimerization catalyst is known to be active for α-olefin trimer production (Figure1d) [ 36–38], chromium-based catalysts that are highly active for ethylene trimerization or tetramerization were screened. Phillips Catalyst (Figure1a), which is commercially used for 1-hexene production, did not display any activity for α-olefin production [39]. An MAO-free catalytic system for ethylene tetramerization (Figure1b) exhibited a reasonably high activity with 1-octene [ 33]. At an optimized reaction temperature of 90 ◦C and after 16 h of operation, a high turnover number (TON) of 17,500 was attained with a high conversion of 1-octene (60%) (see SI for details). However, product analysis by GC indicated that trimers were not selectively generated. Instead, dimers (82%) were predominantly generated, along with a minor number of trimers (18%). A chromium-based catalytic system (Figure1c) that can selectively generate α-olefin trimers was discovered two decades ago [23]. However, the triazacyclohexane chromium catalyst activated with methylaluminoxane (MAO) could be readily deactivated even at room temperature and the attained TON was limited to below 1000 by the need to perform the reaction at 0 ◦C for 20 h in (v/v, 1:1) [24]. Selectivity for the trimer was not complete, being in the range of 80–93%. Later, a higher TON of up to 5000 was realized with improved trimer selectivity (>99%), by performing the reaction in o-C6H4F2 solvent with a triazacyclohexane chromium complex bearing a -CH2CH2CH[CH2CH2CH2CH2CH3]2 fragment. Catalysts 2020, 10, x FOR PEER REVIEW 3 of 13 inCatalysts o‐C20206H,4F102 , 990solvent with a triazacyclohexane chromium complex bearing 3a ‐ of 12 CH2CH2CH[CH2CH2CH2CH2CH3]2 fragment.

O (1 : 3 : 8 ratio) (a) O Cr OH + N AlEt2 +Et3AlꞏClAlEt2

2 R SiBu N R Bu Si 3 3 R NN (c) + MAO Cr [B(C F ) ]- Cl Cl P 6 5 4 Cl Cl (b) N Cr+ Cl P + iBu3Al (d) O Me O N Ti Bu3Si Me Me SiBu3 Me + B(C6F5)3 Figure 1. EthyleneEthylene or or α‐-olefinolefin trimerization (or tetramerization)tetramerization) catalystscatalysts ((aa):): Phillips ethylene trimerization catalyst; ( b): MAO MAO-free‐free ethylene tetramerization catalyst; ( c) and (d): selective α‐-olefinolefin trimerization catalyst.

In the case of titanium-basedtitanium‐based catalysts (Figure 11d),d), itit waswas demonstrateddemonstrated thatthat theirtheir catalyticcatalytic performance could be improved by anchoring the activated catalyst on silica; that is, the supported catalyst, prepared prepared by by reacting reacting the the appropriate appropriate titanium titanium complex complex with MAO with MAOattached attached to silica to (MAO silica‐ silica),(MAO-silica), exhibited exhibited superior superior longevity, longevity, and thus and a 10 thus‐fold a higher 10-fold TON higher of TONup to of6000 up [26,40]. to 6000 Prior [26,40 to]. theirPrior topublication, their publication, we likewise we likewise noted notedthat the that supported the supported catalyst, catalyst, prepared prepared by by reaction reaction of of a triazacyclohexane chromiumchromium complex complex with with MAO-silica, MAO‐silica, exhibited exhibited enhanced enhanced performance performance compared compared to tothat that of of the the homogeneous homogeneous version version of theof the catalyst catalyst [41 [41].].

2.2. α‐α-OlefinOlefin Oligomerization Studies with Supported Catalysts In this this study, study, we we investigated investigated the the performance performance of of a a supported supported catalyst, catalyst, prepared prepared by by reacting reacting a triazacyclohexanea triazacyclohexane chromium chromium complex complex with with MAO MAO-silica.‐silica. MAO MAO-silica‐silica was prepared was prepared by reacting by reacting MAO 3 2 andMAO silica and (pore silica volume, (pore volume,1.6 cm3/g; 1.6 surface cm / g;area, surface 309 m area,2/g; mean 309 particle m /g; mean size, 30 particle m), which size, are 30 bothµm), whichcommonly are used both in commonly the used industry in the polyolefin [42]. When industry MAO and [42 ].silica When were MAO reacted and in 1: silica 2 weight were ratio,reacted the in majority 1: 2 weight (~95%) ratio, of MAO the majorityhad bonded (~95%) with ofsilica MAO to achieve had bonded optimal with MAO silica coverage to achieve (5.6 mmoloptimal‐Al/g). MAO coverageThe supported (5.6 mmol-Al catalyst/g). Thewas supported prepared catalyst by was preparedreacting by(N reacting,N′,N″‐ N N N tridodecyltriazacyclohexane)CrCl( , 0, ”-tridodecyltriazacyclohexane)CrCl3 (R = dodecyl3 (R = indodecyl Figure in1c; Figure [(C12H125c;)3 [(CTAC]CrCl12H25)3TAC]CrCl3) with the3 )prepared with the MAOprepared‐silica MAO-silica in toluene. in The toluene. [(C12H The25)3TAC]CrCl [(C12H25)3 TAC]CrClcontent was3 content varied wasat 25, varied 50, and at 25, 75 50,mg, and while 75 mg,the whileamount the of amount MAO‐silica of MAO-silica was fixed at was 1.0 fixedg (Al/Cr at 1.0= 170, g (Al 84,/Cr 56).= It170, was 84, evident 56). It that was all evident the fed thatchromium all the complexesfed chromium were complexes anchored wereon the anchored silica surface on the in silica all cases, surface as the in alltoluene cases, phase as the of toluene the filtrate phase was of colorlessthe filtrate after was the colorless reaction, after while the reaction, the toluene while solution the toluene of [(C solution12H25)3TAC]CrCl of [(C12H253 )was3TAC]CrCl purple.3 wasThe fabricatedpurple. The supported fabricated catalysts supported (100 catalysts mg) were (100 suspended mg) were in suspended neat 1‐octene in neat (5.0 1-octeneg) at room (5.0 temperature g) at room fortemperature 24 h to determine for 24 h to the determine optimum the loading optimum of the loading chromium of the chromium complex. With complex. a chromium With a chromium complex loadingcomplex of loading 25 mg, of 21% 25 mg, of 21%1‐octene of 1-octene was converted was converted to trimer. to trimer. By increasing By increasing the theloading loading amount amount of chromiumof chromium complex complex to to50 50 mg, mg, the the conversion conversion increased increased to to 30%. 30%. However, However, increasing increasing the loading further to 75 mg did not improve the conversion beyond 30%.

