catalysts

Article Influence of Remaining Acid Sites of an Amorphous Aluminosilicate on the Oligomerization of n- after Impregnation with Nickel Ions

Fabian Nadolny 1, Felix Alscher 1, Stephan Peitz 2 , Ekaterina Borovinskaya 1,3, Robert Franke 2,4 and Wladimir Reschetilowski 1,* 1 Institut für Energietechnik, Technische Universität Dresden, 01069 Dresden, Germany; [email protected] (F.N.); [email protected] (F.A.); [email protected] (E.B.) 2 Evonik Performance Materials GmbH, Paul-Baumann-Straße 1, 45772 Marl, Germany; [email protected] (S.P.); [email protected] (R.F.) 3 Saint-Petersburg State Institute of Technology, Technical University, 190013 St. Petersburg, Russia 4 Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, 44780 Bochum, Germany * Correspondence: [email protected]

 Received: 2 December 2020; Accepted: 18 December 2020; Published: 21 December 2020 

Abstract: Highly linear octene isomers can be produced from n- on industrial scale by using Ni-containing aluminosilicates as heterogeneous catalysts. These catalysts can be prepared by impregnating an aluminosilicate with a Ni(II) salt solution. This leads to a competition between acid-catalyzed and nickel-catalyzed reactions. In this study it is shown that some octene isomers are exclusively formed via an acid-catalyzed mechanism as a result of methyl group migration at the surface of a mesoporous catalyst. Specifically, the isomers 4,4-dimethylhexene (4,4-DMH) and 3-ethyl-2-methylpentene (3E-2MP) exhibit a systematic correlation compared to the amount of 3,4-dimethylhexene (3,4-DMH) formed at acid sites. By analyzing the ratio of 4,4-DMH and/or 3E-2MP to 3,4-DMH in the product spectrum before and after impregnation with a nickel precursor, the extend of acid site covered by nickel ions can be evaluated.

Keywords: oligomerization butene; amorphous aluminosilicate; octene isomer; acid sites; TPAD; nickel impregnation

1. Introduction C4 molecules stemming from thermal or catalytic cracking are used to produce petrochemical intermediates and clean fuel additives on the industrial scale [1]. The C4 stream can be converted directly into gasoline by alkylation [2]. Depending on the composition, can be separated as a pure substance [3] and iso-butene can be selectively converted to, for example, methyl tert-butyl ether [4]. The remaining stream is mainly composed of butanes and n-butenes. The olefins can be converted to low-sulfur gasoline on acid catalysts [5]. Acid-catalyzed oligomerization of n-butenes has been known since the 1930s [6] and has been performed on numerous acid catalysts [7]. The reaction of olefins with protic acids forms a trivalent carbenium ion species, which can interact with the double bond of another olefin and form a C-C bond [8]. Highly branched oligomers are formed by this acid-catalyzed reaction [9]. Alternatively, n-butenes can be converted coordinatively by catalysts based on transition metals, which changes the product range substantially. Early works on a homogeneous nickel system have shown that ethene can be converted to linear oligomers with a selectivity of more than 99% [10]. Oligomerization activity over a homogeneous nickel system has been found for numerous reactants, from ethene to 1-octene [11]. When 1-butene is applied as reactant, selectivity to the formation of linear dimers was found to be over 80%. Formation of the linear products

Catalysts 2020, 10, 1487; doi:10.3390/catal10121487 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 1487 2 of 15

2,5-dimethylhexene and 2-methylheptene was found for homogeneously catalyzed dimerization of iso-butene or co-dimerization of iso-butene with 1-butene [12]. If the ligands are varied in regard to their acidic properties, the activity of the homogeneous catalysts rises with the increasing acidity of the ligands [13]. In this context, a nickel hydride species was identified as a catalytically active species. The DIMERSOL process was developed for industrial application of the homogeneously catalyzed oligomerization of and butene using a catalyst system of the Ziegler-Natta type, based on a nickel derivative and activated by means of an organometallic compound [14]. In the OCTOL process for the selective dimerization of n-butenes to highly linear octenes, which are suitable for the manufacture of isononyl plasticizers, nickel loaded aluminosilicate was used as a heterogeneous catalyst [15]. Over homogeneous and heterogeneous catalysts the mechanism of coordinative oligomerization is assumed to be comparable [16]. The oligomerization process is essential for production of olefinic precursors with respect to synthesis of plasticizer . The obtained octenes are hydroformylated in a homogeneously catalyzed process by applying synthesis gas to form C9 aldehydes, then afterwards hydrogenated to form alcohols, and esterified, for example with phthalic acid, to form PVC plasticizers [17]. Both, the product quality of the plasticizers and the activity of the hydroformylation catalysts depend on the linearity of the octenes. At high linearity, the formation of terminal aldehydes is preferred [18]. Consequently, suitable catalysts for n-butene oligomerization processes have to fulfill several requirements. Nickel oxide on silica exhibits only very low activity regarding the oligomerization of n-butenes; this can be increased through application of Al2O3 [19]. The role played by the aluminum is a subject of controversial discussion. For instance, after impregnation of Ni(NO3)2 solution on aluminosilicate and subsequent calcination, the formation of nickel-aluminum phyllosilicates has been demonstrated. Furthermore, it was shown that the formed amount of nickel-aluminum phyllosilicates decreases with increasing aluminum content [20]. This is accompanied by stabilization of tri- and tetravalent nickel oxide species by aluminum [21], which in the presence of olefins (in the present case primarily 2-butene) are reduced to catalytically active nickel hydride species [22]. In addition, using in situ electron paramagnetic resonance (EPR) spectroscopy correlated with XANES/EXAFS, the formation of a NiI/NiII redox pair was detected during the dimerization of n-butenes, which are present as single sites [23,24]. For the formation of this species, a reaction cycle has been proposed, which requires the interaction of olefins with NiII in the direct vicinity of a Brønsted acid site. Both are approaches for a better understanding of the catalytically active species which presuppose the presence of aluminum. This necessarily leads to a competition between acid-catalyzed and nickel-catalyzed conversion of n-butenes. In respect to the optimization of heterogeneous catalysts for the OCTOL process, both the amount of nickel and the Ni/Al ratio were increased with each catalyst generation, which led to enhanced yields of linear octene isomers [25]. Accordingly, the increased amount of nickel minimizes the acid-catalyzed conversion of n-butenes, but it cannot be suppressed completely. Due to the influence of remaining acid sites, the number of octene skeletal isomers is significantly increased [26]. Recently it was shown that, in addition to amorphous aluminosilicates, highly structured microporous zeolites [27–29] or ordered micro/mesoporous materials [30] suit as supports for the nickel species. A strong metal-support interaction in such catalyst systems leads to the mutual influence of the active nickel and acid sites providing the observed positive trends in catalytic activity and selectivity. Novel catalyst systems based on cobalt oxide on N-doped carbon [31] or cobalt-zeolite catalysts [32] show promising results for a high selectivity to linear products. However, preparation of these catalysts on an industrial scale tends to pose various challenges. Neither in the scientific literature nor in the patent literature any indications that such systems have been examined for possible industrial applications, even on a mini-plant scale, can be found. Therefore, nickel-containing catalysts are still in use for heterogeneous oligomerization in the chemical industry. At present, they are still the industrial systems of choice. In this work, the influence of nickel on the distribution of octene isomers formed by oligomerization of n-butenes is investigated. The change in product distribution is achieved by in situ impregnation of an amorphous aluminosilicate with an organic nickel compound in a differential loop-type Catalysts 2020, 10, 1487 3 of 15 reactor. The findings were verified by analyzing the oligomerization of ex situ impregnated nickel aluminosilicates in a fixed bed reactor. By applying these information, a criterion is established characterizing the number of remaining acid sites after impregnation of an aluminosilicate with nickel ions as well as their effects on the oligomerization process.

