1-s2.0-s0926860x03005532-main

Applied Catalysis A: General 253 (2003) 437–452

The role of the active phase of Raney-type Ni catalysts in theselective hydrogenation ofd-glucose tod-sorbitol

B.W. Hoffera, E. Crezeea, F. Devredb, P.R.M. Mooijmana,W.G. Sloofb, P.J. Kooymanb, A.D. van Langeveldc,∗,

F. Kapteijna, J.A. Moulijna

a Reactor & Catalysis Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlandsb Laboratory of Materials Science, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands

c Charged Particle Optics, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands

Received 7 April 2003; received in revised form 30 June 2003; accepted 30 June 2003

Abstract

Bulk and surface properties of homemade Raney-type Ni catalysts and their Ni–Al alloy precursors have been studiedand compared with commercial samples, both promoted and unpromoted. The two starting alloys had a different Ni2Al3

concentration, resulting in Raney-type Ni catalysts with different Ni–Al composition. All catalysts have been screened in thehydrogenation of an aqueous solution ofd-glucose (10 wt.%) using a three-phase slurry reactor at 4.0 MPa and 393 K. Thepromoters are segregated at the surface of the catalysts and have a beneficial effect on the reaction rate, essentially due toan increased surface area and stability of the active phase while an enhanced interaction ofd-glucose with Ni seems to playa secondary role. The Raney-type Ni catalysts lose Ni and Al at the applied reaction conditions. Mo does not leach at all.Fe leaches severely from the catalyst surface, but about 30% of the Fe at the surface is present as inactive, bulk iron. Themajor cause of deactivation of the Raney-type Ni catalysts is the presence ofd-gluconic acid formed during the reaction. Theeffect ofd-gluconic acid is two-fold: firstly it blocks the Ni sites making them inactive ind-glucose processing. The poisonedcatalytic sites can be recovered again by a regeneration treatment in hydrogen at 393 K. Secondly, the formation ofd-gluconicacid leads to a severe loss of Ni, which makes product purification necessary.© 2003 Published by Elsevier B.V.

Keywords: d-Glucose;d-Sorbitol;d-Gluconic acid; Raney Ni; Promoters; Slurry reactor; Deactivation; XPS; XRD; TPD

1. Introduction

The hydrogenation ofd-glucose (dextrose) tod-sorbitol (Fig. 1) is of great industrial importance,becaused-sorbitol is a valuable additive in food,

∗ Corresponding author. Tel.:+31-152-789176;fax: +31-152-783760.E-mail address: [email protected](A.D. van Langeveld).

drugs, and cosmetics. Moreover,d-sorbitol is an inter-mediate in Vitamin C production. Excellent settlingproperties, a high activity, and low cost price justifythe choice for Raney-type Ni catalysts in this reac-tion. Although it is claimed that Ru/C catalysts are apromising alternative because of their non-leachingbehavior and high activity[1–3], Raney-type Ni cat-alysts still dominate industriald-sorbitol processing.Even though this class of Ni catalysts has alreadybeen applied in industry for decades, many properties

0926-860X/$ – see front matter © 2003 Published by Elsevier B.V.doi:10.1016/S0926-860X(03)00553-2

438 B.W. Hoffer et al. / Applied Catalysis A: General 253 (2003) 437–452

Nomenclature

Ca Carberry numberdp catalyst particle diameter (m)D equimolar diffusion coefficient (m2 s−1)Deff effective diffusion coefficient (m2 s−1)k reaction rate constant (kg−1

cat s−1)

kLa volumetric gas–liquid mass transfercoefficient (s−1)

M sum of all metals at the surfaceSBET catalyst surface area (m2 g−1

cat)Sh Sherwood numberSNi active Ni surface area (m2 g−1

cat)

Greek lettersη catalyst effectiveness factorΦ Wheeler–Weisz modulus (=ηiφ

2)

of Raney-type Ni are not yet fully understood. Also,characterization of these multi-component materialsis difficult, and many discrepancies in literature exist.The key to reproducible synthesis can be found in ab-solute control of preparation of the starting materials,the alloys, and the leaching process. A detailed studyof these processes, coupled with the determination ofthe catalytic properties is then required. In this way,the complete lifetime of a Raney-type Ni catalyst canbe followed in all its different stages. Therefore, therelations between the structure, activity and stabilityof the Raney-type Ni catalysts have been investi-gated in this work. The industrially attractive hydro-genation ofd-glucose was used to establish theserelations.

It is known that Ni catalysts show deactivation af-ter recycling of the catalyst in the hydrogenation of

Fig. 1. Hydrogenation ofd-glucose tod-sorbitol.

d-glucose[2,4–7]. Main reasons for deactivation ofRaney-type catalysts are considered to be:

• loss of active Ni surface by sintering[4,5,7];• leaching of Ni and promoter metal into the acidic

and chelating reaction mixture[5–7];• poisoning of the active Ni surface by organic species

produced by side reactions[4,6].

Addition of Mo and Cr promoters was found tobe beneficial for the catalyst activity and stability inthis reaction[2,4,6,8,9], whereas Fe and Sn promotedRaney-type Ni catalysts deactivate very rapidly[6].Bizhanov et al. studied the promoting effect of Pt,Pd, Ru and Rh on Raney-type nickel and found thatthe activity was enhanced by about 20–30%[10].According to these authors Raney-type Ni/Ru wasthe most promising catalyst. It was also establishedthat addition of the promoters did not lead to anynotable increase in the stability of the Raney-typeNi catalysts[11]. The combined action of Cr and Feon Raney-type Ni is known to be effective in nitrilehydrogenation[12], but is a novel system for thehydrogenation ofd-glucose.