CatalystsCatalysts 20202020,, 1010,, 990x FOR PEER REVIEW 44 of 1213

To investigate the effect of temperature on catalyst performance, conversions were monitored over Totime investigate using optimized the effect supported of temperature catalyst on parameters catalyst performance, (50 mg Cr conversionscomplex/1.0 were g MAO monitored‐silica; Al/Cr over time= 84), using at temperatures optimized supported of ~25 (rt), catalyst 40, 60, and parameters 80 °C, under (50 mg the Cr same complex catalyst/1.0 feed g MAO-silica; (400 mg, 25 Al μ/Crmol= 84),Cr) atin temperatures20 g of neat of1‐octene ~25 (rt), (Table 40, 60, S1). and Plots 80 ◦C, of under conversion the same vs. catalyst reaction feed time (400 clearly mg, 25 indicatedµmol Cr) that in 20 the g ofreaction neat 1-octene rate increased (Table S1). with Plots an increase of conversion in temperature vs. reaction up time to 60 clearly °C, and indicated conversions that the attained reaction at 72 rate h increasedwere 44%, with 59%, an and increase 64% at in~25, temperature 40, and 60 °C, up which to 60 ◦ C,corresponded and conversions to TONs attained of 3100, at 724200, h were and 4600, 44%, 59%,respectively and 64% (Figure at ~25, 40,2). andIncreasing 60 ◦C, which the temperature corresponded from to TONs 60 to of 80 3100, °C 4200,led to and deterioration 4600, respectively in the (Figureconversion2). Increasing rate, likely the attributed temperature to severe from 60 deactivation to 80 ◦C led of to the deterioration catalyst at inhigh the temperature. conversion rate, At likelytemperatures attributed of ~25, to severe 40, and deactivation 60 °C, the ofconversion the catalyst did at not high linearly temperature. increase At with temperatures reaction time, of ~25, but 40,reached and 60a plateau◦C, the following conversion rapid did notgrowth linearly in the increase early stages with reactionof the reaction, time, but which reached indicated a plateau that followingcatalyst deactivation rapid growth was in inevitable the early stageseven at of low the temperatures, reaction, which although indicated the that deactivation catalyst deactivation process was wascurbed inevitable by lowering even at lowthe temperature. temperatures, When although the the oligomerization deactivation process was performed was curbed under by lowering identical the temperature.conditions of When60 °C, thewith oligomerization the same amount was of performed chromium under complex identical and MAO conditions (Cr = of 25 60 μmol;◦C, with Al/Cr the = same100), but amount in the of homogenous chromium complex phase, and31% MAOconversion (Cr = 25wasµ mol;achieved Al/Cr in= a100), relatively but in short the homogenous time of 3 h; phase,however 31% the conversion conversion was could achieved not be in improved a relatively further short by time extending of 3 h; however the reaction the conversion time, indicating could notthat be the improved catalyst was further completely by extending deactivated the reaction within time,3 h. In indicating the homogeneous that the phase, catalyst some was bimolecular completely deactivateddeactivation within process 3 h. is Inproposed the homogeneous to occur, which phase, might some bimolecular be retarded deactivation by immobilization, process isresulting proposed in toan occur, increase which in productivity might be retarded with the by prolonged immobilization, catalyst resulting lifetime. in Deactivation an increase process in productivity might not with be therelated prolonged to the catalystaction of lifetime. residual Deactivation trialkylaluminum process in might MAO, not because be related the addition to the action of Et of3Al residual in the trialkylaluminumtrimerization process in MAO, did becausenot influence the addition the productivity. of Et3Al in the With trimerization the use of process the supported did not influence catalyst, theproducts productivity. could be With isolated the use via of thesimple supported filtration. catalyst, The extended products couldlongevity be isolated and ease via of simple operation filtration. are Theimportant extended advantages longevity of and the ease supported of operation catalyst. are important advantages of the supported catalyst.

FigureFigure 2.2. Conversion vs.vs. time at various temperatures.