2. Results and Discussion

2.1. Change of Octene Isomer Distribution by In Situ Impregnation in a Differential Loop-Type Reactor The effect of nickel on the performance of an oligomerization catalyst can particularly be observed by the linearity of the octene skeletal isomers that are formed. Usage of a mesoporous catalyst prevents any influence of the isomer distribution through shape selectivity [33] and minimizes limitation of diffusion [34]. Two types of mechanism to oligomerize , namely acid-catalyzed and coordinative mechanism, are well known from nickel-containing, mesoporous aluminosilicates [35]. Beside the acidic behavior of the amorphous aluminosilicate, the surface-near aluminum can activate nickel for oligomerization. The catalytic testing was performed at 80 ◦C. Under these conditions, only a small conversion occurs via the strongly temperature-dependent acid-catalyzed mechanism. This underlines the activity of nickel for oligomerization. To distinguish between products of nickel- and acid-catalyzed oligomerization of n-butenes an in situ impregnation in a loop-type reactor can be performed. Through the cycle pump of this reactor type a nickel-containing solution can pass serval times the catalyst bed to achieve an effective impregnation. A high nickel dispersity can be achieved by using organometallic precursors. The oligomerization products over time on stream are shown in Figure1A for the oligomerization over acidic aluminosilicate and after in situ impregnation of the catalyst with Ni(cp)2. Before impregnation, mainly 3,4-dimethylhexene (3,4-DMH) was formed, but after impregnation the proportion of 3,4-DMH was reduced in favor of n-octene (n-O) and 3-methylheptene (3-MH). However, a significant quantity of 3,4-DMH is formed even after the addition of the nickel solution. The coordinative formation of 3,4-DMH is attributed to a reaction of 1-butene with 2-butene and/or dimerization of 2-butenes [25]. Investigations regarding the reactivity of the individual n-butenes via a coordinative mechanism indicated that the reaction of 1-butene is favored [36]. Accordingly, the formation of 3,4-DMH via a coordinative mechanism is depending on the proportion of 1-butene in the feed, the WHSV, and the reaction temperature. Due to a varying feed composition in industrial operation, the ratio of n-O to 3,4-DMH cannot be used as a criterion to assess the saturation of acid sites by nickel ions. Due to the presence of nickel, the conversion suddenly increases from 5 wt.% to over 30 wt.% after impregnation. At 300 h time on stream, the conversion reached a constant level of approx. 25 wt.%. This value is much higher compared to the acid-catalyzed conversion. Nickel can be considered highly active even at 80 ◦C. Figure1A also shows that in acid-catalyzed oligomerization of n-butenes, approx. 10% other skeletal isomers are formed, which can no longer be observed after impregnation with Ni(cp)2 despite a higher conversion-level. These skeletal isomers are described as side products below. Formation of these isomers can be attributed to skeletal isomerization [37]. Migration of a methyl group is based on the formation of stable cyclopropyl carbon cations [38]. On bifunctional catalysts with similar acidity it can be shown that zeolites with one-dimensional channel system and a pore diameter less than 0.7 nm, such as mordenite, do not catalyze skeletal isomerization of butane or pentane [39]. However, n-butene is isomerized to iso-butene on ferrierite with a slightly lower pore diameter to mordenite but with a two-dimensional channel structure [40]. An adequate size or structure of the pores is necessary for the formation of side products. Other investigations indicate that a methyl group migrates to Brønsted acid sites [41]. In this investigation a mesoporous aluminosilicate was used, to prevent any influence caused by pore size. Furthermore, it is known from literature regarding the oligomerization of ethylene that micropores are not necessarily conducive to the activity [42]. Mainly 2,3-dimethylhexene (2,3-DMH), 4,4-dimethylhexene (4,4-DMH), and 3-ethyl-2-methylpentene (3E-2MP) were discovered as side products. With oligomerization of n-butenes on solid phosphoric acid, these isomers were CatalystsCatalysts2020 20, 2010,, 10 1487, x FOR PEER REVIEW 4 of4 of15 15

Catalysts 2020, 10, x FOR PEER REVIEW 4 of 15 temperatures between 150 and 250 °C [43]. The proportion of these skeletal isomers in the product alsospectrum identifiedtemperatures shown at reaction between in Figure temperatures 150 1B and indicates 250 °C between [ 43that]. The none 150 proportion of and these 250 isomersof◦C[ these43]. skeletalare The formed proportion isomers after in ofimpregnation.the these prod skeletaluct isomersHence,spectrum in the the absence productshown inof spectrum theseFigure C8 1B side shownindicates products in that Figure pointsnone1B of indicatesout these the isomerscomplete thatnone are saturation formed of these after of isomers the impregnation. Brønsted are formed acid aftersites impregnation.Hence, by nickel the absence ions. Hence, of these the absenceC8 side products of these points C8 side out products the complete points saturation out the of complete the Brønsted saturation acid of the Brønstedsites by nickel acid sites ions. by nickel ions.

FigureFigure 1. Distribution1. Distribution of of octene octene skeletal skeletal isomers isomers beforebefore and afterafter in in situ situ impregnation impregnation with with Ni(cp) Ni(cp)2 at2 at Figure 1. Distribution of octene skeletal isomers before and after in situ impregnation with Ni(cp)2 at 140140140 hh time timeh time onon on stream;stream; stream; ((AA (A)) distributiondistribution) distribution of ofof the thethe main main products products andand and summarizedsummarized summarized side side side products products products for for the for the the respective conversion, (B) distribution of the side products stemming from the dimerization of n- respectiverespective conversion, conversion, (B) ( distributionB) distribution of the of sidethe side products products stemming stemming from from the dimerization the dimerization of n-butene. of n- butene.butene. Not all C8 side products are equally suitable for assessing the remaining Brønsted acidity. Not all C8 side products are equally suitable for assessing the remaining Brønsted acidity. The The acid-catalyzedNot all C8 side oligomerization products are equally of n-butenes suitable was for repeated assessing in the the remaining differential Brønsted loop-type acidity. reactor The by acidacid-catalyzed-catalyzed oligomerization oligomerization of of n n-butenes-butenes was was repeated in in the the differential differential loop loop-type-type reactor reactor by by varyingvarying the reaction the reaction temperature. temperature. Every Every reaction reaction temperature temperature was was considered considered for at least least 150 150 h h time time on streamvarying to reach the reaction a steady-state. temperature. Figure Every2A showsreaction the temperature ratio of 4,4-DMH was considered and (B) thefor at ratio least of 150 3E-2MP h time to on onstream stream to toreach reach a steadya steady-state.-state. Figure Figure 2A 2A showsshows the ratio ofof 4,44,4--DMHDMH and and (B) (B) the the ratio ratio of of3E 3E-2MP-2MP the mainto the product main product 3,4-DMH 3,4- duringDMH during acid-catalyzed acid-catalyzed conversion conversion at di atff erentdifferent temperatures. temperatures. Due Due to to slight to the main product 3,4-DMH during acid-catalyzed conversion at different temperatures. Due to fluctuationsslight fluctuations in the feed in composition, the feed composition, there is a there slight is change a slight inchange the conversion in the conversion of n-butenes. of n-butenes. However, slight fluctuations in the feed composition, there is a slight change in the conversion of n-butenes. this hasHowever, no influence this has on no the influence ratio of on 4,4-DMH the ratio and of 4,4 3E-2MP-DMH toand 3,4-DMH; 3E-2MP to a 3,4 constant-DMH; ratioa constant was identified ratio However, this has no influence on the ratio of 4,4-DMH and 3E-2MP to 3,4-DMH; a constant ratio at all temperatures.was identified at On all thetemperatures. other hand, On the the ratio other of hand, 2,3-DMH the ratio to 3,4-DMHof 2,3-DMH in to Figure 3,4-DMH2C does in Figure not remain2C was identified at all temperatures. On the other hand, the ratio of 2,3-DMH to 3,4-DMH in Figure 2C does not remain at a constant level at different temperatures. Consequently, the formation of 2,3- at adoes constant not remain level at at di aff erentconstant temperatures. level at different Consequently, temperatures. the formation Consequently, of 2,3-DMH the formation is dependent of 2,3- on otherDMH factors, is dependent such as the on proportionother factors, of such 1-butene as the proportion in the feed, of and1-butene this cannotin the feed, be usedand this as cannot a criterion DMHbe usedis dependent as a criterion on other to assess factors, the such rem asaining the proportionBrønsted acid of 1 sites.-butene The in 3E the-2MP feed, isomer and this behaves cannot to assess the remaining Brønsted acid sites. The 3E-2MP isomer behaves similarly to 4,4-DMH and, be similarly used as toa 4,4 criterion-DMH and,to assess potentially, the rem couldaining also Brønstedbe used as acid a criterion sites. . The 3E-2MP isomer behaves potentially,similarly couldto 4,4- alsoDMH be and, used potentially, as a criterion. could also be used as a criterion.

Figure 2. Ratio of octene skeletal isomer after methyl group migration to the main product 3,4- dimethylhexene at different reaction temperatures in differential loop-type reactor for the acid FigureFigure 2. 2.Ratio Ratio of of octene skeletal skeletal isomer isomer after after methyl methyl group group migration migration to the to main the main product product 3,4- 3,4-dimethylhexene dimethylhexene at at different different rea reactionction temperatures temperatures in in differential differential loop loop-type-type reactor reactor for for the the acid acid catalyzed conversion of n-butenes; ratio of side products (A) 4,4-DMH; (B) 3E-2MP; and (C) 2,3-DMH

to the main product 3,4-DMH. Catalysts 2020, 10, x FOR PEER REVIEW 5 of 15

Catalystscatalyzed2020, 10, conversion 1487 of n-butenes; ratio of side products (A) 4,4-DMH; (B) 3E-2MP; and (C) 2,3-DMH5 of 15 to the main product 3,4-DMH.

