Selective hydrogenation of amides using bimetallic Ru/Re and Rh/Re catalysts

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Selective hydrogenation of amides using bimetallic Ru/Re and Rh/Re catalysts
  Selective hydrogenation of amides using bimetallic Ru/Re and Rh/Re catalysts Graham Beamson b , Adam J. Papworth c , Charles Philipps a , Andrew M. Smith a , Robin Whyman a, ⇑ a Department of Chemistry, Donnan and Robert Robinson Laboratories, University of Liverpool, Liverpool L69 7ZD, UK  b National Centre for Electron Spectroscopy and Surface Analysis, STFC Daresbury Laboratory, Warrington, Cheshire WA4 4AD, UK  c Materials Science and Engineering, Department of Engineering, University of Liverpool, Liverpool L69 3GH, UK  a r t i c l e i n f o  Article history: Received 2 November 2010Revised 14 December 2010Accepted 15 December 2010Available online 26 January 2011 Keywords: Heterogeneous catalysisRutheniumRheniumRhodiumBimetallicHydrogenationAmidePrimary amineCharacterizationReaction mechanism a b s t r a c t Heterogeneous Ru/Re and Rh/Re catalysts, formed in situ from Ru 3 (CO) 12 /Re 2 (CO) 10  and Rh 6 (CO) 16 /Re 2 (CO) 10  respectively, are effective for the liquid phase hydrogenation of cyclohexanecarboxamide(CyCONH 2 )toCyCH 2 NH 2  inupto95%selectivitywithouttherequirementforammoniatoinhibitsecond-ary and tertiary amine formation. Good amide conversions are noted within the reaction conditionregimes 50–100bar H 2  and P 150 (Rh) – 160  C (Ru). Variations in Ru:Re and Rh:Re composition resultin only minor changes in product selectivity with no evidence of catalyst deactivation at higher levelsofRe. InsituHP-FTIRspectroscopyhasshownthatcatalystgenesisoccursviadecompositionofthemetalcarbonyl precursors. Ex situ characterization, using XRD, XPS and EDX-STEM, has provided evidence forthe active components of these catalysts containing bimetallic Ru/Re and Rh/Re nanoclusters, the sur-faces of which become significantly oxidized after use in amide reduction. Potential mechanistic path-ways for amide hydrogenation are discussed, including initial dehydration to nitrile, a pathwaypotentially specifically accessible to primary amides, and evidence for often postulated imineintermediates.   2010 Elsevier Inc. All rights reserved. 1. Introduction Thegeneration,characterizationanduseofbimetallicheteroge-neous Rh/Mo and Ru/Mo catalysts for the selective hydrogenationofamidestoaminesintheliquidphasehasrecentlybeendescribed[1,2].Keyfeaturesofthisworkincludebothhighcatalystselectivityfor the reduction of primary amides to the respective primaryamines, without the necessity for the addition of ammonia and/oramines to inhibit side reactions leading to secondary and tertiaryamine by-products, and considerably milder reaction conditionsrelativetothoserequiredbystandardfirstgenerationcopperchro-mite catalysts [3]. Catalyst performance was shown to be cruciallydependent on Mo:Rh and Mo:Ru composition, respective ratios of >2 and >1 leading to either significant, or complete, inhibition of catalysis.Rhenium-based catalysts have received greater attention thanRh, Ru or Mo for the reduction of ‘difficult’ functional groups suchas amides, and particularly carboxylic acids and esters. Broadbentet al. [4] described the use of Re 2 O 7  as a catalyst precursor forthe hydrogenation of carboxylic acids under the severe reactionconditions (312bar H 2 , 217  C, 6h) typically required by copperchromite catalysts. A step change was described in 1990 byYoshino et al. [5] with a report of the use of promoted bimetallicRe/Os catalysts under considerably milder reaction conditions(25–100bar H 2 , 100–120  C, 6h), withhighest alcohol