KINETICS, CATALYSIS, AND REACTION ENGINEERING

Selective Hydrogenation of Furfural on Ni-P, Ni-B, and Ni-P-BUltrafine Materials

Shao-Pai Lee and Yu-Wen Chen*

Department of Chemical Engineering, National Central University,Chung-Li, 32504 Taiwan, Republic of China

A series of ultafine Ni-P, Ni-B, and Ni-P-B amorphous alloy catalysts with various atomicratios were prepared by a chemical reduction method. The catalysts were characterized withrespect to elemental analysis, nitrogen sorption, XRD, TEM, XPS, and hydrogenation activity.Conventional Raney nickel was included for comparison. The Ni/P/B molar ratio in the startingmaterial significantly affected the concentration of boron and phosphorus bonded to the nickelmetal, subsequently affecting the surface area, the amorphous structure, and the hydrogenationactivity and selectivity of the catalyst. The different electron transfer between nickel metal andthe metalloid elements in Ni-P and Ni-B powders (phosphorus draws electrons and borondonates electrons) results in the extremely different hydrogenation activity of furfural (specificactivity per surface area: Ni85.0P15.0 . Ni71.4B28.9). By regulating a suitable P/B ratio, the ultrafineNi-P-B catalyst dramatically revealed a markedly higher hydrogenation activity of furfuralthan Ni-P and Ni-B. The specific activities per surface area of the catalyst are in the orderNi74.5P12.1B13.4 > Ni72.5P2.0B25.5 > Ni85.0P15.0 . Ni71.4B28.9 > Raney nickel. The phosphorus is anactive component to improve the selectivity of furfuryl alcohol. The hydrogenation of furfural iscatalyzed actively by the Ni-Px-By catalysts, following the first order with respect to theconcentration of furfural. The nature of the ultrafine amorphous structure and the P/B ratioare the keys to manipulate the catalytic properties of Ni-Px-By amorphous alloy catalysts.

Introduction

Ultrafine amorphous alloy powders have attractedextensive interest in recent years,1-11 owing to theirunique isotropic structure and chemical properties. Thematerials, combining the features of amorphous andultrafine particles, have more surface atoms and ahigher concentration of highly coordinated unsaturatedsites. Previous investigators expected such a combina-tion to create properties, particularly for catalytic andmagnetic recording applications.1,12,13 However, thecatalytic properties and the feasibility of applying thesematerials have seldom been investigated. Of relevantinterest when using ultrafine amorphous metal alloysas catalysts is how to increase the surface area of anultrafine amorphous alloy. Generally, the surface areasof ultrafine amorphous alloys prepared by vapor andsputter deposition and melt-quenching are rather small,that is, 0.1-0.01 m2/g.14 This area is too small for usein industry, owing to low productivity per weight of thecatalyst. Ultrafine amorphous alloy particles producedby chemical reduction have received increasing atten-tion in recent years.1,2,5,6,13,15,16 Notably, the ultrafineamorphous alloy particles obtained by chemical reduc-tion have the obvious merits of a larger surface area,applicability for large scale production, and a higherdispersion that can be compacted to serve many pur-poses.

Okamoto et al.13 characterized the surface of Ni-Band Ni-P ultrafine catalysts prepared by a chemicalreduction method with XPS, indicating that a variationin 3d electron density on the nickel metal induced byboron or phosphorus would modify the activity andselectivity of the nickel catalyst for hydrogenation. Shenet al.16 successfully prepared Ni-P-B ultrafine amor-phous particles by chemical reduction methods. Theirresults indicated that different initial pH values affectedthe composition and crystallization behavior of thesample. As generally known, boron or phosphorus canaffect the surface properties of these catalysts and,hence, their catalytic properties. Ni-P-B, which con-sists of two metalloid elements (phosphorus and boron),has seldom been mentioned in terms of catalytic proper-ties and surface state. The determination of the surfacestoichiometry and chemical states of the surface playsa most important role in understanding the activity ofthe Ni-Px-By catalysts.

Furfural, C4H3OCHO, is an important compound inthe fragrance industry. Because of the presence of bothCdO and unsaturated CdC bonds, furfural is a suitablecompound to test the ability of a catalyst to selectivelyhydrogenate a carbonyl bond. There are two ways ofproducing furfuryl alcohol through hydrogenation offurfural, that is, vapor-phase hydrogenation and liquid-phase hydrogenation. Depending on the catalyst, vapor-phase hydrogenation of furfural can give a variety ofproducts such as furfuryl alcohol, 2-methylfuran, andtetrahydrofurfuryl alcohol. Residuous material and ring-

* To whom correspondence should be addressed. Fax: 886-3-4252296. E-mail: ywchen@cc.ncu.edu.tw.