Although Ni is responsible for the hydrogenationcapacity, it is suggested in the literature that residualaluminum (in combination with promoters) plays akey role in the performance of Raney-type Ni cata-lysts, because it is able to act as an electron donorto nickel, which renders the d-band of nickel lesselectron deficient and thus influences the adsorptionof the reacting species[13]. In the present work,the influence of residual Al, and the effect of Mo,and a combination of Cr and Fe promoters on theperformance of Raney-type Ni catalysts in the hydro-genation ofd-glucose has been investigated. Specialattention has been paid to the stability of the cat-alysts in successive hydrogenation runs. Hydrogen

B.W. Hoffer et al. / Applied Catalysis A: General 253 (2003) 437–452 439

chemisorption, temperature-programmed desorp-tion (TPD) and quasi in situ techniques like X-raydiffraction (XRD), X-ray photoelectron spectroscopy(XPS), and energy dispersive X-ray analysis (EDX)were used to characterize the catalysts. The expres-sion ‘quasi in situ’ in this work implies that for thecharacterization of the catalysts transport has beenperformed under protective atmosphere. A prelimi-nary report ond-glucose hydrogenation data has beenpublished previously[2].

2. Experimental

2.1. Catalysts

The applied commercial Raney-type Ni catalystswere provided by Engelhard de Meern (The Nether-lands). Unpromoted as well as promoted Raney-typeNi catalysts were used in this study. The promoterswere Mo and a combination of Cr and Fe, the lat-ter sample being a lab-sample. To vary the residualAl content, two samples from different starting alloys(Engelhard) were leached at mild conditions. Both al-loys (A and B) have a 50/50 (w/w) Ni/Al compositionbut were prepared via different proprietary processes.The alloy powders (dp < 80�m) were exposed to anexcess of 20 wt.% NaOH. Leaching was performed at353 K for 30 min. After leaching, the catalysts werewashed six times with distilled water. A detailed de-scription of the leaching procedure has been givenelsewhere[14].

2.2. Ni and total surface area

The active Ni surface area was determined by meansof volumetric hydrogen chemisorption. The catalystswere washed with ethanol (three times) to removemost of the water. The wet Ni catalysts were dried at323 K for 2 h in He followed by evacuation at 473 Kfor 3 h to remove the hydrogen initially present on thecatalyst. After cooling down in vacuum the hydrogenisotherms were measured at 323 K. The nickel surfacearea was evaluated from extrapolation of the isothermrepresenting strong adsorption to zero pressure (thuscorrected for weakly adsorbed hydrogen). The nickelsurface area was calculated under the assumption thatone Ni surface atom chemisorbs one hydrogen atom.

After the chemisorption measurement the BET sur-face was measured in the same equipment using N2physisorption at 77 K. In between these steps contactwith air was avoided.

2.3. TPD

Desorption of residual hydrogen present on thecommercial catalysts (formed during the leachingprocedure) was studied by TPD in an atmosphericplug–flow reactor. The catalysts were weighed underprotective atmosphere (about 0.2 g of material wasused). The catalyst was introduced into the quartzTPD reactor and a few droplets of ethanol were addedto the catalyst, creating a protective layer of liquidaround the catalyst particles. In this way, the reactorcould be transported to the TPD set-up without expos-ing the catalyst to air. After mounting the reactor inthe set-up, the sample was allowed to dry for a nightin an Ar flow of 0.42 ml s−1. Next, the dry samplewas heated according to a linear temperature-programwith a heating rate of 0.167 K s−1. In the reactor efflu-ent the H2-desorption was monitored with a thermalconductivity detector (TCD).

2.4. Quasi in situ XPS

The Raney-type Ni catalyst samples were washedwith ethanol three times and dried in a glove boxunder protective atmosphere. The dry catalyst waspressed into a soft indium foil mounted on a flat sam-ple holder. The sample holder was put into a transfervessel where the sample remains under the protectiveatmosphere and subsequently the sample was trans-ported into the vacuum chamber of the instrumentfor XPS analysis. The XPS analysis was performedwith a PHI 5400 ESCA system equipped with a dualanode X-ray source (Mg/Al) and a spherical capac-itor analyzer (SCA). The energy scale of the SCAwas calibrated according to a procedure described in[15]. The instrument was set at a constant analyzerpass energy of 35.75 eV and unmonochromatized in-cident Al X-ray radiation (Al K�1,2 = 1486.6 eV)was used for excitation of the sample. The electronsemitted from the sample were detected at an angleof 45◦ with respect to the sample surface. An ellipticarea of 1.1 mm× 1.6 mm was analyzed. Spectra wererecorded from the Ni 2p, Ni 3s, Al 2s (including Al


2p), Fe 3s, Cr 2p and Mo 3d photoelectron lines witha step size of 0.2 eV. The Ni 2p3/2 binding energy ofmetallic Ni (852.80 eV) was used as an internal refer-ence. The spectra shown in this work were correctedfor satellites caused by the non-monochromatic natureof the incident X-ray source[16] and the backgroundwas subtracted from the spectra using the iteratedShirley-method[17]. The ‘surface’ composition of thecatalyst was determined from the integrated intensityof the Ni 3s, Al 2s, Fe 3s, Cr 2p1/2, and Mo 3d5/2photoelectron lines by adopting the elemental sensi-tivity factors [16] of the Multipak software package.It should be noted that in this calculation the effectiveprobe depth, which correlates closely to the inelas-tic mean free path of the electrons, is accounted for.Since the Multipak software assumes the sample to behomogeneous over the volume analyzed, the analysisresults in an average composition of the surface phase,which has a thickness of about 1.5 nm. Consequently,the amount of thin oxidic layers of components presentat the surface of the catalyst will be underestimatedin the calculated results, whereas the occurrence of‘massive’ components as metallic Ni an Al will beoverestimated.