InIn aa didifferentialfferential loop-typeloop-type reactor,reactor,the the productsproducts formedformed areare continuouslycontinuously passedpassed overover thethe catalystcatalyst bed.bed. ThisThis leadsleads toto aa productproduct inhibitioninhibition andand dilutiondilution ofof thethe freshfresh feed.feed. TheThe selectivityselectivity ratioratio mentionedmentioned inin FigureFigure2 2,, 0.023 0.023 for for 4,4-DMH 4,4-DMH and and 0.03 0.03 for for 3E-2MP 3E-2MP to to the the main main product product 3,4-DMH 3,4-DMH at at 100 100 ◦°C,C, cancan bebe seenseen asas thethe maximummaximum methylmethyl groupgroup migrationmigration rate under didifferentialfferential conditions at this temperature. ByBy usingusing aa fixedfixed bedbed reactorreactor thethe concentrationconcentration ofof thethe formedformed productsproducts changedchanged alongalong thethe reactorreactor length.length. ThisThis allowsallows aa higherhigher ratioratio ofof C8C8 sideside toto mainmain products.products. InIn orderorder toto evaluateevaluate thethe amorphousamorphous aluminosilicatealuminosilicate withwith respectrespect toto acid-catalyzedacid-catalyzed oligomerization,oligomerization, aa reactionreaction temperaturetemperature ofof 100100 ◦°CC isis suitable.suitable. AtAt thisthis temperature,temperature, higherhigher conversionsconversions are achievedachieved andand aa variationvariation ofof thethe conversionconversion byby feedfeed raterate isis feasible.feasible.

2.2.2.2. CorrelationCorrelation betweenBetween YieldYield of of Side Side Products Products and and Feed Feed Rate Rate in in Fixed Fixed Bed Bed Reactor Reactor TheThe formationformation ofof 3,4-DMH3,4-DMH comparedcompared toto thethe formationformation ofof sideside productsproducts atat aa constant temperature (100(100 ◦°C)C) and varying varying conversion conversion was was investigated investigated on on acidic acidic aluminosilicate aluminosilicate in fixed in fixed bed bedreactor. reactor. The Theresidence residence time time of the of the olefins olefins in in the the catalyst catalyst bed bed was was influenced influenced by by varying varying the the feed feed rate over a a constantconstant catalyst mass. This This procedure procedure is is well well established established for for assessing assessing catalyst catalyst performance performance [44 [].44 At]. Ata reduced a reduced feed feed rate, rate, the mass the massof olefins of olefins per ho perur per hour gram per catalyst gram catalystwas reduced, was reduced,leading to leading increased to increasedconversion conversion rates. Figure rates. 3 shows Figure the3 showsamount the of amount3,4-DMH of in 3,4-DMH the discharge in the of discharge the reactor of compared the reactor to comparedthe amount to of the side amount products of side 4,4- productsDMH and 4,4-DMH 3E-2MP. and The 3E-2MP.increasing The amount increasing of 3,4 amount-DMH ofis due 3,4-DMH to the isreduced due to thefeed reduced rate. Increasing feed rate. Increasingthe residence the residencetime of the time n-butenes of the n-butenes over the over catalyst the catalyst bed favors bed favorsdimerization dimerization with migration with migration of a methyl of a methyl group groupdisproportionately. disproportionately. Therefore Therefore,, the amount the amount of side ofproducts side products compared compared to 3,4- toDMH 3,4-DMH increases increases quadratically. quadratically. The formulas The formulas given given in Figure in Figure 3 for3 for the thecorrelation correlation between between 3,4-DMH 3,4-DMH and the and respective the respective side product side product apply only apply for only the investigated for the investigated reaction reactiontemperature temperature (100 °C) (100 and◦ theC) anddistribution the distribution of the acid of the sites acid of sites the ofchosen the chosen acidic acidicaluminosilicate. aluminosilicate. Both Bothfactors factors influence influence the ratio the ratio of dimerization of dimerization with with migration migration of a ofmethyl a methyl group group (scheme (scheme in Figure in Figure 3). 3 ).

FigureFigure 3.3. AbsoluteAbsolute amountamount (in(in wt.%)wt.%) ofof 4,4-dimethylhexene4,4-dimethylhexene (4,4-DMH)(4,4-DMH) andand 3-ethyl-2-methylpentene3-ethyl-2-methylpentene (3E-2MP)(3E-2MP) atat didifferentfferent feed feed ratesrates versusversus absoluteabsolute amountamount ofof 3,4-DMH3,4-DMH inin productproduct streamstream atat thethe constantconstant

massmass ofof amorphousamorphous aluminosilicatealuminosilicate inin protonproton form,form, 100100 ◦°CC reactionreaction temperaturetemperature andand 3030 barbar (liquid(liquid phase);phase); trendtrend lineline withwith equations,equations, coecoefficientfficient ofof determinationdetermination andand schemescheme ofof thethe competingcompeting reactions.reactions.

TheThe activityactivity ofof aa methylmethyl groupgroup migrationmigration cancan bebe describeddescribed byby thethe ratioratio ofof thethe sideside toto thethe mainmain product.product. Based Based on on the the results results in in the the differential differential loop loop-type-type reactor, reactor, a constant a constant ratio ratio of 4,4 of-DMH 4,4-DMH and and3E-2MP 3E-2MP to 3, to4-DMH 3,4-DMH could could be foundbe found under under steady steady state state conditions conditions (see (see Figure Figure 2).2 ). In In this this case, case, a a dependence of the ratio of the temperature could be found. It could be found an increased ratio with CatalystsCatalysts 20202020,, 1010,, x 1487 FOR PEER REVIEW 66 of of 15 15 dependence of the ratio of the temperature could be found. It could be found an increased ratio with increasingincreasing temperature. temperature. The The catalytic catalytic testing testing in in the the fixed fixed bed bed reactor reactor w wereere performed performed at at a a constant temperaturetemperature of of 100 100 °C.◦C. However, However, the the results results in in Figure Figure 33 indicateindicate aa correlation ofof feed rate and activity toto methyl methyl group group migration. migration. To To clarify clarify this this dependence, dependence, the the ratio ratio of of 4,4 4,4-DMH-DMH and and 3E 3E-2MP-2MP to to 3,4 3,4-DMH-DMH isis plotted plotted against against the the re residencesidence time time in in Figure 44.. AA linearlinear trendtrend lineline withwith associatedassociated formulaformula isis alsoalso given.given. Both Both C8 C8 side products are formedformed preferentiallypreferentially at at higher higher residence residence time. time. The The rise rise of of the the ratios ratios is iscomparable comparable for for both both side side products. products. The The number number of products of products after after methyl methyl group group migration migration of 3,4-DMH of 3,4- DMHincreases increases 0.003 times0.003 pertimes minute per minute of residence of residence time. time. The ratio The isratio independent is independent of the of catalyst the catalyst mass massbecause because the yield the yield of both of sideboth andside main and main products products is influenced is influenced by the by same the same parameter. parameter.

FigureFigure 4. RatioRatio of of the the side products 4,4 4,4-DMH-DMH and 3E 3E-2MP-2MP to the main product 3,4-dimethylhexene3,4-dimethylhexene

(3,4(3,4-DMH)-DMH) at didifferentfferent residenceresidence times, times, varied varied through through feed feed rate rate in thein the fixed fixed bed bed reactor reactor (100 (100◦C, 30°C, bar); 30 bar);(A) linear (A) linear area uparea to up 7 min; to 7 min; (B) ratio (B) ratio at residence at residence time fromtime from 7 to 25 7 to min. 25 min.

AtAt a residence timetime ofof 6.56.5min min a a ratio ratio of of side side to to main main products products comparable comparable to theto the experiment experiment in thein theloop-type loop-type reactor reactor could could be found. be found. As already As already were discussed, were discussed, the methyl the group methyl migration group suppressed migration suppressedby a high content by a high of C8content molecules. of C8 molecules. Due to the Due increase to the in increase the concentration in the concentration of products of alongproducts the alongcatalyst the bed, catalyst such bed, product such inhibition product inhibition will only occurwill only in the occur back in of the the back catalyst of the bed. catalyst Figure bed.4B showsFigure 4Bthat shows even that at a residenceeven at a residence time of 10 time min, of the 10 ratiomin, the deviates ratio deviates from linear from progression. linear progression. However, However, it can be itassumed can be assumed that if the that residence if the residence time istoo time high, is too there high, will there be nowill further be no further increase increase of this ratioof this since ratio a sincehigher a proportionhigher proportion of products of products is already is already formed formed at the beginning at the beginning of the catalyst of the catalyst bed. This bed. suggest This suggestthat studies that studies on the on formation the formation of side of product side product at nickel-containing at nickel-containing catalysts catalysts should shoul bed carried be carried out outat residence at residence times times of betweenof between 7 and7 and 8 min.8 min. In In this this case, case, quantifiable quantifiable amountsamounts ofof side products are are formedformed at at existing existing acid acid sites sites and and the the product product inhibition inhibition does does not not yet yet significantly significantly affect affect the the migration migration ofof methyl methyl groups. groups.

