Dimerization of 1-Butene Over Nickel Zeolitic Catalyst - A Search for Liunear Dimers

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Applied Catalysis A: General 110 ( 1994) 3948

appliedcatalysis

Dimerization of 1-butene over nickel zeoliticcatalysts: A search for linear dimers

Paolo Beltrame, Lucia Fork, Antonello Talamini, Giovanni ZurettiDi parti mento di Chimi cajGica ed Elett rochimi ca, Uni vemird di M il ano, Via Golgi 19, I- 20133 M il an, It aly

(Received 30 July 1993, revised manuscript received 20 December 1993)

Abstract

Catalysts based both on nickel and ZSM-5 zeolites have been prepared and used for the dimerizationof 1-butene. The reaction was performed at 40 and 80 bar and 100 to 200C in a fixed-bed flow reactorwith gas-chromatographic (CC) determination of alkenes product fractions characterized by their Catom number. Hydrogenation and GC analysis of the alkane Cs isomers allowed determination of thedegree of branching and of the linear isomers content of the CR fraction. The reactions detected wereisomerization of 1 butene to 2-butenes, dimerization and further oligomerization of butenes. FavouredCs products were in every case several dimethylhexenes; however, under appropriate conditions,considerable fractions of linear octenes were obtained.Key wordst dodecenes; isomerization; Ni-ZSM-5 catalysts; octenes

1 Introduction

Dimerization of lower alkenes is a well known upgrading process, leading to more usefulalkenes, from which alcohols and other products can be obtained. Within the dimer fractions,linear alkenes are of special interest and their formation has been pursued with particularattention. Even considering only heterogeneous catalysis and C,-C4 alkenes, a series ofreports on this subject can be mentioned [ l-201, where fractions of linear dimers as lowas 3% and as high as 65% are reported. Most of the catalysts used are based on nickel or(less often) cobalt, although purely acidic catalysts have been employed at times. Consid-ering that the aim of obtaining linear rather than branched alkenes can be related to thesubject of shape selectivity, it is surprising that only a few attempts at using zeolites of theZSM-5 type have been reported [ 11,131. Both these papers refer to propene as the alkene;fractions of linear dimers around 30% are reported, although it is not clear whether acidic*Corresponding author. Fax. ( + 39-2) 706381290926-860X/94/ 07.00 0 1994 Elsevier Science B.V. All rights reserved.SSDIO926-860X (93)E0263-C


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40 P. Beltrum et al. /Applied Catalysis A: General 110 (1994) 39-M

Table 1Main characteristics of the catalysts employedCatalyst A AS B C DType HNaNi-ZSM-5 HNaNi-ZSM-5 NiO-Alz03/SiOzL Co/PSiOZ/A120a 17.5 79.7 23.1 _molar ratioTreatment None Silylated None None Impregnated

with HMDS; withaq.(25I.1 wt.-% wt.-%)soln.ofSiO, added NH,

&&ace acidity (meq/g)IprY,< -3.71 0.148 0.032 0.053 0.03 1 0.033[P&G 1.51 0.999 0.206 0.347Activation Flowing air up to 500C Flowing Nz at 450Cconditions0.81 wt.-% Ni; formula: H,,2Na,.,Ni,~,x(A10,)9,~(SiO~)~~.~.0.45 wt.-% Ni; formula: (H,Na) ,.45Ni0.45(AlO>) z 35( SiOz)s-c.os.2.8 wt.-% Ni + A1?O1 on amorphous silica; 10.2 wt.-% Co on BDH charcoal Lot no.1527630.zeolites are sufficient to reach this goal [ I I] or exchange with nickel is required [ 131.

It seemed appropriate to investigate the dimerization of butenes, on catalysts associatingthe properties of ZSM-5 zeolites and of nickel. This was the aim of the present work, forwhich catalysts A, AS and B (Table 1) were prepared, A from a high alumina ZSM-5,AS by silylating A with hexamethyldisilazane (HMDS) and B from a low aluminaZSM-5. In particular, the silylation of A to AS was performed hoping to block only externalacid sites and force the dimerization to take place within the pores, possibly with shape-selectivity effects. For the sake of comparison, two well proven catalysts have been prepared(C and D in Table , that is NiO-A120, on amorphous silica [ 68,191 and Co0 on carbon[ 2,3]. Reactions were carried out under pressure (40 or 80 bar), at different temperaturesin the range 100_200C, by feeding pure 1 butene.