selectivitiesbeing favoured at the highest pressure. A subsequent very recentdevelopment is the use of titania-supported Pt (and Pt/Re) cata-lysts with optimum reaction conditions at 20bar H 2  and 130  C,although very low reaction rates at as low as 5bar H 2  and 60  Care also quoted [6]. The first account of the use of Re in amidereductionwas also providedby Broadbent et al. [7], whodescribedthe use of Re(VI) oxide for the selective hydrogenation of benzam-ide to benzylamine (205bar H 2 , 220  C, 49h, ethanol solvent) in69% yield, with toluene as the only by-product. Surprisingly, theformationof neither cyclohexylamine nor N-ethyl-substitutedsec-ondary or tertiary amine derivatives were reported [cf. Ref. [2]].Subsequently, a BP patent claimed the use of a Pd/Re/high surfacearea graphite/zeolite 4A combination, dispersed in a solvent suchas 1,4-dioxane, for the reduction of amides at 130bar H 2  and200  C [8]. Using propionamide as substrate, the product distribu-tion comprised a mixture of primary, secondary and tertiaryamines. Fuchikami et al. [9] briefly described the behaviour of Rh/Re catalysts for the reduction of a range of amides. In the oneexample of a primary amide tested,  n -hexanamide required theco-addition of an amine (diethylamine) to induce good selectivityto  n -hexylamine under reaction conditions of 100bar H 2  and180  C. Otherwise di( n -hexyl)amine comprised the predominant 0021-9517/$ - see front matter    2010 Elsevier Inc. All rights reserved.doi:10.1016/j.jcat.2010.12.009 ⇑ Corresponding author. Fax: +44 (0) 151 794 3588. E-mail address: (R. Whyman). Journal of Catalysis 278 (2011) 228–238 Contents lists available at ScienceDirect  Journal of Catalysis journal homepage:  product, in complete contrast to our recent findings with Rh/Moand Ru/Mo catalysts using cyclohexanecarboxamide (CyCONH 2 )as substrate [1,2]. Here, the genesis and performance of both Ru/Re and Rh/Re catalysts in the hydrogenation of CyCONH 2  (and  N  -acetylpiperidine) are reported, together with ex situ characteriza-tion using microanalysis, XRD, XPS and EDX-STEM. 2. Materials and methods With the exception of the additional details described below,experimental and analytical procedures are as outlined in Refs.[1,2].  2.1. Reagents Re 2 (CO) 10  (98%) was purchased from Strem Chemicals and theXPS standard, Re foil (0.025mm thickness, 99.98%), from AldrichChemicals.  2.2. Catalytic procedures Atypical‘single-pot’batchRu/Recatalystpreparation,andeval-uation in CyCONH 2  reduction, was carried out as follows, for anominal Re:Ru atomic composition of 0.25. [Ru 3 (CO) 12 ] (22mg,0.103mmol Ru), [Re 2 (CO) 10 ] (8.4mg, 0.026mmol Re) and cycloh-exanecarboxamide (0.235g, 1.85mmol) contained in a glass linerwere dissolved in 1,2-dimethoxyethane (DME) (30mL) and  n -oc-tane (0.100g) added as an internal standard for GC analysis. Theliner was placed in a ca. 300-mL capacity pressure vessel and thereaction mixture, under agitation, purged three times with N 2  (at5bar), and three times with H 2  (5bar). The autoclave was thenpressurized to 100bar H 2  and heated to 160  C for 16h. A verydark coloured liquid was initially recovered after cooling andventing and from this a black residue slowly settled, leaving a col-ourless solution. The residue was separated by centrifugation(2000rpm, 20min) and the supernatant liquid containing reactionproducts removed. After several washings with DME (10mL), thecatalyst residue (ca 15mg) was dried and the resultant fine blackpowdereitherrecycledusingthesamequantitiesoffreshsubstrateand solvent, or characterized using the techniques described be-low. Product solutions were analysed by GC as previously de-scribed [1].Inadditionto‘batch’ experiments,‘parallel’ catalystevaluationswere also conducted. A typical ‘parallel’ catalyst evaluation wascarried out using the standard autoclave used for batchruns, mod-ified to accommodate four vertically mounted modified test tubesofca.20mLcapacity,allcontainingmini-magneticfollowerstoen-sure effective stirring. Using a 10mL volume of DME solvent, thescale of substrate and catalyst precursors (4mol% Ru) were re-duced accordingly.  