2.3.2.3. Determinatio Determinationn of of Acid Acid-Catalyzed-Catalyzed Oligomerization Oligomerization on on Nickel Nickel-Containing-Containing Catalysts Catalysts TheThe correlation correlation between between 3,4 3,4-DMH-DMH and and the the side side products products in in Figure Figure 33 cancan bebe usedused toto distinguishdistinguish betweenbetween acid acid-- and nickel-catalyzednickel-catalyzed oligomerizationoligomerization of of n-butenes n-butenes over over nickel-containing nickel-containing samples. samples. By By in insitu situ impregnation impregnation an exclusive an exclusive nickel-catalyzed nickel-catalyzed oligomerization oligomerization could be realized could forbe a realized heterogeneous for a heterogeneouscatalyst. The production catalyst. The of such production catalyst of in such industrial catalyst scale in industrial is not feasible. scale To is producenot feasible. oligomerization To produce oligomerizationcatalysts on a ton catalysts scale, inorganic on a ton nickel scale, salts inorganic are preferably nickel salts used are for preferably an ex situ used impregnation. for an ex As situ a impregnation.result, lower dispersities As a result, are lower achieved dispersities and despite are aachieved high nickel and content, despite not a high all acid nickel sites content, are necessarily not all acidmasked sites with are nickelnecessarily ions. Themasked proportion with nickel of side ions. products The proportion formed can of be side used products to assess formed the saturation can be usedof acid to sitesassess with the nickelsaturation by catalytic of acid data.sites with Figure nick4Ael shows by catalytic the part data. of main Figure products 4A shows 3,4-DMH the part and of mainn-O in products the product 3,4-DMH spectrum. and Thesen-O in catalytic the product testings spectrum. were performed These catalytic at 100 testings◦C in liquid were phase performed with a atresidence 100 °C timein liquid of 7.8 phase min. Aswith the a nickel residence content time is increased,of 7.8 min. the As amount the nicke of 3,4-DMHl content decreases is increased, whereas the amount of 3,4-DMH decreases whereas the amount of n-O formed increases. This indicates an

Catalysts 2020, 10, 1487 7 of 15 Catalysts 2020, 10, x FOR PEER REVIEW 7 of 15 theincreasing amount ofpart n-O of formedcoordinative increases. oligomerization This indicates of ann-butenes increasing as the part nickel of coordinative content increases. oligomerization The side ofproducts n-butenes in asthe the product nickel contentstream increases.in Figure 4B The show side productsthat the catalyst in the product with a streamnickel content in Figure of4B 14 show wt.% thatdoes the not catalyst form with any a C8 nickel side content products. of 14 This wt.% suggests does not that, form with any C8 this side catalyst, products. oligomerization This suggests that,of n- withbutenes this catalyst,takes place oligomerization completely via of a n-butenes coordinative takes mechanism. place completely via a coordinative mechanism. TheThe resultsresults fromfrom Figure 55 cancan be used used to to calculate calculate the the mass mass of offormed formed octene octene isomers isomers per pergram gramcatalyst catalyst with withvarying varying nickel nickel content. content. These These values values are summarized are summarized in Table in Table1. Regarding1. Regarding the nickel the - nickel-freefree aluminosilicate, aluminosilicate, a high a high yield yield of the ofthe highly highly branched branched octene octene isomer isomer 3,4 3,4-DMH-DMH can be observed.observed. TheThe overall overall yield yield of of octenes octenes decreases decreases slightly slightly by by the the presence presence of of nickel. nickel. However, However, a a higher higher yield yield of of thethe more more linear linear octene octene isomers isomers can can be be observed. observed. In In comparison comparison to to the the oligomerization oligomerization of of ethylene ethylene overover nickel-containing, nickel-containing, mesoporous mesoporous aluminosilicate, aluminosilicate, the the yield yield of of oligomers oligomers is is at at least least one one magnitude magnitude lowerlower [45 [45].]. This This reduced reduced activity activity can can be attributedbe attributed to the to hindrancethe hindrance of C-C of bondC-C bond formation formation by the by more the stericallymore sterically demanding demanding butene butene molecules molecules in coordination in coordination with active with nickelactive species.nickel species.

Figure 5. Part of octene isomers in product stream after oligomerization of n-butenes on amorphous Figure 5. Part of octene isomers in product stream after oligomerization of n-butenes on amorphous aluminosilicates with varying nickel content between 0 and 14 wt.% in continuous operation (100 C, aluminosilicates with varying nickel content between 0 and 14 wt.% in continuous operation (100◦ °C, liquid phase, average over 180 h on stream); (A) part of main products and (B) part of side products. liquid phase, average over 180 h on stream); (A) part of main products and (B) part of side products. Table 1. Mass of octene isomers formed over catalyst samples with different nickel content per gram

catalystTable 1. per Mass hour of in octene a fixed isomers bed reactor formed (100 over◦C, catalyst 30 bar, residencesamples with time different of 7.8 min). nickel content per gram catalyst per hour in a fixed bed reactor (100 °C, 30 bar, residence time of 7.8 min). Catalyst Sample m3,4-DMH/gcat per h m3-MH/gcat per h mn-O/gcat per h Catalyst Sample m3,4-DMH/gcat per h m3-MH/gcat per h mn-O/gcat per h Ni-free 2.4 0.1 0 Ni1-free wt.% Ni 1.22.4 0.50.1 0.2 0 1 wt.%6 wt.% Ni Ni 0.91.2 0.60.5 0.2 0.2 6 wt.%14 wt.% Ni Ni 0.50.9 1.10.6 0.3 0.2 14 wt.% Ni 0.5 1.1 0.3 Based on the part of side products in Figure5B, the amount of 3,4-DMH formed via a cationic mechanismBased canon bethe calculated. part of side Therefore, products thein Figure correlation 5B, the between amount side of products 3,4-DMH and formed the acid-catalyzed via a cationic mainmechanism product can 3,4-DMH be calculated. can be drawn Therefore, from Figure the correlation4. This dependency between side can products be regarded and as the a kind acid- ofcatalyzed calibration main straight product line. 3,4- BasedDMH oncan the be drawn mentioned from formula, Figure 4. theThis proportion dependency of 3,4-DMHcan be regarded formed as bya kindacid-catalyzed of calibration oligomerization straight line. can Based be calculated. on the mentioned Since the formula, 3,4-DMH the can proportion also be formed of 3,4- viaDMH a coordinativeformed by acid oligomerization,-catalyzed oligomerization a residual acid-catalyzed can be calculated. reaction Since cannot the be 3,4 clearly-DMH assigned can also without be formed the considerationvia a coordinative of the side oligomerization, products. The a measured residual part acid- ofcatalyzed 3,4-DMH reaction formed overcannot catalyst be clearly with di assignedfferent nickelwithout content the consideration should be compared of the side to products. the calculated The measured values using part theof 3,4 proportion-DMH form ofed 4,4-DMH over catalyst and 3E-2MP.with different The results nickel are content summarized should inbe Tablecompared2. to the calculated values using the proportion of 4,4- DMH and 3E-2MP. The results are summarized in Table 2.

Catalysts 2020, 10, 1487 8 of 15

Table 2. Comparison of measured content of 3,4-DMH in product stream with theoretical part of 3,4-DMH generated via carbenium mechanism calculated via the part of 4,4-DMH and 3E-2MP in the product stream, average over 180 h on stream.