2548 Ind. Eng. Chem. Res. 1999, 38, 2548-2556

10.1021/ie990071a CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 06/18/1999

decomposed products are also found in the liquid-phasehydrogenation reaction.17,18 For a copper chromite cata-lyst, gas-phase hydrogenation of furfural gave a selec-tivity for furfuryl alcohol ranging from 35%-80%.19 Inliquid-phase hydrogenation, high temperature and pres-sure are required. The greatest drawback of a Cu-Crcatalyst is its high toxicity, which causes severe envi-ronmental pollution. The Raney nickel catalyst producedtetrahydrofurfuryl alcohol. Previous reports indicatedthat Raney nickel catalysts modified with salts ofheteropolyacids21 or with (NH4)6MoO24

22 and Raneycobalt catalysts modified with various transition met-als23 can improve the catalytic activity and selectivityof the hydrogenation of furfural more than unmodifiedones.

Herein, we report a novel type of ultrafine amorphousalloy material Ni-P-B, which is active and selectivein the hydrogenation of furfural. The materials combinethe effects of metalloid elements and the features ofultrafine amorphous structure. This is expected to resultin special properties, especially for the catalytic applica-tions. Furfural hydrogenation was chosen as a testreaction to probe catalytic behaviors and to allowcomparisons among the ultrafine catalysts Ni-P-B,Ni-P, and Ni-B. Commercial Raney nickel was justincluded for comparison. The catalysts were character-ized with respect to nitrogen sorption, XRD, TEM,elemental analysis, and XPS.

Experimental Section

Chemicals. Furfural, with a purity of >98%, wasobtained from TCI Co. (Tokyo, Japan). High-purityhydrogen gas (>99.99%) from a local vendor was usedwithout further purification. Nickel acetate tetrahydrate(>98%) was supplied by Showa Chemicals (Tokyo,Japan). Sodium hypophosphite (>99%) was obtainedfrom Fisher Co. (NJ). Sodium borohydride (>98%) waspurchased from Lancaster Co. (Morecambe, England).

Catalyst Preparation. A series of Ni-Px-By cata-lysts with various Ni/P/B molar ratios and Ni-B andNi-P catalysts were prepared with chemical reactionmethods. The Ni-Px-By amorphous alloy materialswere prepared by mixing an aqueous solution of nickelacetate (1000 mL, 0.1 M) and sodium hypophosphite (1M) at 30 °C under ultrasonic agitation. The solution ofsodium borohydride (0.1 M) was then added dropwiselyinto the mixture. The black precipitates were washedthoroughly with a large amount of distilled water,followed by an ethanol rinse, and soaked in 99% ethanol.The Ni-B powder was prepared by a similar methodto that for the Ni-P-B powders in the absence ofsodium hypophosphite. The Ni-P powder was pre-pared by heating an aqueous solution that consisted of25.4 g of nickel acetate and 31.8 g of sodium hypophos-phite at 70 °C with vigorous stirring. The pH valueof the solution was adjusted to 11 using a 30 wt %NaOH aqeous solution. The Ni-P black precipitate waswashed with an 8 M NH4OH aqueous solution and thenwashed with distilled water followed by ethanol. Thematerials were also soaked in ethanol. The catalystsamples were blown dry at room temperature, beforeproceeding to the experiments of ICP, BET, XRD, andTEM. The commercial Raney nickel was obtained fromStrem Chemicals Company. It has 85 wt % Ni and 15wt % Al.

Catalyst Characterization. Elemental analysis withinductively coupled plasma atomic emission spectros-

copy (ICP-AES) (Jobin-Yvon Company, France, JY-24)was performed on the Ni-P, Ni-B, and Ni-P-Bmaterials. In general, the weighted samples were dis-solved in nitric acid and diluted with distilled water toa concentration within the calibration range of eachelement. The standard solutions purchased from Merckwere diluted and used to establish the calibrationcurves. The wavelengths in nanometers used for el-emental analysis were 231.604, 214.914, and 249.773for Ni, P, and B, respectively.

X-ray diffraction (XRD) measurements were takenusing a Siemens D5000 powder diffractometer with CuKR radiation (40 kV, 30 mA). The sample was scannedover the range 2θ ) 5-80° to identify the amorphousstructure.