2. Experimental2. I. Reactant and catalysts

I-Butene was a commercial product 99 + which has been checked by our analyticalmethod to be 299.9% pure. Zeolites of the ZSM-5 type were prepared according to theusual procedure [ 211. Catalyst A was obtained by exchange of a high alumina zeolitewith ammonium and then nickel acetates: atomic absorption (AA) analysis (for nickel andsodium) gave mainly Na cations, 17% substituted by Ni, 12% by H. Catalyst AS wassilylated as described by Wilshier et al. [ 111, under nitrogen 99.999 + instead of argon.Catalyst B was obtained from a low alumina zeolite, by exchange with ammoniumacetate and then nickel nitrate: nickel cations were determined by AA analysis, but theamount of sodium was too small to be evaluated precisely. All catalysts were used afterbeing pressed in wafers (2 tons/cm2), crushed and sieved to 0.18-0.36 mm (40-80 mesh)


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particle size. Surface acidity was measured by titration with n-butylamine in an anhydroussolvent [ 221, using proper indicators. Details about the catalysts employed are summarisedin Table 1.2.2. Appar at us and procedure

Reactions were carried out in a stainless steel (AISI 316) continuous flow reactor (6mm I.D.; 250 mm length), having an axial thermowell, so that the available volume was5.3 cm3. The reactor was fed by an ISCO pLC-500 micro flow pump (feed rate [Fl in therange 0.009-0.032 mol/h) through a preheater. Both preheater and reactor were kept in anelectric furnace, with temperature controlled to k 1C. Only a small central portion of thereactor was actually containing the catalyst (usual mass [WI 0.5 g and never more than 1g) : the rest of the volume was filled with inert quartz powder of the same particle size.

Zeolitic catalysts were activated in flowing air at temperatures from 250 to 500C. Whenrequired, they were regenerated in flowing air at 500C. Catalysts C and D were activatedin flowing nitrogen at 450C. The reactor effluent passed through a condensing trap, keptat room temperature, from which gases went through the sampling valve of an in-line gaschromatograph followed by a volumetric gas meter, while liquids were collected (usuallyfor 1 h), weighed and later subjected to the same analysis. This was done on a PONAcapillary column (50 m long, 0.2 mm I.D., 0.5 pm film), operated at temperatures from 10to 200C. A few peaks were identified by comparison with pure samples (isobutane, l-butene, n-butane, 2-butenes) ; other peaks were classified, according to the mass of thecorresponding components, by gas chromatography-mass spectrometry (CC-MS) deter-minations, as C,, Cg, C ,,.. ., C,*, C,,, C,,, alkenes; these fractions had well defined rangesof retention time values. By comparison with a pure sample of isobutene, it was ascertainedthat this compound had a retention time almost identical to that of 1-butene and was notresolved from the latter; however, the formation of isobutene by skeletal isomerization of1 butene could be ruled out (see below).

GC analyses allowed the determination of the overall mass of every component in theeffluent. The mass balance was usually satisfied to > 99.7%. Conversion and selectivitieswere computed on a molar basis. They could be related either to 1 butene or to total butenes;in the former case, 2-butenes were taken as reaction products and the process was actuallythe sum of isomerization and oligomerization; in the latter case, all butenes were consideredas unreacted reagents, so only oligomerization (both of the initial I-butene and of the 2-butenes formed from it) was taken into account.

Samples of liquid alkene products were hydrogenated under pressure (20 bar) over 5%Pd on carbon, obtaining a smaller number of species: the GC peaks corresponding to theC8 fraction were systematically identified, mostly by comparison with pure samples ofoctane isomers and all of them by GC-MS: this allowed the determination of the linearoctene fraction and of the average degree of branching of the CR fraction [ 231 through abranching index defined as BI = [Me,pentanes fraction] x 3 + [ Me,hexanes frac-tion] X 2 + [Me-heptanes fraction]. The hydrogenation procedure was checked on a fewpure octene isomers, among which I-octene, and found to be reliable, without skeletalisomerization. An example of hydrogenated C, isomer determination is given in Table 2.