2.3. Ex situ characterization 2.3.1. EDX-STEM  Sample quantification was determined using the most intense‘pure’ Ru K 1 a  (19.279keV) and Re L 1 a  (8.654keV) lines thatare largely free from interference by signals from other elements.Even so, both trace Oand Si were also detected during the analysisand deconvolution of the Si K and Re L lines became necessary foraccurate quantification. Also, since Ru K and Re L lines are undercomparison, the quoted values are subject to an error of ±5%.  2.3.2. Microanalyses All elemental analyses were performed by Mr. S.G. Apter,Departmentof Chemistry,Universityof Liverpool.Transitionmetalconcentrations were determined using a Spectro Ciros CCD Induc-tivelyCoupledPlasma(ICP)sourcelinkedtoAtomicEmissionSpec-troscopy (AES). Sample digestion was accomplishedusing a matrixcombination containing aqua regia (2eq. HCl: 1eq. HNO 3 ) (5mL)and HF (0.5mL), followed by microwave treatment in a CEMMARS5 oven using the following programme conditions; heatingto 220  C at 20  Cmin  1 , holding at 220  C for 20min followedby a cooling period of 20min. The resultant solutions were dilutedto 100mL in distilled water and referenced against Ru, Rh and Restandards made up in similar HCl/HNO 3 /HF matrix solutions.Application of this procedure overcame the well-known resistanceof Ru in particular to digestion [10] (see Supplementary material S2 for further details).Standard C, H and N determinations were carried out by com-bustion analysis using either a Flash EA-1112 or a Model 1106Carlo Erba thermal elemental analyzer at  2000  C. For the bime-tallic catalyst samples, a V 2 O 5  catalyst was required to aidcombustion. 3. Results and discussion  3.1. N-acetylpiperidine hydrogenation using Ru/Re catalysts A hydrogenation catalyst derived fromRu 3 (CO) 12  and Re 2 (CO) 10 (Re:Ru=1.0, 1mol% Ru) was reported by Fuchikami et al. [9] togive high conversions of   N  -acetylpiperidine into  N  -ethylpiperidine(96% yield) under standard reaction conditions of 100bar H 2  and160  C.Rualonewasessentiallyinactive(1%conversionandamineyield) and Re showed low activity (14% conversion) with only 7%yield of the desired amine. Repetition of this work using Re 2 (CO) 10 has confirmed reproducibility (16% conversion, 53% amine selec-tivity) and revealed that competing C A N bond hydrogenolysis of  N  -acetylpiperidine (to ethanol and piperidine) accounts for theadditional products. No attempts to use Ru/Re catalysts for thereductionof primaryamidessuchas cyclohexanecarboxamide, Cy-CONH 2 , have been reported and this has providedthe focus for ourwork.  3.2. CyCONH   2  hydrogenation using Ru/Re catalysts 3.2.1. Variation of Re:Ru composition Using a series of parallel experiments (10mL scale and 4mol%catalyst, based on Ru), the results of variation in Re:Ru ratio areshown in Fig. 1. From this, it is evident that the addition of onlya small concentration of Re 2 (CO) 10  to Ru 3 (CO) 12 , comparable tothat observed with Ru/Mo (and Rh/Mo) catalysts, is necessary toinitiate analogous overall conversion/product selectivity synergy.In contrast however there is no evidence of significant reductionin conversion with increasing Re content, for