The BET surface area was measured by nitrogenvolumetric adsorption (Micromeritics ASAP 2000) at-196 °C. The temperature of the liquid nitrogen bathwas checked with a thermistor pobe.

The morphologies and particle sizes of the sampleswere determined by transmission electron microscopy(TEM) performed on a JEOL JEM-1200 EX II electronmicroscope operating at 160 kV.

X-ray photoelectron spectroscopy (XPS) measure-ments were taken using a Perkin-Elmer PHI-1600photoelectron spectrometer with Mg KR radiation (15kV and 25 mA). The catalyst sample was mountedquickly onto a grid attached to a sample holder, keepingthe powder soaked in 99% ethanol to minimize theoxidation of the powder by air. After the ethanol wasevacuated, the sample was transferred into the XPSanalyzing chamber. The base pressure in the analyzingchamber was maintained on the order of 10-9 Torr. Thespectrometer was operated at a 23.5 eV pass energy.The spectra were recorded, after an etching of thesurface by Ar+ ions for 30 s. The binding energies (BEs)of the XPS spectra were corrected by contaminantcarbon (C1S ) 285.0 eV) in order to facilitate comparisonof the values between the catalysts and the standardcompounds. The standard compounds were nickel foil,red phosphorus, and elemental boron. In the cases ofthe standard compounds, the reproducibility of thebinding energies was within (0.2 eV. The accuracy ofthe binding energy values for the catalysts examinedhere was (0.3 eV, judged by the reproducibility of thebinding energies for several samples prepared sepa-rately.

Reaction Setup. All the experiments were carriedout in a cyclindrical stirred-tank reactor (Parr Instru-ment Model 4842), with a 150 mm height, a 63.5 mminternal diameter, and a 300 mL capacity. A four-bladedpitched impeller was placed for effective agitation, andthe agitator was connected to an electric motor withvariable speed up to 1700 rpm. A pressure transmitterand an automatic temperature controller were alsoprovided. The gases were supplied from cylinders andintroduced to the base of the reactor; the entrance tubealso served as a sampling tube for the liquid phase.

Reaction Procedure. The catalytic properties of thesamples were tested by the hydrogenation of furfuralin ethanol solution. The reactor was charged with 0.3 gof catalyst and 2 mL of furfural in a 170 mL ethanolsolution. Air was flushed out of the reactor with nitrogenat room temperature; hydrogen was then fed into thereactor. Next, the inlet valve was closed and heatingcommenced with stirring to avoid settling of the cata-lyst. When the designated temperature (80 °C) was

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2549

reached, hydrogen was fed to the predetermined pres-sure of 250 psi (time zero), which was maintainedthroughout the reaction; the stirring speed was 1700rpm. During the run, samples (about 0.5 mL each) werewithdrawn periodically and analyzed with a gas chro-matograph. A gas chromatograph equipped with a flameionization detector and a 3 m × 1/8 in. (1 in. ) 2.54 cm)stainless steel column packed with 25% Carbowax onChromosob W-HP (80-100 mesh) was used for sampleanalysis. Nitrogen was used as a carrier gas.

A polynomial equation was fitted to the conversiondata for each set of reaction conditions. The differentialof the best fit equation directly gave the hydrogenationrate r (dx/dt). The value of the initial rate was obtainedby calculating the rate at zero time. The fractionalconversion XA of a given reactant A is defined as XA )(NAO - NA)/NAO, where NAO is the moles of reactant Aat time t ) 0 and NA is the moles of reactant A attime t.

Results and Discussion

Catalyst Characterization. Table 1 lists the com-positions, surface areas, and particle sizes of the sam-ples. The compositions of the samples determined byXPS (surface atomic composition) and ICP-AES (bulkatomic composition) were Ni71.4B28.9 and Ni69.3B30.7,respectively (sample A), for Ni-B powder, Ni85.0P15.0 andNi83.5P16.5, respectively (sample B), for Ni-P powder,and Ni72.5P2.0B25.5 and Ni69.4P1.0B29.6, respectively (sampleC), and Ni74.5P12.1B13.4 and Ni82.0P9.1B8.9, respectively(sample D), for Ni-P-B powders. The data indicate thatthe surface stoichiometries were similar to the bulk forNi-B and Ni-P; however, the Ni-P-B samples werenot. There are two competing reactions between met-alloids (H2PO4