Measured Content of 3,4-DMH calc. 3,4-DMH calc. Catalyst Sample 3,4-DMH in Product via 4,4-DMH, via 3E-2MP, Stream, wt.% wt.% wt.% 1 wt.% Ni 13.0 14.8 14.3 6 wt.% Ni 9.6 9.6 9.4 14 wt.% Ni 5.1 0 0

After oligomerization, the catalyst with 1 wt.% nickel exhibits an average 3,4-DMH content of 13.0 wt.%. However, based on the amount of side products formed, it should be possible to detect 14.8 wt.% (calculated from 4,4-DMH) or 14.3 wt.% (calculated from 3E-2MP) in the product stream. Accordingly, the calculated part of 3,4-DMH was slightly overestimated. The correlation between 3,4-DMH and side products was determined on acidic aluminosilicate in continuous operation over approx. 500 h time on stream. As a result, it can be concluded that during the long time on stream acid sites are deactivated, probably by deposits of high boiling products. The deactivation of strong acid sites during the oligomerization of propylene is well known since decades [46]. The activity in steady-state is related to acid sites of moderate strength. Investigation of the oligomerization of 1-butene on beta zeolite with different acidities indicates that secondary reactions take place, such as hydrid transfer or migration of a methyl group on strong Brønsted acid sites [47]. These sites may also be responsible for the formation of side products during the oligomerization. Through deactivation, the correlation between 3,4-DMH and side products changes. Figure6A shows the part of 3,4-DMH formed on the catalyst with 1 wt.% nickel over 180 h on stream. The industrial feed applied here shows fluctuations in the olefin concentration. In the first 80 h on stream, the resulting change in WHSV shows a direct change in the amount of 3,4-DMH that is formed. As the strong acid sites are increasingly deactivated from approx. 75 h on stream onwards, less 3,4-DMH is formed at a constant WHSV. Oligomerization of 1-butene on ZSM-5 containing Al and ZSM-5 containing Fe was investigated in the literature [48]. If in ZSM-5 aluminum is substituted with iron, this reduces the acidity, which was measured by the reduced conversion of 1-butene. However, the Fe-ZSM-5 system tends to show a lower deactivation. After initial deactivation of the strong acid sites, this catalyst exhibited stable conversion, probably due to moderately strong acid sites that do not catalyze the formation of deactivation products. Transferring these findings to the mesoporous aluminosilicate discussed here, deactivation should influence the formation of side products. Figure6B shows the measured proportion of 3,4-DMH compared to the part of 3,4-DMH calculated from the side products. As deactivation of strong acid sites increases, activity for migration of methyl group decreases and fewer side products are formed. After 150 h time on stream, the calculated amount of 3,4-DMH is similar to the measured value. The reaction system approaches the stationary state that was already reached when determining the measured values from Figure3. It can be concluded that an increased density of strong acid sites has a positive e ffect on the formation of side products. For the sample containing 6 wt.% nickel, Table1 shows a good correlation between the calculated and measured amounts of 3,4-DMH. It can be assumed that with this catalyst almost the entire amount of 3,4-DMH is formed via a cationic mechanism. This can be attributed to the small amount of active nickel sites for coordinative oligomerization. While with a coordinative mechanism 3,4-DMH is the product of dimerization of 2-butene or co-dimerization of 1-butene and 2-butene, 1-butene is preferably dimerized on active nickel sites [49]. Consequently, with a small number of active nickel sites, almost no 2-butene is coordinatively dimerized to 3,4-DMH as the nickel sites are constantly covered only with excessive 1-butene. Catalysts 2020, 10, x FOR PEER REVIEW 9 of 15 Catalysts 2020, 10, 1487 9 of 15

FigureFigure 6. 6. CatalyticCatalytic results of of n n-butene-butene oligomerization oligomerization on on the the catalyst catalyst with with 1 wt.% 1 wt.% Ni at Ni 100 at °C; 100 (A◦C;) (A3,4) 3,4-DMH-DMH in inthe the product product stream stream in incontinuous continuous operation operation by by varying varying the the content content of of C4 C4 olefins olefins in in the the feed;feed; ( B(B)) comparison comparison between between measured measured andand calculatedcalculated contentcontent of 3,4-DMH3,4-DMH via side products products in in the the productproduct stream. stream.

TheFor number the sample and containing strength 6 ofwt.% acid nickel, sites Table of a 1 solid shows material a good can correlation be determined between the via temperature-programmedcalculated and measured amounts ammonia of desorption 3,4-DMH. It (TPAD). can be assumed The density that of with acid this sites catalyst determined almost with the TPADentire shows, amount for of example,3,4-DMH ais veryformed good via correlation a cationic mechanism. with the catalytic This can activity be attributed for isomerization to the small of n-butaneamount of [50 active]. The nickel desorption sites for profiles coordinative of the aluminosilicatesoligomerization. withWhile and with without a coordinative nickel are mechanism shown in Figure3,4-DMH7. The is desorptionthe product profileof dimerization of the aluminosilicate of 2-butene or in co (A)-dimerization shows a broad of 1 distribution-butene and of2-butene, acid sites 1- withbutene diff iserent preferably strength. dimerized The desorption on active ofnickel ammonia sites [49 at].temperatures Consequently, above with a 500 small◦C number indicates of strongactive acidnickel sites sites, [51]. almost For the no nickel-free 2-butene aluminosilicate is coordinatively a desorption dimerized of to 830 3,4µmol-DMH NH as3 per the gram nickel sample sites was are measured.constantly A covered total of only 130 µwithmol excessive/g strong acid1-butene. sites could be assigned. By adding nickel to the surface, these acidThe sitesnumber are and increasingly strength maskedof acid sites and aof new, a solid narrow material signal can becomes be determined visible. via The temperature formation of- metalprogrammed ammonia ammonia complexes desorption occurs by (TPAD). coordinative The density interaction of acid between sites determined ammonia andwith metal TPAD cations shows, at for example, a very good correlation with the catalytic activity for isomerization of n-butane [50]. The the surface of porous solids [52]. These complexes can decompose at temperatures above 450 ◦C[53]. Increaseddesorption nickel profiles content of the decomposes aluminosilicates more complexes with and without and increases nickel the are intensity shown in of Figure the resulting 7. The signal.desorption The sample profile withof the 1 aluminosilicate wt.% nickel in Figure in (A)7 showsB does a not broad show distribution this narrow of signal.acid sites However, with different in this strength. The desorption of ammonia at temperatures above 500 °C indicates strong acid sites [51]. sample desorption is detected at temperatures above 500 ◦C, comparable to the nickel-free sample. ThisFor showsthe nickel the- presencefree aluminosilicate of strongly a acidic desorption centers, of which830 µmol was NH already3 per gram predicted sample by was the interpretationmeasured. A oftotal the catalyticof 130 µmol/g data regarding strong acid the sites correlation could be between assigned. 3,4-DMH By adding and 4,4-DMH.nickel to the These surface, acid sites these are acid not maskedsites are by increasingly impregnating masked the aluminosilicate and a new, narrow with only signal 1 wt.% becomes nickel. visible. Therefore, The a formation significant of amount metal ammonia complexes occurs by coordinative interaction between ammonia and metal cations at the of 4,4-DMH is formed, which decreases with time by deactivation of strong Brønsted acid sites until surface of porous solids [52]. These complexes can decompose at temperatures above 450 °C [53]. the steady-state (analogous to reference [46]). Increased nickel content decomposes more complexes and increases the intensity of the resulting The catalyst with 6 wt.% nickel shows no significant deactivation within the first 100 h time on signal. The sample with 1 wt.% nickel in Figure 7B does not show this narrow signal. However, in stream and the amount of 3,4-DMH formed by acid catalysis can be calculated directly from the amount this sample desorption is detected at temperatures above 500 °C, comparable to the nickel-free of 4,4-DMH (see Table2). This sample also shows no desorption of ammonia in Figure7B at 600 C in sample. This shows the presence of strongly acidic centers, which was already predicted by◦ the contrast to the sample with 1 wt.% nickel. The strong acid sites were masked with nickel in the catalyst interpretation of the catalytic data regarding the correlation between 3,4-DMH and 4,4-DMH. These with a nickel content 6 wt.%. Through the course of the ratio of 3,4-DMH to 4,4-DMH over time, acid sites are not masked by impregnating the aluminosilicate with only 1 wt.% nickel. Therefore, a the deactivation of strong acid sites during the reaction can be interpreted. In industrial operation, significant amount of 4,4-DMH is formed, which decreases with time by deactivation of strong this information is very helpful in order to be able to make predictions about the conditions of the Brønsted acid sites until the steady-state (analogous to reference [46]). catalyst and about the further run-time. Usually, no sample of the catalyst can be taken from the reactor during the industrial operation, so the interpretation of obtained catalytic data is extremely important.

Catalysts 2020, 10, 1487 10 of 15 Catalysts 2020, 10, x FOR PEER REVIEW 10 of 15

FigureFigure 7. 7.Temperature Temperature programmed programmed desorption desorption of ammonia of ammonia after removal after of removal physisorbed of physisorbed compounds

atcompounds 200 ◦C; (A ) atprofile 200 of°C; amorphous (A) profile aluminosilicate of amorphous in aluminosilicate proton form and in ( B proton) of impregnated form and samples (B) of withimpregnated varying nickel samples content. with varying nickel content.