AA smallsmall amount of dimer was formed along with the major trimer, while other oligomers higher thanthan thethe trimer trimer were were not not observed observed in simulated in simulated distillation distillation gas chromatography gas chromatography (SimDis (SimDis GC) analysis; GC) thatanalysis; is, 0.02, that 0.07, is, 0.02, and 0.07, 0.87 and wt.% 0.87 dimers wt.% were dimers observed were observed with 44, with 59, and 44, 59, 64 wt.%and 64 main wt.% product main product trimer whentrimer oligomerization when oligomerization was performed was performed for 72 h atfor ~25, 72 h 40, at and ~25, 60 40,◦C, and respectively 60 °C, respectively (Table S2). (Table However, S2). theHowever, amount the of dimeramount generated of dimer was generated significant was at significant 80 ◦C (5.0 at wt.% 80 °C at an(5.0 equivalent wt.% at an reaction equivalent time reaction of 72 h). Thetime isomerization of 72 h). The isomerization of 1-octene to of 2-octene 1‐octene was to 2 also‐octene significant was also at significant 80 ◦C, wherein at 80 °C, 20 wt.%wherein of 1-octene 20 wt.% wasof 1‐ convertedoctene was to converted 2-octene within to 2‐octene 72 h (Figure within S1).72 h The (Figure isomerization S1). The isomerization side reaction side was notreaction severe was at ~25,not severe 40, and at 60 ~25,◦C, with40, and only 60 0.8, °C, 1.3 with and only 3.1 wt.% 0.8, of1.3 1-octene and 3.1 beingwt.% convertedof 1‐octene to being 2-octene, converted respectively, to 2‐ atoctene, a reaction respectively, time of 72 at h.a reaction time of 72 h. TheThe microstructuremicrostructure ofof thethe trimertrimer generatedgenerated withwith thethe homogenoushomogenous CrCr catalystcatalyst waswas thoroughlythoroughly 13 13 investigatedinvestigated byby quantitativequantitative 13C NMR spectroscopy using 13C labeled samples by Köhn, and thethe resultsresults areare shownshown inin SchemeScheme1 1[ [23,27].23,27]. The isomer isomer distribution of trimers prepared with homogeneous [(pentyl)[(pentyl)2CHCH22CHCH22)3)TAC]CrCl3TAC]CrCl3 3catalystcatalyst was was reported reported to to be: be: ~60% ~60% of of AA (possibly(possibly including including A’), ~12% of B, ~17% of C, ~4% of D, ~3% of E, and ~1% of F. In the 1H NMR spectrum of trimer generated

Catalysts 2020, 10, 990x FOR PEER REVIEW 55 of of 13 12 at ~25 °C, three vinylic signals were observed at 5.58–5.42, 5.26–5.11, and 4.97–4.90 ppm with ~12%Catalysts of B2020, ~17%, 10, x FOR of CPEER, ~4% REVIEW of D , ~3% of E, and ~1% of F. In the 1H NMR spectrum of5 of trimer 13 integration ratios of 1.0:0.86:1.43 (Figure 3) [27]. The first two signals were assigned to – generated at ~25 ◦C, three vinylic signals were observed at 5.58–5.42, 5.26–5.11, and 4.97–4.90 ppm CH2CH=CHCH2– (A’ in Scheme 1) and –CH2CH=CHCH(CH2–)– (A), respectively, and the third was withat ~25 integration °C, three ratios vinylic of signals 1.0:0.86:1.43 were observed (Figure3 )[at 5.58–5.42,27]. The 5.26–5.11, first two and signals 4.97–4.90 were ppm assigned with to assignedintegration to H 2ratiosC=C(CH of 2–)CH1.0:0.86:1.432– (B and (Figure D) and 3) H[27].2C=C(C TheH 2–)CfirstH (CHtwo 2–)–signals (C) [23].were The assigned allylic protonto – –CH2CH=CHCH2–(A’ in Scheme1) and –CH 2CH=CHCH(CH2–)– (A), respectively, and the third signalsCH2C –CH=CH2HCH=CHCCH2– (A’H in(CH Scheme2–)– in1) Aand, –C –CHH2CH=CHC2CH=CHCH(CHH2– in 2A’–)–, (HA2),C=C(C respectively,H2–)CH and2– in the B andthird D was, and was assigned to H2C=C(CH2–)CH2–(B and D) and H2C=C(CH2–)CH(CH2–)– (C)[23]. The allylic H2C=C(CassignedH 2to–)C HH2C=C(CH(CH2–)–2 –)CHin C were2– (B observedand D) and in theH2C=C(C regionH 2of–)C 2.7–2.0H(CH 2ppm.–)– (C The) [23]. sum The of allylic the integration proton proton signals –CH2CH=CHCH(CH2–)– in A, –CH2CH=CHCH2– in A’,H2C=C(CH2–)CH2– in B and valuessignals of –CtheH 2allylicCH=CHC protonH(CH signals2–)– in Aagreed, –CH2 CH=CHCwell withH 2the– in integration A’, H2C=C(C valuesH2–)C Hof2– thein B vinylic and D ,proton and D, and H2C=C(CH2–)CH(CH2–)– in C were observed in the region of 2.7–2.0 ppm. The sum of the signalsH2C=C(C basedH2 –)Con theH(CH reported2–)– in C product were observed distribution in the andregion aforementioned of 2.7–2.0 ppm. assignment The sum of the(i.e., integration I2.7–2.0 = 2.0 × integration values of the allylic proton signals agreed well with the integration values of the vinylic I5.58–5.42values + 1.5of the× I 5.26–5.11allylic + proton1.5 × I 4.97–4.90signals, where agreed I wellis the with integration the integration value invalues the regionof the vinylicof the subscriptproton proton signals based on the reported product distribution and aforementioned assignment (i.e., I2.7–2.0 numbers).signals based The 1onH theNMR reported spectrum product of trimer distribution generated and aforementioned at 40 °C was almost assignment identical (i.e., Ito2.7–2.0 that = 2.0 of ×the = 2.0 I + 1.5 I + 1.5 I , where I is the integration value in the region of the I5.58–5.42 5.58–5.42+ 1.5 × I5.26–5.11 + 5.26–5.111.5 × I4.97–4.901 , where4.97–4.90 I is the integration value in the region of the subscript trimer× generated at ~25× °C.1 In the H NMR× spectra of trimer generated at 60 °C, a new singlet signal subscript numbers).1 The H NMR spectrum of trimer generated at 40 ◦C was almost identical to numbers). The H NMR spectrum of trimer generated2 at 40 °C was almost identical to that of the was additionally observed in the vinylidene (1H C=C) region at 4.89 ppm (Figure 3). However, the that of the trimer generated at ~251 ◦C. In the H NMR spectra of trimer generated at 60 ◦C, a new integrationtrimer generated ratio of atthe ~25 three °C. Invinyl the signalsH NMR remained spectra of unaltered trimer generated at the three at 60 temperatures °C, a new singlet (i.e., signal I5.58–5.42 : singlet signal was additionally observed in the2 vinylidene (H2C=C) region at 4.89 ppm (Figure3). I5.26–5.11was: additionallyI4.97–4.87 = 1.0:0.83:1.42). observed in the vinylidene (H C=C) region at 4.89 ppm (Figure 3). However, the However,integration the ratio integration of the three ratio vinyl of the signals three vinylremained signals unaltered remained at the unaltered three temperatures at the three (i.e., temperatures I5.58–5.42: (i.e.,I5.26–5.11I5.58–5.42: I4.97–4.87: I5.26–5.11 = 1.0:0.83:1.42).: I4.97–4.87 = 1.0:0.83:1.42).