- and BH4-) and nickel ion.16 In the

synthesis, the composition of the Ni-P-B powder wasdetermined by the relative rates of the two competingreactions. Therefore, it is possible that the rates of thetwo reactions did not remain constant throughout thereaction process. This will result in different composi-tions on the surface. However, the difference was notlarge. Since the intensities of the peak areas of XPSwere used for (semi-) quantitative comparison, theywere measured by planimetry of graphic displays of thespectra, assuming a linear baseline, as described byHonda and Hirokawa.25 The differences in the escapedepths of the photoelectrons would cause error in thedetermination of the surface composition of the cata-lysts, particularly when samples are extensively contam-inated or are heterogeneous in the direction of depth.Accordingly, the boron and phosphorus contents givenhere are semiquantitative rather than absolute. How-ever, the relative values are believed to be correct. Inthis study, the surface stoichiometries were similar tothe bulk of Ni-B and Ni-P; the results are identical tothose reported by Schreifels et al.20 and Okamoto et al.13

The composition significantly influenced the surfacearea of the sample. The sample (A) Ni71.4B28.9 for Ni-Bpowder had the largest surface area (38.8 m2/g). Thesample (B) Ni85.0P15.0 for Ni-P powder had the smallestsurface area (3.3 m2/g). The sample (C) Ni72.5 P2.0B25.5and the sample (D) Ni74.5P12.1B13.4 for Ni-P-B powdersare in the middle (22.7 m2/g and 15.1 m2/g, respectively)between Ni-B powder and Ni-P powder. The surfacearea of the sample (E) Raney nickel, Ni72.3Al27.7, was59.4 m2/g.

Owing to the different starting concentrations ofboron and phosphorus, the distinct differences of themorphology and particle size among these samples wereobserved in the TEM micrograph (Figure 1). The sample(B) Ni85.0P15.0 for Ni-P powder has a spherical morphol-ogy, and the particle size is in the range 50-150 nm.The sample (C) Ni72.5P2.0B25.5 and the sample (D)Ni74.5P12.1B13.4 for Ni-P-B powders and the sample (A)

Table 1. Composition, Surface Area, and Particle Size of the Catalysts

particle size (nm)

catalyst samples (Ni/P/Bmole ratio for initial prep)a

ICP bulk composition(atomic ratio)b

XPS surface composition(atomic ratio)c

BET surfacearea (m2/g)

measuredby TEM

est avgsize

(A) Ni-B (1:0:3) Ni69.3B30.7 Ni71.4B28.9 38.8 10-30 17(B) Ni-P (1:3:0) Ni83.5P16.5 Ni85.0P15.0 3.3 50-150 204(C) Ni-P-B (1:0.3:3) Ni69.4P1.0B29.6 Ni72.5P2.0B25.5 22.7 10-30 30(D) Ni-P-B (1:3:1) Ni82.0P9.1B8.9 Ni74.5P12.1B13.4 15.1 10-30 45

a Ni/P/B mole ratio in the mother solution. b Ni/P/B atomic ratio in the bulk of the ultrafine material. c Ni/P/B atomic ratio on thesurface of the ultrafine material.

Figure 1. TEM of Ni-P-B, Ni-P, and Ni-B samples: (I) sample(C) Ni72.5P2.0B25.5, 1 cm ) 100 nm the sample (D) Ni74.5P12.1B13.4.1and the sample (A) Ni71.4B28.9 are similar to sample (C); (II) sample(B) Ni85.0P15.0.

2550 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999

Ni71.4B28.9 for Ni-B powder have a markedly smallersize than Ni-P. They have a similar diameter rangingfrom 10 to 30 nm (Table 1), and the particles appear tobe interconnected. This is ascribed to the higher surfaceenergy of the small particles. The smaller particle sizeimplies a higher surface energy of the particle. There-fore, the particles tend to connect with each other toreduce the surface energy. Furthermore, by assumingthat the sample particles are spherical and nonporous,their average sizes can be estimated by the formula dh(nm) ) 6/SBETF × 103, where SBET denotes the surfacearea and F represents the density of a particle usingthe value 8.9 g/cm3 (the density of nickel). The resultsof particle size from the TEM micrograph and the N2sorption measurement resemble each other, as shownin Table 1.

The XRD patterns of the samples as shown in Figure2 gave only a broad peak around 2θ ) 45°. This wasassigned to the amorphous state of the nickel-metalloidalloy.12,15 Notably, the patterns contained no distinctpeak corresponding to a crystalline phase. Comparingthe XRD patterns of the Ni-P-B, Ni-B, and Ni-Psamples reveals an obvious difference, indicating thatthe Ni-B powder has a wider disorder range than theNi-P-B powder, and the Ni-P-B powder than theNi-P powder. Moreover, the extent of the wider disor-der range increased with a decrease of phosphoruscontent in the amorphous samples.