2.5. Quasi in situ XRD

The catalyst powder was introduced into a thin glasscapillary under protective atmosphere in a glove box.The capillary was sealed and in this way it could betransported to the diffractometer and analyzed with-out exposure to air. Powder XRD was performed ina Bruker-Nonius D5005 diffractometer using Cu K�radiation.

2.6. Quasi in situ HRTEM coupled with EDX

High-resolution transmission electron microscopy(HRTEM) was performed using a Philips CM30Tequipped with a LaB6 filament, operated at 300 kV.Elemental analysis was performed using a Link EDXsystem. Samples were applied on a micro grid carbonpolymer supported on a copper grid by placing a fewdroplets of a suspension of the sample inn-hexane onthe grid, followed by drying at ambient conditions,all in an Ar glove box. Samples were transferred tothe microscope in a special vacuum-transfer sampleholder under exclusion of air[18].

2.7. Hydrogenation reactions

The catalysts were screened in a 300 ml three-phaseslurry reactor with a gas-induced stirrer (Premex AG)at 393 K and 4.0 MPa using 10%d-glucose (0.56 M)in Milli-Q water. A low concentration ofd-glucosewas used in order to minimize mass transfer limita-tions, which easily occur in this fast reaction[19]. TheRaney-type catalysts were weighed as dry material ina glove box under protective atmosphere. The emptyreactor vessel was transferred into the glove box andthe desired amount of catalyst was introduced in thereactor. To prevent oxidation during transportationof the vessel to the experimental set-up, 100 ml ofsolvent was poured on the catalyst. Above the re-actor an injection vessel was situated. The reactorand injection vessels were purged with N2 and H2at room temperature. When the solvent with catalystwas at the desired temperature in the reactor (at about1.0 MPa), the injection vessel, containing 100 ml ofconcentratedd-glucose solution, was opened. Theexact starting time of the reaction could in this waybe assured. The temperature, total pressure and hy-drogen consumption were recorded during reaction.The starting pH of the reactant solution was about6. High performance liquid chromatography (HPLC)was used to analyze samples of the product mixture.The HPLC was equipped with a refractive index (RI)detector and a RCM monosaccharide column oper-ated at 358 K to separate the components. To studythe stability of the catalysts, several consecutive hy-drogenation runs were performed. After each run thereactor was emptied through a filter (5 nm) underhydrogen atmosphere. The catalyst was washed insitu with 150 ml water and a fresh reaction mixturewas introduced for the following run. In this way,it was assured that the catalyst was not exposed toair.

2.8. Leaching of the catalysts

The amount of metal ions present in the reactionmixture after the hydrogenation run was determinedby means of inductively coupled plasma-optical emis-sion spectroscopy (ICP-OES). The analysis was car-ried out with a Perkin-Elmer 5100PC apparatus. A10 wt.% d-sorbitol solution was used as a matrix inthe calibration samples.


3. Results and discussion

3.1. Bulk composition of the Ni–Al alloys andRaney-type Ni catalysts

Fig. 2shows the XRD patterns of the alloy samples.The two starting alloys A and B, used for the prepara-tion of the homemade catalysts, contain the same typeof phases (Ni2Al3, NiAl 3 and pure Al), but in differentconcentrations. Although quantification of the phasedistribution within the alloy has not been performed,Fig. 2 indicates that the Ni2Al3 phase is predominantin both alloys. More Ni2Al3 and less metallic Al ispresent in alloy B in comparison with alloy A. TheNi2Al3 ratio of the two alloys could be estimated fromthe intensity ratio of the{1 0 1} and{0 0 1} reflections:

(Ni2Al3)B-alloy

(Ni2Al3)A-alloy= 1.2

This suggests that the A-type alloy can be transformedinto activated catalyst more easily than the B-typealloy, because Ni2Al3 is reported to be the most dif-ficult phase to leach[20]. A previous electron mi-croscopy study of the alloys showed that both alloyshave a kernel of NiAl3, surrounded by Ni2Al3 [14].Fig. 3shows the XRD patterns of the resulting home-made catalysts and the commercial samples. The XRDpatterns of the leached alloys (referred to as RaNi-Aand RaNi-B) show, apart from elemental Ni, traces ofbayerite, Al(OH)3. The presence of metallic Ni indi-

Fig. 2. X-ray diffraction patterns of the starting alloys: (�) Ni2Al3, (�) Al; the remaining peaks are NiAl3 reflections.

cates that the alloys have been activated relatively fastat the applied leaching conditions, since the originalNi–Al phases are not detected, but traces of interme-diates like bayerite point out that the leaching pro-cess was not yet in its the final state. Formation ofbayerite is unwanted because its presence could causepore blocking. Composition analysis of the homemadecatalysts with EDX supports the suggestion that al-loy B was more difficult to leach due to the largeramount of Ni2Al3: after 30 min leaching the B-catalysthas a larger amount of residual Al than the A-catalyst(Table 1). It has been shown that at the applied condi-tions already after 10 s of leaching, the leaching of alu-minum from the alloy has finished and the Al is presentas alumina[14]. Extended leaching time is required tocompletely dissolve the aluminum oxide species andoptimize the catalytic performance. This phenomenonis more pronounced for the B catalyst compared tothe A catalyst: due to the lower leaching rate, morebayerite is present for RaNi-B after 30 min of leachingas was shown with XRD (Fig. 3). For the commer-cial samples, only the Ni reflections can be observedin XRD. The absence of Al reflections could indicatethat the residual Al consists of small particles unde-tectable using XRD. A more plausible explanation isthe existence of a solid solution of Al in the Ni lattice.If that were the case, the lattice parameters would belarger than bulk Ni, leading to a shift to lower 2θ val-ues in the XRD diagram.Fig. 3shows that indeed thelattice parameter of the Ni is enlarged by the presenceof Al, indicated by the small shift of the Ni reflections


Fig. 3. X-ray diffraction patterns of the catalysts: (�) Ni, (�) Al(OH)3. The dashed lines represent the Ni{1 1 1} and{2 2 0} reflectionsof bulk Ni.

of the catalyst samples compared to bulk Ni, whichposition is indicated by the dashed lines[21].