Scheme 1. Rationale for the observed trimer isomer distribution. SchemeScheme 1. 1.Rationale Rationale forfor thethe observed trimer trimer isomer isomer distribution. distribution.

1 Figure 3. H1 NMR spectra of 1-octene trimer generated at (a) 25 C and (b) 60 C. FigureFigure 3. 3.1H H NMR NMR spectra spectra of of 1 1‐‐octeneoctene trimertrimer generated at at ( a(a) )25 25 °C °C◦ and and (b ()b 60) 60 °C. °C.◦

Catalysts 2020, 10, x FOR PEER REVIEW 6 of 13 Catalysts 2020, 10, 990 6 of 12 The reactivity of 1‐decene and 1‐dodecene was investigated and compared with that of 1‐octene by performingThe reactivity the of oligomerization 1-decene and 1-dodecene at 60 °C wasunder investigated an identical and catalyst compared feed with (400 that‐mg of 1-octenesupported by catalyst/20‐g neat monomer) and monitoring the conversion over time (Figure S2 and Table S3). The performing the oligomerization at 60 ◦C under an identical catalyst feed (400-mg supported catalyst/20-g neatuse of monomer) 1‐decene and provided monitoring 77% the conversion conversion (TON, over time4400) (Figure within S2 a and reaction Table S3).time The of use72 h, of which 1-decene is providedsignificantly 77% higher conversion than that (TON, observed 4400) within with a1‐ reactionoctene (64% time within of 72 h, 72 which h; TON, is significantly 4600). The higherconversion than thatwas observedfurther improved with 1-octene with (64% the withinemployment 72 h; TON, of 1‐ 4600).dodecene The conversion(80% within was 72 furtherh; TON, improved 3800). After with thereaching employment 70% conversion of 1-dodecene in 24 h, (80% the within reaction 72 rate h; TON, slowed 3800). down After severely, reaching requiring 70% conversion another in48 24h to h, thereach reaction 80% conversion. rate slowed This down is in severely, accordance requiring with lowering another 48 monomer h to reach concentration 80% conversion. at the This stage is inof accordancehigh conversion. with loweringHowever, monomer in the plot concentration of TON vs. attime the (Figure stage of 4), high the conversion. three lines However,representing in the 1‐ plotoctene, of TON 1‐decene, vs. time and (Figure 1‐dodecene4), the threeoverlap lines each representing other up 1-octene,to a reaction 1-decene, time of and 24 1-dodecene h, which indicates overlap 1 eachthat monomer other up toreactivity a reaction is comparable time of 24 h, for which all three indicates α‐olefins. that The monomer H NMR reactivity signal patterns is comparable of vinylic for alland three allylicα -olefins.protons and The the1H integration NMR signal value patterns ratios of of vinylicthe three and vinylic allylic signals protons (i.e., and I5.58–5.42 the: integrationI5.26–5.11: I4.97– 4.87) were identical for all the three samples of 1‐octene‐, 1‐decene‐, and 1‐dodecene‐generated trimers value ratios of the three vinylic signals (i.e., I5.58–5.42: I5.26–5.11: I4.97–4.87) were identical for all the three samples(Figure S3). of 1-octene-, 1-decene-, and 1-dodecene-generated trimers (Figure S3).

Figure 4. Turnover number (TON) vs. time for various monomers.