up to Re:Ru=1.8.Moreover, product selectivity remains essentially constant be-tween Re:Ru values of 0.25 and 1.1, with uniformly trace amountsof (CyCH 2 ) 2 NH present across the range Re:Ru=0.3–1.8, and onlyat Re:Ru=1.8 are slight divergences of product distribution in fa-vour of CyCH 2 OH apparent. A Re:Ru compositionof ca. 0.3 appearsoptimum for a combination of conversion and selective formationof CyCH 2 NH 2  (both >90%). The conversion and selectivity recordedfor the Re:Ru=0.34 catalyst in Fig. 1 are those obtained from thefinal reaction solution from the in situ HP-FTIR experiment usingthe complete CyCONH 2  reduction system, described in Section3.6.1, thus confirming internal consistency between the two setsof experimental data.Furthermore, for the purposes of checking reproducibility be-tween ‘parallel’ and ‘batch’ processing, three Ru/Re catalysts wereexamined inbatchexperiments, using5mol%Ru. The resultssum-marizedinTable1provideconfirmation, withessentiallythesamereaction product profile and only a marginal improvement in G. Beamson et al./Journal of Catalysis 278 (2011) 228–238  229  selectivity towards CyCH 2 NH 2 . No secondary amine production isevident with Re:Ru=0.34 and 0.54 catalysts (Entries 2 and 3)and only traces at Re:Ru=0.15 (Entry 1), in which theCyCH 2 NH 2 /CyCH 2 OH product distribution is a close parallel withthat observed with Ru/Mo catalysts of equivalent composition [2].  3.2.2. Variation of total catalyst concentration A preformed Ru/Re (Re:Ru=0.25) catalyst previously preparedin the absence of the amide substrate was used, in batch experi-ments, to establish a conversion/selectivity profile for CyCONH 2 reduction as a function of total catalyst concentration. The results(Table 2 and Supplementarymaterial Fig. S1) showthat CyCH 2 NH 2 selectivity (93–96%) is essentially unaffected and that amide con-version displays an approximately first-order dependence on cata-lyst concentration. Hence, an apparent discrepancy between theincomplete amide conversions evident throughout the ‘parallel’experiments shown in Fig. 1, and the 100% conversions noted inTable 1, may be rationalized in terms of the slightly higher (5 vs.4mol%) Ru catalyst concentration used in the batch experiments.  3.2.3. Variation of pressure and temperature The effects of variation in pressure and temperature on Cy-CONH 2  hydrogenation during batch experiments are summarizedinTable3. Fromthisdata, itisclearthatthestandardreactioncon-ditions of 100bar H 2  and 160  C are necessary for highestCyCH 2 NH 2  selectivity, which undergoes a significant reduction at50bar H 2 , notwithstanding essentially complete CyCONH 2  conver-sion (cf. Entries 1 and 2). Reduction in reaction pressure to 20barH 2  (Entry 3) results in much lower amide conversion, a furtherslight reduction in selectivity towards CyCH 2 NH 2 , and increased(CyCH 2 ) 2 NH production. Furthermore, reaction temperatures be-low 160  C are also clearly deleterious to catalytic performance,with 20% conversion at 150  C (Entry 4) and zero activity evidentat 140  C (Entry 5). This behaviour contrasts markedly with thelimiting parameters of 20bar H 2 /160  C and/or 100bar H 2 /130  Cavailable to Ru/Mo catalysts [2]. Any potential explanation interms of decreased availability of H 2  in DME solution at the lowerpressures using Ru/Re catalysts seems therefore to be unlikely (forfurther information on H 2  solubility in DME, see Supplementarymaterial S1, Table S1 and Figs. S2 and S3).  