3.2. Surface composition of the Raney-type Nicatalysts

The nature and the occurrence of the differentphases at the catalyst surface can be evaluated fromthe position of the XPS emission lines and their (rel-ative) intensities.Fig. 4 shows the XPS spectra of theNi 2p region of the Raney-type Ni catalysts and areference material, which is a single crystal of NiAl(�-NiAl), cleaned in situ by sputtering with Ar. Theposition of the Ni 2p3/2 emission line of the singlecrystal is found at 852.8 eV, corresponding to metal-lic nickel (Ni0). This is confirmed by the doublet

Table 1Metal distribution of the catalysts (at.%)

Catalyst Bulk (from EDX) Surface (from XPS)

Ni Al Mo Cr Fe Ni Al Mo Cr Fe

RaNi 84 16 – – – 69 31 – – –RaNi-Mo 79 19 1.4 – – 66 25 8.7 – –RaNi-Cr/Fe 83 11 – 2.5 3.6 57 26 – 3.8 5.9RaNi-A 87 13 – – – 60 40 – – –RaNi-B 74 26 – – – 62 38 – – –

separation (17.2 eV), which is typical for metallicnickel [16]. The positions of the Ni 2p3/2 emissionlines of all catalysts indicate the presence of Ni0, butalso other Ni species can be observed. The emissionline at 854.6 eV corresponds to NiO, with a doubletseparation of about 18.2 eV[22]. For the homemadecatalysts, a third Ni contribution is observed at abinding energy of 858.8 eV. In literature, Ni2O3 [23],Ni(OH)2 [24] or NiAl2O4 [24] have been claimedto be present at the surface of Raney-type Ni cata-lysts. Both Ni2O3 and Ni(OH)2 compounds have a Ni2p3/2 binding energy at about 856.0 eV[25] and theNi 2p3/2 binding energy of NiAl2O4, is reported tobe 857.0 eV[16]. Thus, the photoelectron contribu-tion at 858.8 eV does not correspond with one of theaforementioned phases. However, this high binding


Fig. 4. Ni 2p spectra of the various samples of Raney-type Ni anda �-NiAl single crystal.

energy suggests that it is related to a Ni2+ containingspecies (referred to as Ni−Al−O), which strongly in-teracts with its environment.Fig. 5 gives some moredetails. At the high binding energy side in the Al 2sregion of the homemade catalysts, a large contribu-tion can be seen at 122 eV. Because the spectrum of�-NiAl shows that the Al 2s line of metallic Al canbe found at 117.6 eV, and aluminum oxides and hy-droxides can be found around 119 eV[26], the highAl 2s binding energy contribution can be assignedto Al3+ in a strong ligand field. All catalyst sam-ples have a contribution of metallic Al (117.6 eV).Deconvolution of the Al 2s region shows that RaNicontains two types of aluminum oxides (most likelyAl(OH)3 and Al2O3). In contrast, XPS analysis of thepromoted RaNi catalysts shows that only one type ofaluminum oxide is present. The Al 2s binding energyof 119 eV suggests that this species is rather an alu-minum hydroxide than an oxide, because aluminumoxides have a higher Al 2s binding energy than hy-droxides [26]. The overlapping of the Al contribu-tions in the spectra of the homemade catalysts doesnot allow an unambiguous identification of all thephases.

Fig. 5. Al 2s and Ni 3s spectra of the various samples of Raney-typeNi and a�-NiAl single crystal.

The Mo in the Mo promoted RaNi is mainly oxidic,although some metallic Mo is present. Hamar-Thibaultand Masson studied the RaNi-Mo system and con-cluded that molybdenum was in an oxidized state ofthe type MoO2 corresponding with a Mo 3d5/2 bindingenergy of 229.9 eV[27]. In the present study, the Mo3d5/2 binding energy was found at 232.0 eV, makingMoO3 more likely to be present at the catalyst surfacethan MoO2 [16]. The Cr 2p3/2 line at 577.2 eV corre-sponds to Cr2O3 [28]. Because the Fe 2p lines overlapwith Ni Auger lines, the Fe 3s emission line has beenused in this study. From its binding energy position(93.5 eV), it can be concluded that iron is present asan oxide (most likely Fe2O3) rather than as metallicFe (91.3 eV)[16].

The surface composition of the catalysts has beenestimated from the recorded XPS spectra and is re-ported inTable 2. Note that in the current work it isnot the aim to establish the dispersion of the variousphases present in the catalyst, but to acquire infor-mation of the relative amount of the various species.Since the software assumes the sample to be homoge-neous over the volume analyzed, the analysis resultsin an average composition of the surface phase with athickness of about 1.5 nm. Consequently, the presence


Table 2Phase distribution at the catalyst surface (at.%) from XPS

RaNi RaNi-Mo RaNi-Cr/Fe RaNi-A RaNi-B

Ni0 48.4 57.1 57.1 19 15NiO 20.3 8.8 7.1 18 16Ni–Al–O 0 0 0 23 31Al0 12.4 17.0 17.9Al2O3 10.8 – – 40a 38a

AlO(OH) 8.2 8.2 8.3Mo0 – 0.8 – – –MoO3 – 7.9 – – –Cr2O3 – – 3.8 – –Fe2O3 – – 5.9 – –

a This number represents the sum of all Al species.

of thin oxidic layers at the surface will be underes-timated in the calculated results, whereas the occur-rence of ‘massive’ components as metallic Ni an Alwill be overestimated, depending on the thickness ofthe oxide layers. At the surface of the RaNi catalyst arelatively large amount of oxides is present, both NiOand aluminum oxides. The presence of promoters ap-parently leads to suppression of the formation of NiOand Al2O3, leaving larger amounts of metallic Ni andAl at the surface.