2.3. Preparation of α-olefin Trimers and Lubricant Properties 2.3. Preparation of α‐olefin Trimers and Lubricant Properties To evaluate lubricant properties of trimers, ~200 g of α-olefin trimers was prepared using To evaluate lubricant properties of trimers, ~200 g of α‐olefin trimers was prepared using the the supported catalyst. Thus, 1-octene, 1-decene, and 1-dodecene monomers (300 g), as well as supported catalyst. Thus, 1‐octene, 1‐decene, and 1‐dodecene monomers (300 g), as well as 1‐ 1-octene/1-dodecene mixed monomers (300 g) in 2/1, 1/1, and 1/2 mole ratios were stirred for 72 h at octene/1‐dodecene mixed monomers (300 g) in 2/1, 1/1, and 1/2 mole ratios were stirred for 72 h at 60 60 C under neat conditions in the presence of the optimized supported catalyst (6.0 g, MAO-g/silica-g °C ◦under neat conditions in the presence of the optimized supported catalyst (6.0 g, MAO‐g/silica‐g = 0.45, Al/Cr = 84). Conversions exceeding 60% were attained in all cases. After oligomerization, = 0.45, Al/Cr = 84). Conversions exceeding 60% were attained in all cases. After oligomerization, the the catalyst could be completely removed by filtration. After removing the unreacted monomer catalyst could be completely removed by filtration. After removing the unreacted monomer by by vacuum distillation, the hydrogenation reaction was performed under neat conditions without vacuum distillation, the hydrogenation reaction was performed under neat conditions without feeding an additional solvent, using a Pd/C catalyst (0.5 wt.% per trimer) under 25 bar H2 gas at rt feeding an additional solvent, using a Pd/C catalyst (0.5 wt.% per trimer) under 25 bar H2 gas at rt for for 12 h. Complete hydrogenation was confirmed by 1H NMR spectral analysis, wherein a complete 12 h. Complete hydrogenation was confirmed by 1H NMR spectral analysis, wherein a complete disappearance of vinylic and allylic signals was observed after hydrogenation (Figure S4). disappearance of vinylic and allylic signals was observed after hydrogenation (Figure S4). The prepared α-olefin oligomers were analyzed by a SimDis GC (Figure5). Selective generation of The prepared α‐olefin oligomers were analyzed by a SimDis GC (Figure 5). Selective generation trimer was evident; a set of signals assignable to the trimers (i.e., C24H50,C30H62, and C36H74) of trimer was evident; a set of signals assignable to the trimers (i.e., C24H50, C30H62, and C36H74) was was observed predominantly in the cases of 1-octene, 1-decene, and 1-dodecene trimerization observed predominantly in the cases of 1‐octene, 1‐decene, and 1‐dodecene trimerization (Figure 5a– (Figure5a–c). Each set of signals was composed of three signals in a ratio of 74 : 24 : 3, c). Each set of signals was composed of three signals in a ratio of 74 : 24 : 3, which is in good agreement which is in good agreement with the trimer isomer distribution results revealed by quantitative with the trimer isomer distribution results revealed by quantitative 13C NMR spectroscopy using 13C 13C NMR spectroscopy using 13C labeled samples [27]. Hydrogenation of the isomers of A (possibly labeled samples [27]. Hydrogenation of the isomers of A (possibly including A’, ~60%) and B (~12%), including A’, ~60%) and B (~12%), derived from 1,3,5-alkyl-substituted chromacycloheptane, which is, derived from 1,3,5‐alkyl‐substituted chromacycloheptane, which is, in turn, derived from 1,3‐alkyl‐ in turn, derived from 1,3-alkyl-substituted chromacyclopentane, resulted in the generation of substituted chromacyclopentane, resulted in the generation of 1,3,5‐alkyl‐substituted hexane as the 1,3,5-alkyl-substituted hexane as the major product (~72%) (Scheme1). Hydrogenation of the major product (~72%) (Scheme 1). Hydrogenation of the isomers of C (~17%) and D (~4%), derived isomers of C (~17%) and D (~4%), derived from 2,3,5-alkyl-substituted chromacycloheptane via from 2,3,5‐alkyl‐substituted chromacycloheptane via 1,2‐insertion, which is, in turn, derived from