TheThe catalyst catalyst with with 14 6 wt.%wt.%nickel nickel has shows a higher no significant number of deactivation active nickel within sites and, the asfirst a result, 100 h 2-butenetime on isstream also coordinatively and the amount converted of 3,4-DMH to 3,4-DMH formed with by acid this catalysis catalyst canas more be calculated nickel centers directly are from available the thatamount are not of 4,4 completely-DMH (see covered Table 2). by This 1-butene. sample This also is shows the reason no des whyorption no prediction of ammonia can in be Figure made 7B about at the600 part °C in of contrast acid catalysis to the sample based with on the 1 wt.% amount nickel. of 3,4-DMHThe strong as acid it can sites obviously were masked be formed with nickel also in by coordinativethe catalyst catalysis.with a nickel This content catalyst 6 exhibits wt.%. Through no formation the course of side of products, the ratio which of 3,4 means-DMH thatto 4,4 complete-DMH saturationover time, of the the deactivation acid sites with of strong nickel acid ions sites can beduring assumed. the reaction This information can be interpreted. would not In industrial have been availableoperation, without this information consideration is very of helpful the side in products. order to be able to make predictions about the conditions of theThe catalyst interaction and about of nickel the furth cationser run with-time. the surfaceUsually, of no acidic sample aluminosilicate of the catalyst results can be in taken a decrease from ofthe Brønsted reactor acidity, during whereas the industrial the Lewis operation, acidity increases so the interpretation [54]. As already of discussed, obtained catalytic these new data Lewis is extremely important. acid sites can form complexes with ammonia, which in case of nickel are decomposed at about 500 ◦C and causeThe catalyst a corresponding with 14 wt.% signal nickel in the has TPAD. a higher The number interaction of active of nickel nickel with sites aluminosilicates and, as a result, can2- bebutene described is also by coordinatively the HSAB theory converted (hard and to 3,4 soft-DMH acids with and bases).this catalyst The nickel as more cations nickel interacting centers are as electrophilicavailable that compound are not completely with the electron-rich covered by surface-oxygen1-butene. This is in the vicinity reason to why aluminum no prediction [55]. A classicalcan be replacementmade about ofthe protons part of acid by nickelcatalysi cationss based at on Brønsted the amount acid of sites 3,4-DMH are conceivable as it can obviously [56] as wellbe formed as the interactionalso by coordinative of nickel cationscatalysis. with This silanol catalyst groups exhibits [57 no]. Information contrast of to side the oligomerizationproducts, which means of ethylene that acomplete complete saturation substitution of the of protonsacid sites with with nickel nickel cations ions can favors be assumed. the oligomerization This information of butenes would to not the desiredhave been products. available without consideration of the side products. The interaction of nickel cations with the surface of acidic aluminosilicate results in a decrease 3.of Materials Brønsted andacidity, Methods whereas the Lewis acidity increases [54]. As already discussed, these new Lewis acid sites can form complexes with ammonia, which in case of nickel are decomposed at about 500 3.1.°C and Catalysts cause a corresponding signal in the TPAD. The interaction of nickel with aluminosilicates can be describedAcid-catalyzed by the HSAB oligomerization theory (hard of and n-butenes soft acids was and investigated bases). The nickel on granulated cations interacting amorphous as aluminosilicateelectrophilic compound (Grace, DAVICAT with the O electron 701, Columbia,-rich surface MD,-oxygen USA) within vicinity particle to sizes aluminum between [55 1]. and A classical replacement of protons by nickel cations at Brønsted acid sites are conceivable [56] as well 2.5 mm, a Si/Al ratio of 6.4 and an average pore diameter of 11 nm. The average pore diameter was as the interaction of nickel cations with silanol groups [57]. In contrast to the oligomerization of determined using nitrogen physisorption and evaluation according to Barret, Joyner, and Halenda [58] ethylene a complete substitution of protons with nickel cations favors the oligomerization of butenes as well as porosimetry with mercury [59]. To assess the influence of nickel on the formation of to the desired products. different octene isomers, the amorphous aluminosilicate was impregnated by incipient wetness method with an aqueous Ni(NO ) solution and then calcined at 550 C for 10 h under a nitrogen stream. 3. Materials and Methods3 2 ◦ For impregnation, the volume of nickel solution was adjusted to suit the pore volume. The concentration of3.1. nickel Catalysts ions in the solution was adjusted in the way that, after calcination, catalysts with a nickel content of 1, 6, and 14 wt.% were obtained. Acid-catalyzed oligomerization of n-butenes was investigated on granulated amorphous aluminosilicate (Grace, DAVICAT O 701, Columbia, MD, USA) with particle sizes between 1 and 2.5

Catalysts 2020, 10, 1487 11 of 15

3.2. Characterization The acid strength distribution and number of acid sites of the used amorphous aluminosilicate were determined by temperature-programmed desorption of ammonia. The measurement was carried out with the TPDRO 1100 device from Thermo Scientific (Waltham, MA, USA). For this purpose, 1 g of the sample was weighed into a measuring cell and calcined at 550 ◦C. When the sample had cooled to room temperature, it was loaded with ammonia for 2 min. The physically and weakly adsorbed part of the ammonia was removed at 200 ◦C with argon. After the sample had cooled again, the desorption profile from 40 ◦C to 600 ◦C was determined with a heating rate of 5 K/min and a flow rate of 20 mL/min argon. The maximum temperature was maintained for 30 min. Detection took place via a thermal conductivity detector. To quantify the amount of chemically adsorbed ammonia, the sample was heated to the temperature of the desorption maximum and a defined quantity of ammonia was injected into the system with a gas syringe. The measurement signal was used as a calibration point for the respective temperature. The catalysts with different nickel content were characterized in the same way.

3.3. Analysis of Products and Feed The composition of feed and reaction product was determined online via a sample loop with two gas chromatography systems (GC). Distribution of the C4 fraction is determined separately by GC. Distribution of the octene isomers and the proportion of the C4 fraction over the complete product spectrum were analyzed by hydrogenating the evaporated components via a Pd catalyst in the GC liner with hydrogen as a carrier gas [60]. An inlet temperature of 190 ◦C was used to avoid cracking of oligomers in the liner. The distribution of the octene skeletal isomers is directly accessible through hydrogenolytic gas chromatography. The arrangement of the isomers was verified with the corresponding pure substances and based on literature values [61]. The supporting information provides more detailed information regarding the calculation of conversion and selectivity.

3.4. Testing and In Situ Impregnation in a Loop-Type Reactor Acid catalyzed conversion of n-butenes was tested in a differential loop-type reactor. The ratio of feed rate and cycle was set to 1:10 to ensure adequate back mixing [62]. Four grams of aluminosilicate was added to the reactor for this purpose. A mixture of n-butenes, iso- and n-butane was used as the feed. The feed was set to a weight hourly space velocity (WHSV) of 7.5 g olefin per gram aluminosilicate. To ensure a reaction in liquid phase, the reaction temperature was varied between 80 and 100 ◦C at a pressure of 30 bar. For in situ impregnation, 0.5 g bis(cyclopentadienyl) nickel(II) (nickelocene, Ni(cp)2) was dissolved in 100 mL dry n-heptane and stirred for 1 h. The solution was then filled into a pressure-resistant vessel under argon and fed via a by-pass into the reactor with fresh feed under reaction conditions. During impregnation at 80 ◦C, the feed and discharge of the reactor were closed and the mixture was stirred with circulation for 1 h. Following this, the feed and discharge were opened and the conversion and distribution of the C8 isomers were analyzed. A typical composition of feed and a diagram of the reaction apparatus can be found in the supporting information.

3.5. Testing in a Fixed Bed Reactor Oligomerization of n-butenes on acidic aluminosilicate and catalysts based on this with different nickel content were investigated in a fixed bed reactor in continuous operation. For this purpose, 12.5 g catalyst was filled into a tube with an inside diameter of 6 mm. Glass beads in front of and behind the catalyst filling acted as preheating and cooling zones. Oligomerization was carried out at 100 ◦C with 30 bar in the liquid phase with a WHSV of 7.5 g olefin per hour per gram catalyst.

4. Conclusions Heterogeneous catalysts for coordinative oligomerization of n-butenes are produced on an industrial scale through impregnation of acidic aluminosilicates with a nickel precursor. Remaining acid Catalysts 2020, 10, 1487 12 of 15 sites can convert n-butenes via an acid-catalyzed mechanism and thus influence product distribution. It was shown that certain octene isomers are exclusively formed via an acid-catalyzed mechanism. Some octene side products, namely 4,4-dimethylhexene (4,4-DMH) and 3-ethyl-2-methylpentene (3E-2MP), are formed by a methyl group migration of 3,4-dimethylhexene (3,4-DMH) at strong acid sites. Sufficient pore size to form transition states is required. The amount of these side products depends on the reaction temperature, space velocity, and density of strong acid sites. For a specific acidic aluminosilicate, a distribution between side products and 3,4-DMH can be determined under different reaction conditions. By using this aluminosilicate for oligomerization after impregnation with a nickel precursor, the amount of 4,4-DMH and/or 3E-2MP indicates the degree of saturation of active acid sites with nickel ions. The proportion of 3,4-DMH formed through an acid-catalyzed mechanism can be calculated. This allows the effectiveness of an impregnation method and the influence of different nickel precursors and quantities to be determined by catalytic testing without any further characterization. Furthermore, information about the deactivation of strong Brønsted acid sites can be obtained by the shifting ratio of 3,4-DMH to 4,4-DMH over time. With increasing deactivation, the relative part of 4,4-DMH decreases. As a result, predictions can be made about the further catalyst activity in industrial operation. During continuous operation, where no catalyst samples can be taken, the assessment of how to proceed depends on such important catalytic information.