3.3. N-acetylpiperidine hydrogenation using Rh/Re catalysts In a preliminary single-pot reaction using  N  -acetylpiperidine assubstrate, a Rh/Re catalyst (Re:Rh=1.0, 1mol% Rh) derived fromRh 6 (CO) 16  and Re 2 (CO) 10  gave >80% conversion to  N  -ethylpiperi-dine in >85% selectivity, using the standard reaction conditions of 100bar H 2 , 160  C and 16h, in confirmationof the report of Fuchi-kami et al. [9]. As found with the Rh/Mo [1] and Ru/Mo [2] cata- lysts, the solid material generated during this single-pot reactioncould be recovered most easily from the product solution by cen-trifugation, repeated washing with DME solvent, and subsequentdrying. Following re-charging with  N  -acetylpiperidine after thistreatment catalyst recycle could be conveniently monitored usingHP-FTIR spectroscopy, by following the decay of the amide car-bonylabsorptionbandat1658cm –1 asafunctionoftime(seeSup-plementary material, Fig. S4), from which it is clear that initiation of hydrogenation occurs without an induction period on reachingthe operational temperature of 160  C (at time=0min).  3.4. CyCONH   2  hydrogenation using Rh/Re catalysts In Fuchikami’s work [9], the one experiment reporting the useof a primary amide ( n -hexanamide), at a Re:Rh composition of 2,required the addition of an amine (diethylamine) to induce goodselectivity to the primary amine. This contrasts with our more re-cent results using Rh/Mo, Ru/Mo and Ru/Re catalysts prepared in asimilar manner, in which the addition of neither ammonia noramine is required for high primary amine selectivity. Our standardsubstrate CyCONH 2  was therefore used to determine whether Rh/Re catalysts do indeed exhibit fundamentally different behaviourtowards the reduction of primary amides.  3.4.1. Variation of Re:Rh composition ResultsofvariationinRe:RhcompositiononCyCONH 2  hydroge-nationare shownin Fig. 2, fromwhichit is evident that in contrasttoRu/Recatalysts(alsoRh/MoandRu/Mo),thesequentialaddition Fig. 1.  CyCONH 2  hydrogenation: (a) overall conversion and (b) product selectivityvs.Re:Rucomposition(100barH 2 ,160  C,16h). Key: j Conversion, } CyCH 2 NH 2 , +(CyCH 2 ) 2 NH, 4 CyCH 2 OH.  Table 1 CyCONH 2  hydrogenation. Dependence of conversion and product distribution onRe:Ru composition in batch experiments. Entry Re:Ru Conversion (%) Product selectivity (%)CyCH 2 NH 2  CyCH 2 OH (CyCH 2 ) 2 NH1 0.15 100 86 12 Trace2 0.34 100 92 7 03 0.54 100 93 6 0Reaction conditions: 100bar H 2 , 160  C, 16h, 5mol% Ru.  Table 2 CyCONH 2  hydrogenation. Dependence of conversion and product distribution oncatalyst concentration. Catalyst concentration(mol%)Conversion(%)Product selectivity (%)CyCH 2 NH 2  CyCH 2 OH (CyCH 2 ) 2 NH4.0 90 95 3 22.2 50 93 6 11.2 32 96 3 <1Reaction conditions: 100bar H 2 , 160  C, 16h, Re:Ru=0.25 catalyst.  Table 3 CyCONH 2  hydrogenation: conversion and product selectivity vs. pressure andtemperature. Entry  T  (  C) P  H2 (bar)Conversion(%)Product selectivity (%)CyCH 2 NH 2  CyCH 2 OH (CyCH 2 ) 2 NH1 160 100 95 91 8 12 160 50 98 81 16 33 160 20 60 77 13 94 150 100 20 80 17 25 140 100 0 – – –Reaction time:16h, Re:Ru=0.25 catalyst.230  G. Beamson et al./Journal of Catalysis 278 (2011) 228–238  of Re to Rh only leads to a gradual increase in conversion over theRe:Rh range 0–0.67, and quite possibly beyond. Consequently, amore substantial concentration of Re (ca. Re:Rh=0.7) is requiredfor 100% CyCONH 2  conversion. Nevertheless, CyCH 2 NH 2  is stillthe major product throughout, formed in 90% selectivity using aRe:Rh=0.80 catalyst composition. These results provide a distinctcontrast with that of Fuchikami et al. [9] and confirm our belief that high primary amine selectivities comprise an intrinsic charac-teristic of all the bimetallic systems investigated (Ru/Mo, Ru/Re,Rh/Mo and Rh/Re), at least when using CyCONH 2  as a primaryamide substrate. Nevertheless, in displaying a gradual increase inconversion as a function of Re content, the Rh/Re catalysts clearlydifferinrelationtothedegreeofsynergismthatwassoclearlyevi-dent, and necessary, for the high amide conversions and primaryamine selectivities associated with the first three members of theseries.  