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42 P. Beltrame et al. /Applied Catalysis General 110 (1994) 39-48Table 2Hydrogenation of the products of run B-l: GC analysis in the C8 regionr.t. (min) Corresponding octane isomer Sample L3 Sample L5ca. 23.8 2Jdimethylhexane 0.93% 0.94%25.0-25.2 2,4-dimethylhexane 8.32% 8.36%25.7-25.8 3,3-dimethylhexane 4.25% 4.27%ca. 26.6 2,3,4-trimethylpentane 0.65% 0.62%27.9-28.0 2,3_dimethylhexane 11.11% 11.23%28.0-28.1 probably 3-ethyl-2- 2.47% 2.39%

methylpentane28.9-29.1 3,4_dimethylhexane 60.08% 60.06%29.4-29.5 3-methylheptane 10.05% 10.05%32.3-32.5 n-octane 2.14% 2.08%

branching index BI = 1.89 1.89cat.B (501 mg); T= 180C; P=40 bar; W/F=28 g h/mol; at TOS 4248 h, on the average, with reference tototal butenes: conversion 49%. C, selectivity 82%.

Gas analysis and liquid sampling were performed at time-on-stream (TOS) values of 18,21, 24, 42, 45, and 48 h. This allowed the determination of the degree of stability of thecatalyst, by comparison of its behaviour on the first and the second day on-stream. Usuallyvalues of fractional conversion and selectivities on the second day on-stream were fairlyconstant and were averaged.

An example of the detailed analytical results on a given sample is shown in Table 3.Table 3Example of GC analysis during a run: sample at TOS 48 h of run A-12 (gas mass 125.46 mg/h; liquid mass874.30 mg/h; total mass fed 999.77 mg/h)Fraction Found

(mg/h) (mmol/h)Reference: I-ButeneFract. conv. Select.

Reference: Total butenesFract. conv. Select.

i-Butane 1.126 0.019n-Butane 2.498 0.043I-Butene 32.119 0.5722-Butenes 296.853 5.291Total butenes 328.972 5.863CS 0.443 0.006C, 1.118 0.013C, 1.441 0.015C, 545.768 4.864Cg 2.1.5s 0.017Cl0 0.448 0.003C,, 0.000 0.000C12 115.486 0.686C,, 0.000 0.000C 14f 0.303 0.002

0.12%0.26%

96.79%30.68%

0.05%0.12%0.15%

56.40%0.22%0.05%0.00%

11.93%0.00%0.03%

0.17%0.37%

67.10%0.07%0.17%0.21%

81.36%0.32%0.07%0.00%

17.22%0.00%0.05%

cat.A (500 mg); 7= 140C; P= 80 bar; W/F=28 g h/mol; at TOS 4248 h, on the average, with reference tototal butenes: conversion 67%. Cx selectivity 8 1 .Sum of gas and liquid analyses.


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3 Results and discussion

Dimerization catalysts are also active for the isomerization of butenes. From a thermo-dynamic point of view, isomerization of 1-butene by double bond migration is favoured,since equilibrium constants evaluated for the ideal gas state [24] for the reactions l-butene F1 cis-2-butene and 1-butene F? trans-2-butene correspond to 2-butenesl l-buteneequilibrium ratios of 18.3 at 370 K, 12.1 at 420 K, and 8.9 at 470 K, i.e. values well aboveunity for the temperature range of our runs. During the experimental runs, the actual ratiosoften approached the equilibrium value (in a few cases almost reaching it), showing thatthe isomerization activity of the catalysts was usually predominant over the dimerizationactivity. This justifies the current use of total butenes as reference for conversion andselectivities.

Reference to total butenes would be correct even in the case of a partial skeletalisomerization of 1 butene to isobutene. However, in a run carried out by feeding isobuteneinstead of I-butene on one of our catalysts (cat. A, T= 18OC, P= 80 bar, W/F= 28 g hlmol) it was found that some of the most abundant peaks in the CC analysis referred toproducts different from those commonly found when feeding I-butene. Therefore, anyisomerization of 1-butene to isobutene, followed by dimerization of the latter, would havebeen easily detected.