Table 1shows that the surface of the catalysts isenriched with Al. Also, the promoters are segregatedat the surface of the catalyst at the expense of Al.The total amount of Ni at the surface of the commer-cial samples is comparable. The amount of aluminumspecies at the surface of the homemade catalysts islarge compared to the commercial RaNi catalysts, aswas expected since the leaching time was relativelyshort. Although the bulk compositions of the home-made catalysts differ significantly in Ni/Al ratio, thesurface composition is comparable.

Table 3Properties of the Raney-type Ni catalysts

Catalyst SNi (m2 g−1cat)

a SBET (m2 g−1cat)

b SNi /SBET (Ni0/M)surfc Activity (kg−1 s−1)

RaNi 19 56 0.34 0.48 0.35RaNi-A 10 56 0.18 0.19 0.16RaNi-B 7 62 0.11 0.15 0.14RaNi-Mo 21 77 0.27 0.57 0.50RaNi-Cr/Fe 10 112 0.09 0.57 0.90

a Determined from hydrogen chemisorption.b Determined from nitrogen physisorption.c Determined from XPS,M is the sum of all metals at the surface.

3.3. Surface morphology of the Raney-type Nicatalysts

Table 3gives a summary of the characteristics ofthe studied catalysts. The amount of metallic Ni at thecatalyst surface as determined by XPS for the Ni-onlymaterials follows the trend of the active Ni surfacearea. The homemade catalysts have a lower Ni sur-face area as measured by hydrogen chemisorption thanthe commercial RaNi due to incomplete leaching, re-sulting in a relatively large amount of Al at the sur-face. Also, the strongly interacting third Ni species,found in the homemade samples, might explain therelatively small amount of metallic Ni. Thus, for theunpromoted catalyst systems the two applied methodsto estimate the fraction Ni at the catalyst surface, i.e.hydrogen chemisorption and XPS, give the expectedtrends. However, the presence of promoters gives con-tradictory results for the determination of the Ni sur-face fraction with the different techniques. Saliently,the surface area of active material as determined byhydrogen chemisorption (SNi) is at most 34% of thetotal surface area (SBET). Addition of Mo results inan increased total surface area, butSNi is comparableto that of unpromoted RaNi. Promotion with Cr andFe lowersSNi dramatically, but gives a more porousstructure to the catalyst, since theSBET increases witha factor 2. These observations are remarkable becausea higher BET surface area is likely to correspond to ahigher Ni surface since the catalysts consist essentiallyof Ni (∼85%). Possible explanations are (i) the seg-regation of the promoter atoms to the nickel surface,thus hindering hydrogen chemisorption (Table 1) and(ii) an underestimation of the hydrogen chemisorptionsurface area due to the presence of strongly boundhydrogen formed during catalyst preparation, which


Fig. 6. TPD profiles of the commercial Raney-type Ni catalysts. The profiles are normalized for the amount of sample used.

could not be removed during the catalyst pretreat-ment in the chemisorption experiment. To resolve this,TPD was performed.Fig. 6 shows the TPD profilesof the commercial catalysts. Martin et al.[29] demon-strated, by measuring the saturation magnetization ofRaney-type Ni in an electromagnetic field, that theevolved hydrogen during TPD cannot be the result ofthe reaction of water with metallic Al, as was proposedby Mars and coworkers[30]. Thus, the hydrogen de-tected during the TPD, originates from adsorbed hy-drogen on the Ni, formed during preparation of thecatalysts. The profiles of the promoted catalysts showone large peak at about 460 K, while the unpromotedRaNi has its largest contribution peaking at 435 K, andsmaller peaks at 480 and 525 K. These small peakscould be related to a different type of Ni species, butadditional experiments are required to understand thisphenomenon. The Cr/Fe promoted catalysts releasesthe largest amount of hydrogen. Possibly, the appliedtemperature and time (473 K, 2 h) to evacuate the sam-ples in the determination of the nickel surface area byhydrogen chemisorption was insufficient to remove allthe strongly bonded hydrogen initially present on thecatalyst. Consequently, the hydrogen chemisorptionresults in an underestimation of the availability of theNi sites, most pronounced for the RaNi-Cr/Fe sample.

3.4. Performance of the Raney-type catalysts in thehydrogenation of d-glucose—mass transfer effects

Preliminary measurements were performed withthe Raney-type Ni catalysts to study whether mass

transfer or the chemical reaction controls the overallreaction rate. To study the impact of gas–liquid masstransfer, the stirrer speed was varied between 1600and 2500 rpm. The difference in reaction rate constantwas less than 5%, indicating the absence of gas–liquidmass transfer limitations. In addition, previous experi-ments in the applied three-phase batch reactor showedthat gas–liquid mass transfer coefficient (kLa) valuesup to 0.13 s−1 could be reached in a 10%d-glucosesolution at reaction conditions[19]. Knowing thisvalue, it is possible to calculate the ratio between theobserved reaction rate and the gas to liquid maxi-mum transfer rate, defined as the Carberry number,CaG–L, and the utilization of the catalyst consideringgas–liquid mass transfer,ηG–L [31]. In the calcula-tions, the initial reaction rate has been consideredin order to deal with a worst-case scenario.Table 4shows that in all cases, the effectiveness factorηG–L isclose to unity, confirming that gas–liquid limitations

Table 4Evaluation of the presence of H2 transport limitationsa

Gas–liquid Liquid–solid Internal

Ca η Ca η Φ η

RaNi 0.038 0.96 0.020 0.98 0.232 0.87RaNi-Mo 0.056 0.94 0.029 0.97 0.338 0.82RaNi-Cr/Fe 0.062 0.94 0.032 0.97 0.377 0.78RaNi-A 0.023 0.98 0.011 0.99 0.136 0.92RaNi-B 0.015 0.99 0.008 0.99 0.092 0.95

a Reaction conditions: 10 wt.%d-glucose, 4.0 MPa, 393 K,0.9 gcat.


can be neglected. The liquid–solid mass transfer canbe estimated by means of the dimensionless Sher-wood number,Sh [31]. The effectiveness factor forliquid–solid mass transferηL–S is above 0.95.