3.4.2. Variation of amide concentration Using batch experiments, the dependence of amide concentra-tion on reaction selectivity, using a Re:Rh=0.80 catalyst composi-tion (5mol% Rh), are shown in Table 4, from which it is clear thathigh dilution is beneficial, with the highest selectivity towardsCyCH 2 NH 2  occurring at the lowest amide concentration (Entry 1),the standard concentration used in all the Re:Rh, pressure andtemperature variation experiments (see Section 3.4.3). The in-creased amount of secondary amine formation in solutionscontaining higher amide concentrations is presumably a conse-quence of the greater probability of condensation reactions occur-ring on the catalyst surface. In contrast, CyCH 2 OH formationremains largely constant throughout.  3.4.3. Variation of pressure and temperature Using a Re:Rh=0.80 catalyst and standard batch experiments,the results of the effects of variation in pressure and temperatureon CyCONH 2  hydrogenation are summarized in Table 5. Althoughhighamideconversionsaremaintainedat50barH 2 (Entry2),asig-nificant reduction of primary amine selectivity in favour of (CyCH 2 ) 2 NH, relative to that noted at 100bar (Entry 1) is evident,CyCH 2 OH remaining unchanged. A considerable reduction in con-version is noted at 20bar (Entry 3), with selectivity towards Cy-CH 2 OH significantly increased in relation to that at 50bar,notwithstandingaminorincreasein(CyCH 2 ) 2 NHformation.Inspec-tionofEntries4–6showsthatanyreductioninreactiontemperaturebelow 150  C is deleterious to catalyst performance, with no reac-tionevidentat 130  Cduringthe standard16hreactiontime. Nev-ertheless,slowreductiondoesoccurduringaconsiderablyextendedreactiontime(Entry7).Thiscouldpossiblybeaconsequenceoflow-er rates of Re 2 (CO) 10  decomposition, and active catalyst formation,at 130  C. Overall, reaction conditions of 100bar H 2  and 160  C (cf.Entry 1) do appear necessary to give a combination of the highestamideconversions andprimaryamineselectivities.  3.5. Similarities and differences in catalyst behaviour  Fromtheresultsdescribedearlier,itappearsthatRu/ReandRh/Re catalysts both provide excellent intrinsic selectivities for thereduction of CyCONH 2  to CyCH 2 NH 2 . Although they are actuallysuperior in terms of primary amine selectivity, the minimumpres-sure and temperature requirements (cf. Tables 3 and 5, Entry 1)dictatetheuseof100barH 2 and160–150  C,respectively,forgoodamide conversions, and in this respect, they are of inferior perfor-mance to their Ru/Mo and Rh/Mo counterparts. With Ru/Re cata-lysts, an increase in the concentration of Re above the thresholdvalue of ca. Re:Ru=0.15 has little effect on catalyst selectivity,although there is a general trend of decreasing conversion and in-creasedsecondaryamineproductionat higherRe:Rucatalystload-ings. With the Rh/Re systems, low initial concentrations of Reenhance the high primary amine selectivity above that observedwiththebaseRhcase(70%,Fig.2),butadditionalRecontentisnec-essary to allow 100% CyCONH 2  conversion under the standardreaction conditions, cf. Fig. 2.  