Catalysts were used both as fresh samples and after regeneration, A comparison of theresults of identical runs, on fresh and on regenerated catalysts, showed very similar levelsof conversion and selectivity. Only a slight increase of the yield of C8 alkenes was noticedafter regeneration of the catalyst or when comparing e.g. a catalyst after its fourth regen-eration with a sample after the first regeneration.

Few tests were directed at evaluating the effect of changing flow conditions. Two runson catalyst A, at W/F= 14 g h/mol, T= 180C P = 40 bar, were carried out with differentF-values (0.0178 mol/h for run A-4; 0.03 18 mol/h for run A-6) : results were similar, sothat external diffusion effects may be considered negligible. Taking into account the valuesof the critical temperature and pressure of I-butene and 2-butenes, and the low volatility ofreaction products, the reaction mixtures of most of our runs can be considered to besupercritical fluids at the reactor entrance and liquids down the reactor. In the analogouscase of a previous research on the dimerization of I-butene over a nickel-exchanged Xzeolite [ 251, evidence of a diffusion-controlled mechanism had been found.A change of time factor, of course, has a marked effect, as shown in Table 4. At increasingvalues of W/F, conversion increases and so does stability of the catalyst (smaller differencesbetween first and second day on-stream), while dimerization selectivity decreases and thebranching index of the C8 product fraction rises from 1.95 to 1.99. This shows that longertime factors favour both trimerization of butenes and formation of higher-branched dimersby isomerization.

Most of the runs were carried out at a constant value of W/F (28 g h/mol), in order tocompare different catalysts at different temperature and pressure values. When catalysts A,AS and B have been used at 180C in runs at 40 and/or at 80 bar, they proved roughlysimilar as to conversion, stability, and C8 selectivity, although differences indicate that ASis a less active catalyst, as shown in Table 5. It can be noticed that yields are higher at thehigher pressure.


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Table 4Runs on catalyst A (T= 180C; P= 40 bar; F= 17.8 mmol/h), at increasing WIFvalues; reference for conversionand selectivities: total butenesW/F (g h/mol) 14 28 56Run no.Fractional conversion (I = 24 h)At TOS 4248 h:

Fractional conversionSelectivity to C, alkenesSelectivity to C1* alkenes

Within the C, hydrogenatedfraction:n-Octane3-Me-heptaneMe,-hexanesBI

A-3 A-2 A-l0.3 1 o.s4 0.820.14 0.49 0.780.82 0.77 0.580.14 0.18 0.28

0.48% 1.08% 0.38%8.50% 5.65% 4.58%

86.29% 88.23% 90.72%1.95 1.97 I 99

Branching index defined as [Me,pentanes fraction] X 3 + [Me,hexanes fraction1 X 2 + [Me-heptanes fraction].At a pressure of 80 bar, comparison runs were also carried out at lower temperatures: at

140C yields of C, alkenes [ Y] at TOS = 4248 h were in the following order:cat. A (run A- 12) (fract. conv. = 0.67; Y= 0.54) > cat. B (run B-7) (fract. conv. = 0.5 1;Y= 0.46) > cat. AS (run AS-4) (fract. conv. = 0.28; Y= 0.24)while at 120C a comparison limited to catalysts A and B gave:

cat. A (run A- 13) ( fract conv. = 0.62; Y = 0.48) > cat. B (run B-8) ( fract. conv. = 0.28;Y= 0.24)differences in yield being due mainly to differences in conversion. Remarkably, the activityof the catalysts is lowered in a different way when the temperature drops.Table 5Comparison of catalysts in standard runs (T= 180C; Wl F= 28 g h/mol); reference: total butenes; average valuesat TOS 4248 h

P=40 barRun no. A-2 B-lCatalyst sample fresh freshFractional conversion 0.49 0.49Selectivity to C, alkenes 0.77 0.82Selectivity to C,? alkenes 0.18 0.12Yield of CR alkenes 0.38 0.40