The absence of external mass transfer does not ruleout diffusion limitations in the pores of the Raney-typecatalysts. The average particle diameter of the appliedcatalysts is in the order of 40�m. The Wheeler–Weiszcriterion was used to evaluate the absence of internaldiffusion limitations based on experimental data. Theeffective diffusivity Deff was estimated to be 0.1D.For d-glucose no pore diffusion limitations occurred,but in most cases the reaction is partly controlled byinternal mass transfer limitation of hydrogen. Onlythe experiment performed with RaNi-B is completelygoverned by kinetics, due to the low activity of thiscatalyst, as will be discussed further on. To check thereproducibility of the experiments, the RaNi-Mo cat-alyst was tested three times. The reaction rate was re-produced with an error of only 2%.

3.5. Performance of the Raney-type catalysts in thehydrogenation of d-glucose—activity

All catalysts show a high selectivity tod-sorbitol(>99%). Main by-products during reaction ared-fructose andd-mannitol.d-Fructose is formed dueto isomerization ofd-glucose. d-Mannitol is pro-duced by hydrogenation ofd-fructose (also yieldingd-sorbitol). The catalytic hydrogenation ofd-glucosecan usually be described by Langmuir–Hinshelwoodkinetics with a transition from zero-order dependencyin d-glucose at high concentrations to first-order be-havior at low concentrations[19]. At the appliedd-glucose concentrations (10 wt.%) in this study, thereaction can be described by a first-order dependencyin d-glucose for all catalysts[2]. The activity of thecatalysts can therefore be expressed by the pseudofirst-order reaction rate constants (kg−1

cat s−1) and is

given in Table 3. The mildly leached RaNi sampleshave lower activity than the commercial RaNi cat-alyst, in line with the XPS analysis and hydrogenchemisorption data, from which it was establishedthat the Ni surface area is smaller for the mildlyleached catalysts. RaNi-A contains more metallicNi after 30 min leaching and less of the intermedi-ate Ni–Al–O species due to lower amount of initial,less reactive, Ni2Al3 in comparison with RaNi-B.

Although RaNi-B has a significantly different bulkcomposition than RaNi-A (Table 1), the activity datashow that the surface properties determine the cat-alytic performance. Unfortunately, the role of metallicAl remains unknown since its surface quantity couldnot be determined by means of XPS. The promotedcatalysts show higher reaction rates than unpromotedRaNi. The Cr/Fe promoted Raney-type Ni performsbetter than the RaNi-Mo catalyst, which is the tra-ditional catalyst used in this process. To understandthe role of the promoters on the catalytic activity, theresults have been expressed per unit surface area asdetermined by hydrogen chemisorption (SNi), nitro-gen physisorption (SBET) and a combination ofSBETand XPS (Fig. 7). It can be seen that the activity ofthe commercial Raney-type catalysts correlates withthe specific surface area as determined by nitrogenphysisorption. This result suggests than the mainrole of the promoters is to increase and stabilize thespecific surface area of the catalyst. Based on thechemisorption data, however, it can be seen that theenhanced BET surface area of the promoted catalystscannot fully account for the increase in activity. Espe-cially, the activity of the Cr/Fe promoted Raney-typeNi catalyst does not correlate withSNi. Therefore,the following working hypothesis is proposed. Thepromoters are present at the surface in the form ofoxide rafts or patches[32]. At the edges of the oxidicrafts, a positive charge is induced on the neighboringNi, thus enhancing the adsorption ofd-glucose. Thehydrogen adsorbs dissociatively on the Ni sites andmigrates by a spill over effect to the Ni sites with theadsorbedd-glucose molecules, initiating the reaction.For the Cr and Fe promoted catalysts it is difficult toconclude on their role as promoter[6].

3.6. Performance of the Raney-type catalystsin the hydrogenation of d-glucose—stability

Stability of catalyst performance is of extremeimportance in industriald-sorbitol processing. Thecatalysts should be resistant to metal sintering andpoisoning. Moreover, due to formation of smallamounts ofd-gluconic acid during the reaction, whichis a strong chelating agent, metal atoms tend to leachfrom the catalyst surface into the liquid phase. Thisphenomenon is highly undesirable, firstly because ofthe resulting metal impurities in the product, which


Fig. 7. Activity of the Raney-type catalysts expressed per unit surface area as determined by hydrogen chemisorption, nitrogen physisorption(SBET) and a combination ofSBET and XPS (393 K, 4.0 MPa, 10 wt.%d-glucose).

renders product purification necessary (d-sorbitol isused in food and personal care products) and, sec-ondly, loss of active catalyst sites. The amount ofdissolved metal ions in the reaction mixture aftereach hydrogenation run was determined by ICP-OES.Table 5 shows the results, expressed as the loss oforiginal metal content (wt.%). About 0.8 wt.% Ni hasleached from the catalyst surface for all Raney-typeNi catalysts (ca. 350 mg kg−1

sorbitol). Mo does not leachat all, within limits of accuracy, while Al and Crleach a little, comparable with Ni. Fe dissolves quiteeasily in the reaction mixture. This loss of Fe has asconsequence that also Ni and Al leach more severelyas compared to RaNi. RaNi-Mo is in this perspectivethe most stable catalyst; Mo does not leach and bothNi as well as Al leaching is reduced in the presenceof Mo. This is in agreement with work of Gallezotet al. [6]. They found that iron promoted Raney-typeNi showed deactivation after successive runs due toleaching of the iron from the Ni surface. Leaching of

Table 5Amount of leached metal ions (wt.%)a

Ni Al Mo Fe Cr

RaNi 0.8 0.8 – – –RaNi-Mo 0.6 0.5 0.0 – –RaNi-Cr/Fe 1.1 1.5 – 27.7 1.2RaNi-A 0.9 0.5 – – –RaNi-B 0.7 0.1 – – –

a Relative to the composition of the starting catalyst material.

molybdenum or chromium was less pronounced. Thecombined effect of Cr and Fe was not studied.