3.6. Catalyst genesis using in situ HP-FTIR spectroscopy 3.6.1. Ru/Re catalysts Inspection of absorbances of the amide carbonyl band at1693cm –1 in spectra of the complete CyCONH 2  amide reductionsystem, using Ru 3 (CO) 12  and Re 2 (CO) 10  (Re:Ru=0.34) as catalystprecursors, and a standard heating time of ca. 15min to 160  C(see Supplementary material Fig. S5, first entry at 160  C shownas time=0min), reveals that amide reduction commences imme-diately on reaching the operational reaction temperature. Calcu-lated as previously described [2], the amide decay curvecorresponds to an initial TON (mmol. CyCONH 2  consumed vs.mmol. Ru) of 1.9h –1 . Thus, in sharp contrast to Rh/Re catalysts(see Section 3.6.2) and to the Rh/Mo and Ru/Mo catalyst systemsdescribed previously [1,2], no catalyst induction period appearsnecessary for this combination. All evidence of Ru 3 (CO) 12  disap-pears during the initial heating period, and the residual  m (CO)absorptions, corresponding withthe spectrumof Re 2 (CO) 10 , slowlydegrade during the subsequent 2h.A control experiment, also using a 0.34 Re:Ru precursor ratiobut with stepped, rather than continuous heating (see Fig. 3), pro-vides some amplification of the initial stages of catalyst genesis. Atroom temperature, both Ru and Re precursors are clearly identifi-able by comparison with dominant features of reference spectra,cf. Ru 3 (CO) 12  (2062cm  1 ), Re 2 (CO) 10  (2069, 2014 and 1971cm –1 ).On heating to 80  C, the 2062-cm  1 band has been displaced byabsorptions at 2036 and 2000cm –1 , consequent upon the forma-tion of [H 3 Ru 4 (CO) 12 ]  , as found in the Ru/Mo systems [2]; the  Table 4 CyCONH 2  hydrogenation. Variation of conversion and product selectivity with amideconcentration. Entry Amide (mol/L)Conversion(%)Product selectivityCyCH 2 NH 2  CyCH 2 OH (CyCH 2 ) 2 NH1 0.072 97 89 3 72 0.112 100 80 6 113 0.147 100 82 6 114 0.181 93 74 6 18Reaction conditions: 100bar H 2 , 160  C, 16h, Re:Rh=0.80 catalyst. Fig. 2.  CyCONH 2  hydrogenation: (a) conversion and (b) product selectivity vs.Re:Rh composition (100bar H 2 , 160  C, 16h). Key:  j  Conversion,  }  CyCH 2 NH 2 , +(CyCH 2 ) 2 NH, 4 CyCH 2 OH. G. Beamson et al./Journal of Catalysis 278 (2011) 228–238  231  additional absorption expected at 2018cm –1 being masked by theintense band due to Re 2 (CO) 10 . All Ru-containing species decom-pose before 160  C is reached, at which point ca. Thirty percentageof the Re 2 (CO) 10  initially charged remains in solution. A combina-tionof70%oftheRe 2 (CO) 10  initiallypresentwithcompletedecom-position of Ru 3 (CO) 12  would in fact correspond to a catalystcomprising the optimum Re:Ru value of 0.25 (cf. Fig. 1). Conse-quently, decomposition of the remaining 30% of Re 2 (CO) 10  duringthe subsequent 2-h period shown in Fig. 3, throughout which slowreduction of CyCONH 2  is taking place, may be largely superfluoustooverallcatalystperformance. It mayindeedbedeleterious, sinceinspection of  Fig. 1 reveals a slight decrease in conversion withincreasing Re content, which may be a reflection of the onset of the considerably more extensive catalyst poisoning previouslyencountered with the Ru/Mo and Rh/Mo catalyst systems. Thegreater stability of Re 2 (CO) 10 , relative to Ru 3 (CO) 12 , with respectto decomposition is entirely consistent with the position of Re(comprising a third row transition element) in the Periodic Table.Thus, the situation is not directly comparable to that in whichMo(CO) 6  was used as co-catalyst precursor with Ru 3 (CO) 12 , andwhere the former species was found to be the first component toundergo rapid decomposition [2].The results of a further control experiment, namely the reac-tion of H 2  with a DME solution of Re 2 (CO) 10  alone under thestandard amide reduction conditions, are also revealing. Here,full decomposition of the organometallic molecule to metallicRe does not occur during the standard 16-h reaction