P=80 barRun no.Catalyst sampleFractional conversionSelectivity to Cs alkenesSelectivity to C,, alkenesYield of C, alkenesRegenerated catalyst,

A-11 AS-5 B-5 B-9fresh regen. fresh regen.0.81 0.5 1 0.65 0.730.76 0.87 0.89 0.830.20 0.11 0.10 0.140.61 0.44 0.58 0.60


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Analyses of the branching of octene isomers gave the results shown in Table 6. Thevalues of the Branching Index (BI), not specified in the table, were in most cases large(> 1.87); the lowestvalues were: 1.85 (A-13); 1.84 (B-9); 1.75 (B-6); 1.66 (B-7); 1.49(AS-4) ; 0.77 (B-S). When on a given catalyst different temperatures were tested, a regularincrease of the linear C, alkene fraction has been recorded at decreasing values of r, since,on the other hand, conversion was obviously lower at lower temperature, the yield of n-octenes did not increase to the same extent. Linear octenes were more abundant, in general,in runs at 80 than at 40 bar; therefore the most convenient condition for lineardimers provedto be high pressure and a relatively low temperature. However, a limit to lowering thereaction temperature is met, when considering the catalyst stability. By defining a StabilityIndex (SI) as fract. conv. ( t = 42-48 h) / fract. conv. (t = 24 h) , it has been found that, ina series of runs under analogous conditions, for each catalyst there is a range of temperatureswhere SI is close to 90-lOO%, but a temperature exists, below which SI falls to low values(Fig. 1).

It seems appropriate to compare catalysts A, AS and B in the region where they are allstable, as has been done in Table 5 (runs at 180C). Looking in detail at the results of runsunder a pressure of 80 bar, the activity order appears to be A > B > AS. This is also theTable 6Production of linear isomers of CR alkenes (W/F= 28 g h/mol unless otherwise indicated); reference: totalbutenes; values on the second day on streamCatalyst T

(C)P=40 bar P=80 barRun no. n-Octane Y(n-C,) Run no. n-Octane Y( n-C*)

A 120 A-13 3.6% 1.74%140 A-12C 3.0% 1.62%180 A-2d 1.1% 0.41% A-l Id 2.4% 1.48%200 A-l 0.5% 0.24%

AS 140 AS-4 7.8% 1.83%160 AS-3 0.9% 0.34%180 AS-5 0.5% 0.23%

B 120 B-8 36.6% 8.90%140 B-7C 5.0% 2.31%160 B-6 2.5% 1.18%180 B-ld 2.1% 0.84% B-5d 2.2% 1.29%180 B-4 2.1% 0.97% B-9 3.0% 1.81%

C 100 C-2d 23.6% 5.99%180 C-l 4.1% 2.14%

D 180 D-l 23.8% 2.83%n-Octane within the C, hydrogenated fraction,Yield of n-octenes, calculated from the yield of CI, alkenes and the fraction of linear isomers.Regenerated catalyst.dFresh catalyst sample.W/F=56 g h/mol.


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order of surface acid titer of the catalysts (Table l), when measured both for strong acidsites (pK,< - 3.7) and for medium-to-high strength acid sites (pK, < 1.5). The surfaceacid titer, as determined by the aluminium content of the zeolite, seems to control the greateractivity of catalyst A with respect to B. As to catalyst AS, silylated with hexamethyldisi-lazane, it can be argued that, besides restricting the pore mouth width, so reducing thecatalyst activity by hindering the access of reactant to the inner sites, the HMDS moleculecould have penetrated, at least in part, inside the pores, so reducing the tetrahedral aluminiumcontent and therefore the overall surface acidity of the zeolite (see data in Table 1).

For the same runs at 80 bar (Table 5) one can appreciate that the overall selectivity toC, and C,* alkenes falls in the range 0.96-0.99; within these overall values, the higheractivity of catalyst A corresponds to higher Cl2 and lower Cx selectivity, as expected forconsecutive reactions. The low-Al catalyst B presents a higher level of C, selectivity, bothat 80 and at 40 bar, with a small loss of activity (80 bar) or no loss at all (40 bar). Fromthis point of view, the silylation of A does not prove really effective, since it gives to catalystAS about the same CX selectivity of B, but at the expense of a lower activity.