Author Contributions: Conceptualization, W.R. and R.F.; methodology, F.N., F.A., and W.R.; software, F.N. and S.P.; validation, F.N., F.A., S.P., W.R., E.B., and R.F.; formal analysis, E.B.; investigation, F.N. and F.A.; resources, S.P.; data curation, F.N.; writing—original draft preparation, F.N. and F.A.; writing—review and editing, W.R., S.P., R.F., and E.B.; visualization, F.N.; supervision W.R., E.B., and R.F.; project administration, R.F. and S.P.; funding acquisition, W.R. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors acknowledge support by the Open Access Publication Funds of the TU Dresden. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Geilen, F.M.A.; Stochniol, G.; Peitz, S.; Schulte-Körne, E. Butenes. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH: Weinheim, Germany, 2013; pp. 1–13. 2. Hommeltoft, S.I. Isobutane alkylation: Recent developments and future perspectives. Appl. Catal. A 2001, 221, 421–428. [CrossRef] 3. Broellos, K.; Volkamer, K.; Lindner, A. Process for Obtaining a Conjugated Diolefine from a C4-C5 Mixture. Patent DE 3119540 A1, 5 May 1982. 4. Collignon, F.; Mariani, M.; Moreno, S.; Remy, M.; Poncelet, G.J. Gas Phase Synthesis of MTBE from Methanol and Isobutene over Dealuminated Zeolites. J. Catal. 1997, 166, 53–66. [CrossRef] 5. Catani, R.; Mandreoli, M.; Rossini, S.; Vaccari, A. Mesoporous catalysts for the synthesis of clean diesel fuels by oligomerisation of olefins. Catal. Tod. 2002, 75, 125–131. [CrossRef] 6. Ipatieff, V.N.; Corson, B.B.; Egloff, G. , a New Source of Gasoline. Ind. Eng. Chem. 1935, 27, 1077–1081. [CrossRef] 7. Nicholas, C.P. Applications of light olefin oligomerization to the production of fuels and chemicals. Appl. Catal. A Gen. 2017, 543, 82–97. [CrossRef] 8. Busca, G. Acid Catalysts in Industrial Hydrocarbon Chemistry. Chem. Rev. 2007, 107, 5366–5410. [CrossRef] 9. Silva, A.F.; Fernandes, A.; Antunes, M.M.; Neves, P.; Rocha, S.M.; Ribeiro, M.F.; Pillinger, M.; Ribeiro, J.; Silva, C.M.; Valente, A.A. Kinetics and Thermochemistry of C4-C6 Olefin Cracking on H-ZSM-5. Fuel 2017, 209, 371–382. [CrossRef] 10. Keim, W.; Kowaldt, F.W.; Goddard, R.; Krüger, C. Novel coordination of (benzoylmethylene)- triphenylphosphorane in a nickel olgomerization catalyst. Angew. Chem. Int. Ed. Engl. 1978, 17, 466–467. [CrossRef] 11. Keim, W.; Hoffmann, B.; Lodewick, R.; Peuckert, M.; Schmitt, G.; Fleischhauer, J.; Meier, U. Linear Oligomerization of olefins via nickel chelate complexes and mechanistic considerations based on semi-empirical calculations. J. Mol. Catal. 1979, 6, 79–97. [CrossRef] Catalysts 2020, 10, 1487 13 of 15

12. Behr, A.; Rentmeister, N.; Seidensticker, T.; Faßbach, T.A.; Peitz, S.; Maschmeyer, D. Nickel catalyzed dimerization reactions of vinylidene compounds: Head-to-head couplings and catalyst stabilization. J. Mol. Catal. A 2014, 395, 355–363. [CrossRef] 13. Keim, W.; Behr, A.; Kraus, G. Influences of olefinic and diketonate ligands in the nickel-catalyzed linear oligomerization of 1-butene. J. Organomet. Chem. 1983, 251, 377–391. [CrossRef] 14. Forestière, A.; Olivier-Bourbigou, H.; Saussine, L. Oligomerization of Monoolefins by Homogeneous Catalysts. Oil Gas. Sci. Technol. 2009, 64, 649–667. [CrossRef] 15. Nierlich, F.; Neumeister, J.; Wildt, T.; Droste, W.; Obenaus, F. Process for Oligomerising Olefins. Patent EP 0395857, 4 May 1990. 16. Forget, S.; Olivier-Bourbigou, H.; Delcroix, D. Homogeneous and Heterogeneous Nickel-Catalyzed Olefin Oligomerization: Experimental Investigation for a Common Mechanistic Proposition and Catalyst Optimization. ChemCatChem 2017, 9, 2408–2417. [CrossRef] 17. Franke, R.; Selent, D.; Börner, A. Applied Hydroformylation. Chem. Rev. 2012, 112, 5675–5732. [CrossRef] 18. Börner, A.; Franke, R. Hydroformylation—Fundamentals, Processes and Applications in Organic Synthesis; Wiley-VCH: Weinheim, Germany, 2016; Volume 2. 19. Wendt, G.; Finster, J.; Schöllner, R. Untersuchungen an Nickeloxid-Mischkatalysatoren. VIII. Katalytische

Eigenschaften von NiO-Al2O3/SiO2-Katalysatoren. Z. Anorg. Allg. Chem. 1982, 488, 197–206. [CrossRef] 20. Wendt, G.; Jusek, M.; May, M. Untersuchungen an Nickeloxid-Mischkatalysatoren. XVI. Reduktionsverhalten

amorpher NiO-Al2O3/SiO2-Katalysatoren. Z. Anorg. Allg. Chem. 1987, 550, 177–183. [CrossRef] 21. Wendt, G.; Jusek, M.; Schöllner, R. Untersuchungen an Nickeloxid-Mischkatalysatoren. VII. Strukturelle

Eigenschaften von NiO-Al2O3/SiO2-Katalysatoren. Z. Anorg. Allg. Chem. 1982, 488, 187–196. [CrossRef] 22. Kießling, D.; Wendt, G.; Jusek, M.; Schöllner, R. Reduction of an Amorphous NiO-Al2O3/SiO2 Catalysts by Different Olefins and Hydrogen. React. Kinet. Catal. Lett. 1991, 43, 255–259. [CrossRef] 23. Rabeah, J.; Radnik, J.; Briois, V.; Maschmeyer, D.; Stochniol, G.; Peitz, S.; Reeker, H.; La Fontaine, C.; Brückner, A. Tracing Active Sites in Supported Ni Catalysts during Butene Oligomerization by Operando Spectroscopy under Pressure. ACS Catal. 2016, 6, 8224–8228. [CrossRef] 24. Brückner, A.; Bentrup, U.; Zanthoff, H.; Maschmeyer, D. The role of different Ni sites in supported nickel catalysts for butene dimerization under industry-like conditions. J. Catal. 2009, 266, 120–128. [CrossRef] 25. Albrecht, S.; Kießling, D.; Maschmeyer, D.; Nierlich, F.; Wendt, G. Oligomerisierung von n-Butenen. Chem. Ing. Tech. 2005, 77, 695–709. [CrossRef] 26. Nierlich, F.; Neumeister, J.; Wildt, T. The oligomerization of lower olefins by heterogeneous catalysis. Prepr. Am. Chem. Soc. Div Pet. Chem. 1991, 36, 585–595. 27. Ehrmaier, A.; Liu, Y.; Peitz, S.; Jentys, A.; Chin, Y.-H.C.; Sanchez-Sanchez, M.; Bermejo-Deval, R.; Lercher, J. Dimerization of Linear Butenes on Zeolite-Supported Ni2+. ACS Catalysis 2019, 9, 315–324. [CrossRef] 28. Ehrmaier, A.; Peitz, S.; Sanchez-Sanchez, M.; Bermejo-Deval, R.; Lercher, J. On the role of co-cations in nickel exchanged LTA zeolite for butene dimerization. Microporous Mesoporous Mater. 2019, 284, 241–246. [CrossRef] 29. Ehrmaier, A.; Löbbert, L.; Sanchez-Sanchez, M.; Bermejo-Deval, R.; Lercher, J. Impact of Alkali and Alkali-Earth Cations on Ni-Catalyzed Dimerization of Butene. ChemCatChem 2020, 12, 3705–3711. [CrossRef] 30. Alscher, F.; Nadolny, F.; Frenzel, H.; Knossalla, J.; Peitz, S.; Borovinskaya, E.; Breitkopf, C.; Franke, R.; Reschetilowski, W. Determination of the prevailing n-butenes dimerization mechanism over nickel containing Al-MCM-41/ZSM-5 mixed-phase catalysts. Catal. Sci. Technol. 2019, 9, 2456–2468. [CrossRef] 31. Zhao, D.; Xu, Z.; Chada, J.P.; Carrero, C.A.; Rosenfeld, D.C.; Rogers, J.L.; Hermans, I.; Huber, G.W. Cobalt Oxide on N-Doped Carbon for 1-Butene Oligomerization to Produce Linear Octenes. ACS Catalysis 2017, 7, 7479–7489. [CrossRef] 32. Franken, J.; Kirschhock, C.E.A.; Mathys, G.M.; Martens, J.A.; Franken, J.; Kirschhock, C.E.A.; Mathys, G.M.; Martens, J.A. Design of a Cobalt–Zeolite Catalyst for Semi-Linear Higher-Olefin Synthesis. ChemCatChem 2012, 4, 1245–1248. [CrossRef] 33. Martens, J.A.; Ravishankar, R.; Mishin, I.E.; Jacobs, P.A. Tailored oligomerization with H-ZSM-57 zeolite. Angew. Chem. Int. Ed. 2000, 39, 4376–4379. [CrossRef] 34. Kärger, J.; Freude, D. Mass Transfer in Micro- and Mesoporous Materials. Chem. Eng. Technol. 2002, 25, 769–778. [CrossRef] 35. Andrei, R.D.; Popa, M.I.; Fajula, F.; Hulea, V. Heterogeneous oligomerization of ethylene over highly active and stable NiAlSBA-15 mesoporous catalysts. J. Catal. 2015, 323, 76–84. [CrossRef] Catalysts 2020, 10, 1487 14 of 15