period, inwhich Re 2 (CO) 10  is converted, possibly via H 4 Re 4 (CO) 12  ( m (CO)2042s, 1990mcm –1 ) [11] in the O-donor solvent, into Re 4 (OH) 4 (-CO) 12  ( m (CO) 2027s and 1923br cm –1 ) [12], which could be recov-ered in the form of a clear pale yellow solution after cooling anddepressurization. Furthermore, from an inspection of the relativeband intensity ratios associated with the spectrum of Re 2 (CO) 10 ,there is no evidence for the intermediate formation of significantamounts of HRe(CO) 5  ( m (CO) 2015s and 2006mcm –1 ) [13] duringthe reaction of H 2  with Re 2 (CO) 10  at 100bar H 2  and 160  C.In summary therefore, the presence of both Ru 3 (CO) 12  and sub-sequent reaction products, e.g., [H 3 Ru 4 (CO) 12 ] – appear to initiateand enhance partial decomposition of Re 2 (CO) 10  during the initialstages of catalyst genesis. Onset of catalytic activity appears tobe dependent only on complete decomposition of the Ru carbonylintermediates, which occurs very rapidly, and is largely indepen-dent of the overall rate of decomposition of Re 2 (CO) 10 .  3.6.2. Rh/Re catalysts In situ HP-FTIR spectra of the Rh/Re systems (Fig. 4) were lessconclusive thanthose of Ru/Re in terms of identifyinga parallel ef-fect, namely the role of Rh carbonyl decomposition in initiatingandenhancingdecompositionof Re 2 (CO) 10  duringcatalyst genesis,simply because the decay of   m (CO) absorption bands associatedwith Rh 6 (CO) 16  could not be readily identified due to its limitedsolubility in DME. Nevertheless, a peak at 1755cm –1 may corre-spond with the formation and presence of either polynuclear Rhcarbonyl anions such as [Rh 6 (CO) 15 ] 2– [14], or [Rh 13 (CO) 24 H  x ] n – (  x  =2, n  =3;  x  =3, n  =2,etc.),thecarbonylhydride-containingfam-ily proposed in the Rh/Mo catalyst precursor systems [1]. Signifi-cant differences from the Ru/Re systems are however readilyevident.GenesisofaRh/Re(Re:Rh=1)catalyst,chosenonthebasisof the information in Fig. 2 to be above the threshold of 0.8 forcompleteamide hydrogenationunder thestandardreactioncondi-tions, requires an induction period of ca. 500min, with initiationapparently dependent on complete decomposition of Re 2 (CO) 10 , asituation in close parallel to that observed with Rh/Mo catalysts[1] (cf. Fig. 4, in which  m (CO) absorptions at 2069, 2014 and1971cm –1 correspond to the presence of Re 2 (CO) 10 ). Internal con-sistency with catalyst performance as a function of Re:Rh (cf.Fig. 2) is also evident.  Table 5 CyCONH 2  hydrogenation: conversion and product selectivity vs. pressure andtemperature (Re:Rh = 0.80 catalyst). Entry  T  (  C) P  H2 (bar)Conversion(%)Product selectivity (%)CyCH 2 NH 2  CyCH 2 OH (CyCH 2 ) 2 NH1 160 100 98 90 6 32 160 50 98 78 7 133 160 20 61 69 14 174 150 100 97 84 8 65 140 100 64 83 8 76 130 100 0 – – –7 a 130 100 60 81 9 6Reaction time: 16h, Re:Rh=0.80 catalyst. a Reaction time 68h. 22 °C2000 19501970199020102030205020702090 (cm -1 )       A      B      S 60 °C80 °C100 °C120 °C140 °C160 °C206920142036206219710.22030 Fig. 3.  In situ HP-FTIR spectra (2100–1950cm –1 ) during initial stages of catalystgenesis. Reaction conditions: 100bar H 2 , 22–160  C, Re:Ru=0.34 catalyst, DMEsolvent. time (min)       A      B      S Fig. 4.  InsituHP-FTIRspectra(2100–1600cm –1 )showingdecayanddisappearanceof   m (CO) absorptions during catalyst genesis followed by initiation of hydrogena-tion. Reaction conditions: 100bar H 2  and 160  C, Re:Rh=1 catalyst. Key:  j CyCONH 2  (1693cm –1 ), } 2014cm –1 , 4 2069cm –1 , s 1971cm –1 , + 1755cm –1 .232  G. Beamson et al./Journal of Catalysis 278 (2011) 228–238
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