The loss of Ni from the catalyst into the reactionmixture is an indication for the catalyst’s structuralstability. To test the catalyst performance stability, suc-cessive experiments were carried out. As an example,the activity of successive recycle runs of the commer-cial RaNi sample is shown inFig. 8. The successiveruns show a steady decrease in activity. After threeconsecutive runs 48% of the initial activity is lost.Promoting Raney-type Ni gives initially a better cata-lyst stability (Table 6). The activity of the Raney-typeNi-Mo catalyst has lost 30% of the initial activity af-ter three runs and RaNi-Cr/Fe 16%. Hence, althoughthe promoters make Raney-Ni catalysts more stable,still a significant loss in activity after a few successiveruns is observed. van Gorp et al. also studied the sta-bility of the commercial RaNi-Mo and observed a de-crease in activity of about 17% after three runs, whichis much less than the present results[3]. The tests were

Table 6Activity of the catalysts after successive recycles (kglucose,kg−1 s−1)

RaNi RaNi-Mo RaNi-Cr/Fe

Run 1 0.35 0.46 1.02Run 3 0.18 0.32 0.86Run 5 – 0.25 0.61

Loss run (1− 3) (%) 48 30 16Loss run (3− 5) (%) – 21 29


Fig. 8. Conversion ofd-glucose as function of time in three successive hydrogenation runs for RaNi.

performed at identical temperature and hydrogen pres-sure, but with a 2.78 Md-glucose solution instead ofthe present 0.56 M solution. A first-order reaction rateof 0.025 kg−1

cat s−1 was found, which is 20 times lower

than the value found in this work. Most likely, thesedata were disguised by mass transfer limitations ofhydrogen, which masked the deactivation. The ques-tion arises what is causing the decrease in activity.Fig. 9shows the loss in activity of the Cr/Fe promotedRaney-type Ni during the stability tests as a functionof iron content in the catalyst. Initially a large amountof Fe is lost (almost 30%), whereas only 10% loss inactivity is observed. After run 3, the decrease in hy-drogenation activity is proportional to the decrease inFe content. Apparently, about 30% of the Fe is present

Fig. 9. Relation between leached Fe and catalytic activity after eachhydrogenation run relative to starting conditions. The pre-leachedsample is indicated with (�).

as catalytically inactive, cluster material. This resultsupports the proposed mechanism of the promotingeffect of Fe. The promoting Fe is relatively stronglyinteracting with the matrix of the catalyst, whereasless interacting bulk-like Fe-species are catalyticallyinactive. AlsoTable 6shows that the deactivation isaccelerated once the bulk Fe has been removed. Thisshows that although chromium is expected to have astronger promoting effect, the importance of the pres-ence of iron at the surface in this system cannot be dis-regarded. To further investigate the influence of the Fecontent on the activity, also a pre-leached Raney-typeNi-Cr/Fe catalyst was prepared by treatment in 0.01 MHCl under reflux conditions. Whereas after this treat-ment hardly any Cr, Ni, and Al was leached, about14% of Fe was removed from the catalyst. The activitywas only slightly lowered (seeFig. 9), in agreementwith the stability experiment. Hence, for the Cr/Fepromoted catalyst leaching of Fe could be causing thedeactivation. However, also the bare Raney-type Niand Mo promoted catalyst show deactivation, so Feleaching is not the only reason for the loss in activity.Other possible explanations of deactivation are metalsintering or poisoning of the active sites. The Ni sur-face area of the Mo promoted RaNi was therefore ex-amined before and after reaction (Fig. 10). The BETsurface area remains intact after repeated use, indi-cating that significant sintering is not occurring. Ap-parently, the active Ni sites are poisoned by stronglyadsorbed species. The hydrogen chemisorption resultsshow that after run 5, a large part of the Ni surface


Fig. 10. Ni surface area from hydrogen chemisorption and total surface area for nitrogen physisorption as a function of the condition ofthe RaNi-Mo catalyst.

area is not accessible for hydrogen anymore. Remark-ably, the decrease in Ni surface area (85%) is not pro-portional to the loss of activity (46%), which suggeststhat not all sites are identical in terms of activity. One

Fig. 11. Formation ofd-gluconic acid via transfer hydrogenation.

explanation could be that there are Ni clusters in closevicinity to promoter metal and Ni-only clusters, moresensitive to poisoning. Regeneration by reduction for2 h at 573 K and consequent hydrogen chemisorption


Fig. 12. Influence of initiald-gluconic acid on thed-glucose hydrogenation rate of RaNi and RaNi-Mo (0.56 Md-glucose, 393 K and4.0 MPa). The black bars represent glucose only and the gray bars with additionald-gluconic acid (0.1 M).

shows that most of the original Ni surface area is re-gained (Fig. 10); the adsorbed species have been re-moved resulting in a clean surface. The compoundmost likely to be poisonous isd-gluconic acid, formedvia a Cannizzaro-type of reaction[8]. This reactionis an oxido-reduction, induced by an alkaline envi-ronment in the presence of nickel and leads to theformation of a mixture ofd-gluconic acid (∼50%),d-sorbitol, andd-mannitol (Fig. 11) [33]. This dehy-drogenation reaction involves transfer hydrogenationwith d-glucose as hydrogen donating agent[33,34].