Catalysts 2020, 10, x FOR PEER REVIEW 7 of 13

2,3‐alkyl‐substituted chromacyclopentane, results in the generation of the second major isomer, 2,3,5‐ alkyl‐substituted hexane (~21%). Hydrogenation of the isomers of E (~3%) and F (~1%), derived from 1,4,5‐alkyl‐substituted chromacycloheptane, which is, in turn, derived from 1,4‐alkyl‐substituted chromacyclopentane, results in the generation of a minor product, 1,4,5‐alkyl‐substituted hexane (~4%). The ratios of the three signals (i.e., signals for 1,3,5‐alkyl‐, 2,3,5‐alkyl‐, and 1,4,5‐alkyl‐ substituted hexane) were the same for 1‐octene, 1‐decene, and 1‐dodecene trimers (74:24:3). Four fractions were observed for the samples prepared with 1‐octene/1‐dodecene mixed monomers. The four fractions corresponded to C24H50 (trimer of three 1‐octene units), C28H58 (trimer of two 1‐octene and one 1‐dodecene), C32H66 (trimer of one 1‐octene and two 1‐dodecene), and C36H74 (trimerCatalysts 2020of three, 10, 990 1‐dodecene). As previously mentioned, monomer reactivity did not differ between7 of 1 12‐ octene and 1‐dodecene, and the molar ratio of the four fractions (i.e., C24H50 : C28H58 : C32H66 : C36H74) 1,2-insertion,can be estimated which by statistics. is, in turn, The derived estimated from ratios 2,3-alkyl-substituted were in good agreement chromacyclopentane, with the ratios results measured in the ongeneration SimDis ofGC; the for second example, major for isomer, the sample 2,3,5-alkyl-substituted prepared in a 2:1 hexane molar (~21%). ratio of Hydrogenation 1‐octene/1‐dodecene of the (Figureisomers 5d), of E the(~3%) estimated and F (~1%), C24H50 derived: C28H58 : from C32H 1,4,5-alkyl-substituted66 : C36H74 molar ratios chromacycloheptane,are 0.296:0.444:0.222:0.037 which (i.e., is, 0.67in turn, × 0.67 derived × 0.67:3 from × 0.67 1,4-alkyl-substituted × 0.67 × 0.33:3 × 0.67 chromacyclopentane, × 0.33 × 0.33:0.33 × 0.33 results × 0.33), in while the generation the ratios ofmeasured a minor byproduct, SimDis 1,4,5-alkyl-substituted GC were 0.26:0.45:0.21:0.04 hexane (Table (~4%). S4). The In ratios each of fraction, the three three signals isomer (i.e., signals signals were for 1,3,5-alkyl-, observed with2,3,5-alkyl-, ratios similar and 1,4,5-alkyl-substituted to those of 1‐octene, 1 hexane)‐decene, were and the1‐dodecene same for trimer 1-octene, signals 1-decene, (74–77:21–25:2; and 1-dodecene Table S4).trimers (74:24:3).

Figure 5.5. SimulatedSimulated distillation distillation Gas Gas Chromatography Chromatography (SimDis (SimDis GC) charts GC) forchartsα-olefin for trimersα‐olefin prepared trimers withprepared (a)1-octene, with (a ()1b)‐octene, 1-decene, (b and) 1‐decene, (c) 1-dodecene and (c) monomers, 1‐dodecene as monomers, well as 1-octene as well/1-dodecene as 1‐octene/1 mixed‐ dodecenemonomers mixed in (d) monomers 2:1, (e) 1:1 andin (d (f) )2:1, 1:2 ( molee) 1:1 ratios. and (f) 1:2 mole ratios.

LubricantFour fractions properties were observed (kinematic for theviscosity samples at 40 prepared and 100 with °C, 1-octeneviscosity/1-dodecene index, and mixedpour point) monomers. were measuredThe four fractions for samples corresponded prepared to on C24 aH ~20050 (trimer g scale of at three a certified 1-octene institute units), C (entries28H58 (trimer 1–6 in of Table two 1-octene 1). The propertiesand one 1-dodecene), of commercial C32H‐grade66 (trimer PAOs of (PAO one 1-octene‐2.0, PAO and‐2.5, two and 1-dodecene), PAO‐4.0), fractionated and C36H74 (trimer in the cationic of three 1-dodecene).oligomerization As previously process of mentioned, 1‐decene, were monomer also measured reactivity did for not comparison differ between (entries 1-octene 7–9). and The 1-dodecene, measured valuesand the of molar kinematic ratio ofviscosity the four at fractions 40 and 100 (i.e., °C, C 24viscosityH50 :C 28index,H58 :C and32H pour66 :C point36H74 for) can commercial be estimated‐grade by PAOsstatistics. were The in estimated agreement ratios with were the in goodliterature agreement values. with However, the ratios measuredthe measured on SimDis values GC; for for 1 example,‐decene trimersfor the sample(kinematic prepared viscosity in a at 2:1 40 molar °C, 15.1 ratio cSt; of 1-octenekinematic/1-dodecene viscosity at (Figure 100 °C,5d), 3.9 the cSt; estimated viscosity C index,24H50 161):C28 differedH58 :C 32appreciablyH66 :C36H 74frommolar reported ratios data are 0.296:0.444:0.222:0.037 (kinematic viscosity at (i.e., 40 0.67°C, 12.1–13.80.67 0.67:3cSt; kinematic0.67 × × × × 0.67 0.33:3 0.67 0.33 0.33:0.33 0.33 0.33), while the ratios measured by SimDis GC were × × × × × × 0.26:0.45:0.21:0.04 (Table S4). In each fraction, three isomer signals were observed with ratios similar to those of 1-octene, 1-decene, and 1-dodecene trimer signals (74–77:21–25:2; Table S4). Lubricant properties (kinematic viscosity at 40 and 100 ◦C, viscosity index, and pour point) were measured for samples prepared on a ~200 g scale at a certified institute (entries 1–6 in Table1). The properties of commercial-grade PAOs (PAO-2.0, PAO-2.5, and PAO-4.0), fractionated in the cationic oligomerization process of 1-decene, were also measured for comparison (entries 7–9). The measured values of kinematic viscosity at 40 and 100 ◦C, viscosity index, and pour point for commercial-grade PAOs were in agreement with the literature values. However, the measured values for 1-decene trimers Catalysts 2020, 10, 990 8 of 12