Standard runs at 18OC, although useful for the above comparison of activities andselectivities, were not interesting for the production of linear dimers (Table 6) and thissuggested to perform runs at lower temperatures, with better results in terms of lower degreeof branching of the dimers, as already discussed. To compare our catalysts, on this point,with some whose behaviour had already been published, tests have been done on catalystsC (NiO-Al,O,/SiO,) [ 191 and D (Coo/C) [ 31 and results are reported in Table 6. Valuesof the Branching Index BI were found to be: 1.79 (C- 1) ; 0.99 (C-2) ; 0.95 (D- 1) . WhiIethe behaviour of catalysts C and D, under conditions similar to those indicated by therespective authors, is confirmed to be good in terms of linear dimers, it can be noticed thatcatalyst B, at 120C and 80 bar, seems even better. Unfortunately, this good behaviour isonly observed under conditions where the conversion values are low, a fact which reflectsin low values of the yields of n-octenes, in the range from 5 to 10% in the best cases. Fig.2 shows (for catalyst B at constant time factor) how lower temperatures cause lower overall

100 110 120 130 140 150 160 170 1Temperature PC)Fig. 1. Stability Index [SI=fract. cow. (t=42-48 h)/fract. cow. t=24 h) ] of catalysts W), AS 0) andB ( + ) as a function of temperature (runs at 80 bar; W/F= 28 g hlmol).


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0.6 --

120 130 140 150 160 170 ITemperature Z

Fig. 2. Yields to total ( W) and linear (0) C, alkenes in runs on catalyst B at 80 bar (W/F= 28 g h/mol)Cs yields, but higher linear Cs yields than those observed in runs at 180C. Catalyst AS,similar to B as regards the surface acidity, could not be tested at temperatures less than14OC, owing to its instability (Fig. 1).

Run B-8, the best case, gave an interesting result as to the molar ratio 2-butenes/ 1-butenein the effluent: such a ratio was around 2 on the first day on-stream and around 0.3 on thesecond day, against an equilibrium value of 15 at 390 K. So, high selectivity to linear dimersis associated with low conversion in the butene isomers equilibration. The latter observationsuggests that a prerequisite for obtaining linear dimers is that 1-butene must not isomerizeto 2-butenes. Under conditions that limit the carbenium ion mechanism and favour thecoordinative mechanism of dimerization on nickel [ 191, I-butene is likely to give lessbranched dimers and the latter are more able to avoid further isomerization.

The typical shape selectivity of ZSM-5 zeolites does not seem to play an important rolein this case, since the blocking of external acid sites (silylated catalyst AS), which shouldhave enhanced any shape-selectivity effect, did not give particularly good results. Thereaction to linear (or at least to less branched) dimers seems to depend on a correct balanceof the dimerization and isomerization activities of a catalyst that combines metal (Ni) andacid centres.

4. ConclusionsI-Butene has been contacted with some catalysts based on nickel and ZSM-5 zeolite,

finding good levels of conversion. Two main processes compete: isomerization to 2-butenesand dimerization to octene isomers; in addition, 2-butenes are also able to dimerize. Otherreactions are present, in particular the formation of trimers (Cr 2 alkenes) .

From the point of view of the yields of C, alkenes, catalysts of ZSM-5 exchanged withnickel and not silylated (A, B) proved to be more active than a silylated sample (AS). Asto the yields of linear octenes, the best results were obtained in runs at 80 (rather than 40)bar, and at relatively low temperature ( 120-140C were required in order to obtain 2 3%


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of linear isomers in the C, alkene fraction), the best result (ca. 37% of linear isomers) wasobtained with catalyst B, based on a low-alumina ZSM-5 zeolite.

AcknowledgementsWe acknowledge Praoil SpA for GC-MS identification of the alkene fractions of different

C number in the reaction product. Thanks are due to Dr. Casalini for GC-MS identificationof the C, alkane isomers from the hydrogenation of the product.

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Dimerization of 1-Butene Over Nickel Zeolitic Catalyst - A Search for Liunear Dimers