It is known that carboxylic acids have a strong ten-dency to bind to Ni. The effect ofd-gluconic acid for-mation is suspected to be two-fold: (i) the acidity of thereaction mixture increases, resulting in dissolution ofmetals; (ii) blocking of active sites by stronger adsorp-tion. To prove this hypothesis, experiments were car-ried out in whichd-gluconic acid (0.1 M) was addedto the reaction mixture.Fig. 12shows the results forboth RaNi and RaNi-Mo. The activities ofd-glucosehydrogenation have dramatically decreased for bothtypes of catalyst. The product mixture was completelygreen, indicating large amounts of dissolved Ni. Thiswas confirmed by ICP-OES: almost 20% Ni and 3% Alof the original catalyst had dissolved into the productmixture. The BET surface area of the spent RaNi-Mocatalyst was examined and showed that the area wasdecreased from 77 to 53 m2 g−1 due to the presenceof d-gluconic acid. Thus, although large amounts of

Ni and Al were leached, a large part of the originalporous structure has not been destroyed. It can there-fore be concluded that blocking of the active sites byd-gluconic acid is the main cause for the loss in ac-tivity. Crezee et al. showed that also Ru/C catalystsdeactivate due to poisoning of active species, althoughto a lesser extent[35]. The authors were able to re-generate part of the catalyst activity by treatment ofthe catalyst in vacuum thereby removing the adsorbedproducts. Apparently, for RaNi additional hydrogena-tion at elevated temperature is required. Industrially,this procedure can be performed, but the regenerationmust be preceded by a very severe washing procedure.The regeneration treatment is not possible in the pres-ence ofd-glucose, because the high temperature ofthe regeneration step will cause formation of polysac-charides, which will strongly adsorb on the catalyst,resulting in even further deactivation, as was observedexperimentally.

In summary, it can be concluded that poisoning ofthe active sites byd-gluconic acid is the main causeof deactivation of (promoted) Raney-type nickel in thehydrogenation ofd-glucose.d-Gluconic acid is alsoresponsible for leaching of Ni into the product mix-ture. It can be expected that the loss of Ni and Al afterlong-term processing eventually leads to a decrease insurface area resulting in loss of activity. The Cr/Fe pro-moted system loses Fe, which also contributes to thedeactivation. Considering the constant loss of both Ni


and Al, this could eventually lead to significant loss ofsurface area in long-term industrial applications, es-pecially when more concentratedd-glucose solutionsare used. It should, however, be noted that in indus-trial d-sorbitol processing higher hydrogen pressures(typically 20 MPa) are applied[36], possibly advanta-geous for the stability of the catalyst with respect topoisoning. Buffering the reaction mixture to keep thepH neutral may minimize leaching of metals into theproduct, while maintaining reasonable selectivity.

4. Conclusions

The phase composition of the starting alloys inRaney-type Ni manufacturing is highly important, aswas demonstrated by using relatively short digestiontimes in the leaching process of Al from the Ni–Alalloy. The alloy with a high initial amount of Ni2Al3gives an Al-rich catalyst after leaching. But indepen-dent of the starting alloy, the surface properties arecomparable. The homemade catalysts have a low Nisurface area and contain remains of bayerite, formedduring the leaching, as compared to commercial RaNi.As a result, these catalysts are less active ind-glucosehydrogenation. Metallic Ni and Al are present at thesurface of all examined catalysts. The surfaces of Moand Cr/Fe promoted catalysts contain a small amountof Ni and Al oxides, but the promoters are essentiallypresent as oxides. The promoters and Al are segre-gated to the surface in the form of their oxides, whichresults in a lower chemisorption area as compared toSBET. Furthermore, the evacuation temperature andtime (473 K, 2 h) as applied for the chemisorption ex-periments might be insufficient to remove the stronglybonded hydrogen initially present on the catalyst.All catalysts show high selectivity towardsd-sorbitol(>99%). The main role of the promoters is to enhanceand stabilize the BET surface area. The activity of thecommercial Raney-type of catalysts is mainly depen-dent on their specific surface area. Additionally, thereis a promoting effect based on the enhanced interac-tion of d-glucose with Ni due to neighboring oxidicpromoter metal. The Raney-type Ni catalysts lose Niand Al at the applied reaction conditions. Mo doesnot leach at all. About 30% of the Fe at the surfaceof the examined RaNi-Cr/Fe catalyst is present asinactive, bulk iron. This Fe leaches severely from the

catalyst surface, but only once the promoting, morerefractory, Fe has been leached into the product mix-ture the activity ind-glucose hydrogenation decreasessignificantly. The major cause of deactivation of thecommercial Raney-type Ni catalysts is the presence ofd-gluconic acid formed during the reaction. The effectof d-gluconic acid is two-fold: firstly, it poisons the Nisites making them inactive ind-glucose processing.The sites can be recovered again by a regenerationtreatment in hydrogen at 393 K. The formation ofd-gluconic acid also leads to a relatively large loss ofNi, which is highly undesired ind-sorbitol processing.

Acknowledgements

This work has been supported by The NetherlandsFoundation for Technical Research (STW), Engel-hard de Meern, and DSM Research. Johan Groen(TUDelft) is gratefully acknowledged for performingthe hydrogen chemisorption and nitrogen physisorp-tion measurement, and Niek van der Pers (TUDelft)for the XRD analysis. We also thank Michiel Makkee(TUDelft) for the fruitful discussions aboutd-glucosehydrogenation.

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