(kinematic viscosity at 40 ◦C, 15.1 cSt; kinematic viscosity at 100 ◦C, 3.9 cSt; viscosity index, 161) differed appreciably from reported data (kinematic viscosity at 40 ◦C, 12.1–13.8 cSt; kinematic viscosity at 100 ◦C, 3.1–3.4 cSt; viscosity index 125–137), which had been measured for samples prepared on a small scale (<10 g), at a low temperature (0 ◦C), using various homogeneous [R3TAC]CrCl3 catalysts [24]. The higher the viscosity index, the less the viscosity is affected by changes in temperature, and the better a lubricant base oil will be. With a marginal increase in the viscosity index relative to that of PAO-4.0, which is fractioned in the cationic oligomerization process of 1-decene (125–137 vs. 124), 1-decene trimers, selectively generated via a metallacyclic intermediate, have been regarded negatively as a lubricant base oil [1]. However, the viscosity index measured herein for 1-decene trimers, prepared on a relatively large scale with the supported chromium catalyst, was significantly higher than that of the commercial-grade PAO-4.0 (161 vs. 123). The kinematic viscosity and viscosity index gradually increased with the switch from the 1-octene trimer to the 1-decene trimer and further to the 1-dodecene trimer (entries 1–3). The pour point of 1-octene and 1-decene trimers was sufficiently below the instrument limiting value (< 57 C), while that of 1-dodecene trimer was disadvantageously high at − ◦ 39 C. − ◦ Table 1. Lubricant properties of prepared α-olefin trimers and commercial-grade poly(α-olefin)s (PAOs).

Kinematic Viscosity at Kinematic Viscosity at Entry Monomer Viscosity Index Pour Point (◦C) 40 ◦C (cSt) 100 ◦C (cSt) 1 1-C H 8.13 2.4 110 < 57 8 16 − 2 1-C H 15.1 3.9 161 < 57 10 20 − 3 1-C H 21.6 5.0 171 39 12 24 − 4 1-C /1-C (2 : 1) 11.5 3.0 122 < 57 8 12 − 5 1-C /1-C (1 : 1) 14.7 3.6 137 < 57 8 12 − 6 1-C /1-C (1 : 2) 16.5 4.1 160 51 8 12 − 7 PAO-2.0 (1-C ) 4.98 1.7 93 < 57 10 − 8 PAO-2.5 (1-C ) 7.71 2.3 109 51 10 − 9 PAO-4.0 (1-C ) 17.4 3.9 123 < 57 10 −

It has also been demonstrated that a courier prepared with a 1-octene/1-dodecene mixed monomer exhibited lubricant properties similar to those prepared with 1-decene; for example, the trimer prepared with a 1:2 1-octene/1-dodecene monomer mix exhibited lubricant properties comparable to those of 1-decene trimer (kinematic viscosity at 40 ◦C, 16.5 vs. 15.1 cSt; kinematic viscosity at 100 ◦C, 4.1 vs. 3.9 cSt; viscosity index, 160 vs. 161), although the pour point was inferior ( 51 C vs. < 57 C ; entry 6 − ◦ − ◦ vs. entry 2). Lubricant properties could also be tuned by varying the 1-octene/1-dodecene molar ratio, wherein viscosity as well as viscosity index increased with a decrease in the 1-octene/1-dodecene ratio (entries 4 and 5). Superior lubricant base oils may be obtained by fractionation of the four fractions generated in the selective trimerization of 1-octene/1-dodecene mixed monomers.

3. Material and Methods

3.1. Materials All manipulations were performed in an inert atmosphere using standard glove box and Schlenk techniques. Toluene and hexane were purchased from ThermoFischer Scientific (Seoul, Korea), purified over Na/ketyl, distilled, and stored under N2 gas. 1-Octene, 1-decene, and 1-dodecene were purchased from SigmaAldrich (Seoul, Korea) and purified by passing through an activated alumina column, dried over Na/K alloy, distilled, and stored under N2 gas prior to use. MAO (10 wt.% in toluene; 4.6 wt.%-Al) and silica (SYLOPOL-2410) were purchased from Grace (Yeosu, Korea). 1H NMR (600 MHz) spectra were recorded using a JEOL ECZ 600 spectrometer. SimDis GC analysis was performed using a YL instrument 6500 GC (Seoul, Korea) system equipped with a DB-2887 column (10 m 0.53 mm 3 µm; purchased from Agilent) and a flame ionization detector (FID). × × Lubricant properties of α-olefin trimers and PAO were measured at K-petro Korea (Cheongju, Korea) according to ASTM D445-18, ASTM D2270-10 (2016), and ASTM D6749-02 (2012). Catalysts 2020, 10, 990 9 of 12

3.2. Preparation of Supported Catalyst MAO (10 wt.% in toluene, 25 g, 43 mmol Al) was added to a flask containing silica (SYLOPOL-2410, 5.0 g) suspended in toluene (10 mL). The mixture was heated to 70 ◦C and stirred for 3 h. The solid was collected by filtration and washed with toluene (10 mL 3) and hexane (20 mL 2). The residual × × solvents were completely removed using a vacuum line to obtain a white powder (MAO-silica, 7.4 g). The prepared MAO-silica (16.0 g), (C12H25)3TAC)CrCl3 (0.800 g, 1.07 mmol), and toluene (80 mL) were added to a flask, and the resulting suspension was stirred for 2 h at rt. The solid was collected by filtration and washed with toluene (50 mL) and hexane (30 mL). The filtrate was colorless, indicating that the chromium complex was completely anchored. Residual solvents were removed using a vacuum line to obtain a green powder (16.6 g, 64 µmol Cr/g-cat, Al/Cr = 84). The prepared supported catalyst was dispersed in toluene (3.5 g) and aqueous nitric acid (7.0 g, 10 wt.%) was added. After stirring overnight, aqueous phase was taken for inductively coupled plasma (ICP) analyses, through which the Al and Cr contents were determined.