36. Kießling, D.; Wendt, G.; Hagenau, K.; Schöllner, R. Dimerization of n-butenes on amorphous NiO-Al2O3/SiO2 catalysts. Appl. Catal. 1991, 71, 69–78. [CrossRef] 37. Brouwer, D.M.; Hogeveen, H. Electrophilic Substitutions at and in Alkylcarbonium Ions. Progr. Phys. Org. Chem. 1972, 9, 179–240. 38. Olah, G.A.; Jeuell, C.L.; Kelly, D.P.; Porter, R.D. Stable carbocations. CXIV Structure of cyclopropylcarbinyl and cyclobutyl cations. J. Am. Chem. Soc. 1972, 94, 146–156. [CrossRef] 39. Liu, H.; Lei, G.D.; Sachtler, W.M.H. isomerization over solid acid catalysts—Effects of one-dimensional micropores. Appl. Catal. A Gen. 1996, 137, 167–177. [CrossRef] 40. Hu, H.; Ke, M.; Zhang, K.; Liu, Q.; Yu, P.; Liu, Y.; Li, C.; Liu, W. Designing ferrierite-based catalysts with improved properties for skeletal isomerization of of n-butene to isobutene. RSC Adv. 2017, 7, 31535–31543. [CrossRef] 41. Kim, Y.T.; Chada, J.P.; Xu, Z.; Pagan-Torres, Y.J.; Rosenfeld, D.C.; Winniford, W.L.; Schmidt, E.; Huber, G. Low-temperature oligomerization of 1-butene with H-ferrierite. J. Catal. 2015, 323, 33–44. [CrossRef] 42. Lallemand, M.; Rusu, O.A.; Dumitriu, E.; Finiels, A.; Fajula, F.; Hulea, V. NiMCM-36 and NiMCM-22 catalysts for the ethylene oligomerization: Effect of zeolite texture and nickel cations/acid sites ratio. Appl. Catal. A Gen. 2008, 338, 37–43. [CrossRef] 43. Bekker, R.; Prinsloo, N.M. Butene Oligomerization over Phosphoric Acid: Structural Characterization of Products. Ind. Eng. Chem. Res. 2009, 48, 10156–10162. [CrossRef] 44. Reschetilowski, W. Einführung in Die Heterogene Katalyse; Springer Spektrum: Berlin/Heidelberg, Germany, 2014; pp. 91–107. 45. Hulea, V.; Fajula, F. Ni-exchanged AlMCM-41—An efficient bifunctional catalyst for ethylene oligomerization. J. Catal. 2004, 225, 213–222. [CrossRef] 46. Misono, M.; Yoneda, Y. The Acid Strength and the Catalytic Activity of Solid Catalysts in the Oligomerization of Propylene. Bull. Chem. Soc. Jap. 1967, 40, 42–49. [CrossRef] 47. Villegas, J.I.; Kumar, N.; Heikkilä, T.; Lehto, V.-P.; Salmi, T.; Murzin, D.Y. A study on the dimerization of 1-butene over Beta zeolite. Top. Catal. 2007, 45, 187–190. [CrossRef] 48. Kresnawahjuesa, O.; Kühl, G.H.; Gorte, R.J.; Quierini, C.A. An Examination of Bronsted Acid Sites in H-[Fe]-ZSM-5 for Oligomerization and Adsorption. J. Catal. 2002, 210, 106–115. [CrossRef]

49. Wendt, G.; Kießling, D. Dimerization of n-butenes on NiO-Al2O3/SiO2 catalysts. Chem. Tech. 1995, 47, 136–143. 50. Katada, N.; Niwa, N. Analysis of Acidic Properties of Zeolitic and Non-Zeolitic Solid Acid Catalysts Using Temperature-Programmed-Desorption of Ammonia. Catal. Surv. Asia 2004, 8, 161–170. [CrossRef] 51. Arena, F.; Dario, R.; Parmaliana, A. A characterization study of the surface acidity of solid catalysts by temperature programmed methods. Appl. Catal. A Gen. 1998, 170, 127–137. [CrossRef] 52. Wu, M.-C.; Truong, C.M.; Goodman, D.W. Interactions of ammonia with a nickel oxide (100) surface studied by high-resolution electron energy loss spectroscopy and temperature programmed spectroscopy. J. Phys. Chem. 1993, 97, 4182–4186. [CrossRef] 53. Vaughan, J.S.; O´Connor, C.T.; Fletcher, J.C.Q. High-Pressure Oligomerization of Propene over Heteropoly Acids. J. Catal. 1994, 147, 441–454. [CrossRef] 54. Li, X.; Han, D.; Wang, H.; Liu, G.; Wang, B.; Li, Z.; Wu, J. Propene oligomerization to high-quality liquid fuels over Ni/HZSM-5. Fuel 2015, 144, 9–14. [CrossRef] 55. Reschetilowski, W. Metal-support interaction in bifunctional metal zeolite catalysts and the HSAB principle. Erdöl Erdgas Kohle 1996, 112, 467–470. 56. Hoang, D.L.; Berndt, H.; Miessner, H.; Schreier, E.; Völter, J.; Lieske, H. Nickel modified H-ZSM-5 catalysts. Appl. Catal. A Gen. 1994, 114, 295–311. [CrossRef] 57. Moussa, S.; Concepión, P.; Arribas, M.A.; Martínez, A. Nature of Active Nickel Sites and Initiation Mechanism for Ethylene Oligomerization on Heterogeneous Ni-Beta Catalysts. ACS Catal. 2018, 8, 3903–3912. [CrossRef] 58. Barret, E.P.; Joyner, L.G.; Halenda, P.H. The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms. J. Am. Chem. Soc. 1951, 73, 373–380. [CrossRef] 59. Rouquérol, J.; Avnir, D.; Fairbridge, C.W.; Everett, D.H.; Haynes, J.H.; Pernicone, N.; Ramsay, J.D.F.; Sing, K.S.W.; Unger, K.K. Recommendations for the characterization of porous solids. Pure Appl. Chem. 1994, 66, 1739–1758. [CrossRef] Catalysts 2020, 10, 1487 15 of 15

60. Beroza, M.; Sarmiento, R. Determination of the Carbon Skeleton and Other Structural Features of Organic Compounds by Gas Chromatography. Anal. Chem. 1963, 35, 1353–1357. [CrossRef] 61. Soják, L.; Addová, G.; Kubinec, R.; Kraus, A.; Boháˇc,A. Capillary gas chromatography—Mass spectrometry of all 93 acyclic octenes and their identification in fluid catalytic cracked gasoline. J. Chrom. A 2004, 1025, 237–253. [CrossRef] 62. Reisener, G.; Schreier, M.; Adler, R. Korrektur reaktionskinetischer Daten des realen Differential-Kreislaufreaktors. Chem. Ing. Techn. 2000, 72, 1192–1195. [CrossRef]

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).