3.3. 1-Octene Trimerization on A Small Scale for the Data in Figures2 and3 A Schlenk flask (50 mL size) was charged with the prepared supported catalyst (400 mg) and 1-octene (20 g). The flask was sealed with a Teflon valve and immersed in an oil bath with a temperature set to r.t (~25 ◦C), 40 ◦C, 60 ◦C, and 80 ◦C. During the reaction, a small amount of solution (200 mg) was sampled for SimDis GC analysis.

3.4. Preparation of α-Olefin Trimer on a Large Scale for the Samples in Table1 A Schlenk flask (1 L size) was charged with the supported catalyst (6.0 g), 1-octene (120 g), and 1-dodecene (180 g). After the flask was sealed with a Teflon valve, it was immersed in an oil bath with a temperature of 60 ◦C. After 72 h had elapsed, the resulting mixture was filtered. The filtrate was distillated at 80 ◦C under vacuum to remove unreacted monomers. A colorless residue was obtained (207 g, 69% conversion), which was transferred to a bomb reactor (2 L size) containing a Pd/C catalyst (1.0 g, 10 wt.%-Pd). The reactor was pressurized with 25 bar H2 gas and stirred for 12 h at room temperature. After venting off H2 gas, the Pd/C catalyst was removed by filtration over Celite to obtain a colorless oil, which was used without further treatment for the measurement of lubricant properties.

3.5. SimDis GC Analysis A sample (200 mg) drawn from the reaction was diluted with nonane (300 mg) and syringe-filtered (pore size, 0.2 µm). The filtrate (2 µL) was injected into a GC equipped with a DB-2887 column. N2 gas flowed at a constant rate (10 mL/min). The inlet and FID temperatures were set to 320 ◦C. The oven temperature was controlled by holding at 40 ◦C for 4 min, increasing to 200 ◦C at a rate of 10 ◦C/min, holding at 200 ◦C for 4 min, increasing to 320 ◦C at a rate of 10 ◦C/min, and finally holding at 320 ◦C for 6 min.

4. Conclusions

(N,N0,N”-tridodecyltriazacyclohexane)CrCl3 complex was reacted with MAO-silica for the preparation of a supported catalyst. By the immobilization of the chromium catalyst on the silica surface, the deactivation process was diminished and the activity was sustained, although it decreased gradually with time. After 72 h, even at an elevated temperature of 60 ◦C, the supported catalyst provided 60–80% conversion (TON, 3800–4600), enabling the preparation of α-olefin trimers on ~200 g scale, while the corresponding homogeneous catalyst was completely deactivated within 3 h at 60 ◦C. SimDis GC analysis performed after hydrogenation confirmed the selective generation of trimers. Each trimer signal was split into three signals, in all cases, in a ratio of ~74:~24:~3, which were assigned to 1,3,5-, 2,3,5-, and 1,4,5-alkyl-substituted hexanes. Because of the simplified skeleton, the prepared 1-decene trimer displayed superior lubricant properties compared to those of commercial-grade PAO-4, Catalysts 2020, 10, 990 10 of 12 produced via the cationic oligomerization of 1-decene. 1-Octene/1-dodecene mixed couriers were also prepared, and SimDis GC analysis indicated that the ratio of C24H50,C28H58,C32H66, and C36H74 fractions was statistically controlled according to the variation in 1-octene/1-dodecene feed ratios. A mixed courier prepared with the 1-octene/1-dodecene mixed monomer exhibited lubricant properties comparable to those of the 1-decene trimer, although the pour point was inferior. The activity of the reported catalyst (TON, ~4000) is unfortunately inadequate for application in a commercial process. However, this study offers valuable insights for the future development of catalysts with higher activity for selective α-olefin trimerization, which could have a substantial impact on the commercial production of lubricant base oils.

5. Patents A patent was filed (KR. Patent 10-1449474, 13 Oct 2014, assigned to Ajou University) and a patent was applied (Kr 10-2020-0061874, 22 May 2020, assigned to Ajou University).

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/10/9/990/s1, Table S1. 1-Octene trimerization at various temperatures (data for Figure2), Table S2. Trimer /dimer selectivity in 1-octene trimerization performed at various temperature (Figure2), Table S3. Trimerization of 1-octene, 1-decene, and 1-dodecene performed at 60 ◦C (Figure4), Table S4. SimDis GC data for 1-octene /1-dodecene mixed trimerization reactions, Figure S1: SimDis GC chart for 1-octene trimerization performed at 80 ◦C for 72 h, Figure S2: Conversion vs. time for 1-octene, 1-decene, and 1-dodecene trimerization performed at 60 ◦C, Figure S3. 1H NMR spectra of 1-octene, 1-decene, and 1-dodecene trimmers, Figure S4. 1H NMR spectra before and after hydrogenation of 1-octene trimer. Author Contributions: Conceptualization and design of experiments, B.Y.L.; trimerization of α-olefins and preparation of supported catalyst, J.W.B. and Y.B.H.; SimDis GC studies, H.J.L., J.C.L., and S.M.B.; NMR studies, J.W.B., Y.H.S., and D.G.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the C1 Gas Refinery Program (2019M3D3A1A01069100) and by the Priority Research Centers Program (2019R1A6A1A11051471) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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