The XPS spectra of the sample (A) Ni74.1B28.9 for Ni-Bpowder are shown in Figure 3. The Ni2p3/2 binding energyfor Ni74.1B28.9 powder at 852.4 eV, compared with thespectrum of pure nickel metal foil (852.2 ( 0.3 eV), isascribed to metallic Ni.13,15,33,34 In the B1s level, thereexist two kinds of boron species on the surface of theNi74.1B28.9 powder. The peaks at the lower and higherbinding energies are assigned to boron interacting withnickel (188.0 eV) and oxidized boron (193.0 eV), respec-tively. The lower binding energy at 188.0 eV shiftedpositively by 1.0 eV from that for elementary boron(187.0 eV); it is concluded that the boron speciesinteracting with nickel are positively charged and thatthe boron donates electrons.13,15,33,34 The XPS spectraof the sample (B) Ni85.0P15.0 for Ni-P powder are shownin Figure 4. The Ni2p3/2 binding energy for the Ni85.0P15.0powder is 852.3 eV, consistent with that for pure nickelmetal. In the P2p level, two kinds of phosphorus speciesappeared on the surface of the Ni85.0P15.0 powder. Thepeaks at the lower and higher binding energies areassigned to phosphorus interacting with nickel (129.6

eV) and oxidized phosphorus (133.5 eV), respectively.The lower binding energy peak at 129.6 eV shiftednegatively by 0.8 eV from that for red phosphorus (130.4eV). It is concluded that the phosphorus species inter-acting with nickel are negatively charged and that thephosphorus accepts electrons;13,15,33,34 similar negativeshifts have been reported for MnP (-0.8 eV), CrP (-1.3eV), and Cu3P (-0.4 eV).26 The XPS spectra of the sam-ple (C) Ni72.5P2.0B25.5 and the sample (D) Ni74.5P12.1B13.4for Ni-P-B powders are shown in parts I and II,respectively, of Figure 5. The Ni2p3/2 binding energies forNi72.5P2.0B25.5 (852.3 eV) and Ni74.5P12.1B13.4.1 (852.4 eV)are consistent with that for pure nickel metal. The B1sbinding energies of Ni72.5P2.0B25.5 (188.0 eV, 192.7 eV)and Ni74.5P12.1B13.4 (187.9 eV, 192.9 eV) are consistentwith the B1s binding energy in the Ni-B powder. TheP2p binding energies of Ni72.5P2.0B25.5 (129.5 eV, 133.3eV) and Ni74.5P12.1B13.4 (129.6 eV, 133.4 eV) are consis-tent with the P2p binding energy in the Ni-P powder;but the P2p peak is not obvious for Ni72.5P2.0B25.5. Boroncombined with nickel metal in the Ni-P-B powder wasfound to donate electrons to the nickel metal, whereasphosphorus bonded to nickel metal accepted electronsfrom the nickel metal. The electron transference be-tween nickel and the metalloids in the Ni-P-B powderis complex; the different electron transfer mechanismsbetween nickel and the metalloid elements (boron

Figure 2. XRD patterns of Ni-P-B, Ni-P and Ni-B samples:(A) Ni71.4B28.9; (C) Ni72.5P2.0B25.5; (D) Ni74.5P12.1B13.4.1; (B) Ni85.0P15.0. Figure 3. XPS spectra of the sample (A) Ni71.4B28.9 for Ni-B

powder.

Figure 4. XPS spectra of the sample (B) Ni85.0P15.0 for Ni-Ppowder.

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2551

donates electrons and phosphorus draws electrons) existat the same time.

In this study, keeping the catalyst soaked in 99%ethanol can minimize the oxidation of the catalyst byair. Phosphorus oxide and boron oxide were producedduring the preparation of the catalyst; Schreifels et al.20

indicated the oxygen was from the soluble oxygen in thewater solution. The same results have been reported byChen et al.13,15,16,20,24 It was evidenced that all the XPSdata of phosphorus for Ni-P-B and Ni-P catalysts andof boron for Ni-P-B and Ni-B catalysts demonstratedan element state (P, B) and an oxide state, even thoughthe samples were reduced in situ.

Mass-Transfer Considerations. Catalytic reactionin a slurry reactor involves processes such as gas toliquid mass transfer, liquid to particle mass transfer,intraparticle diffusion, adsorption, surface reaction, anddesorption of products.27 To evaluate the extent of mass-transfer limitation related to diffusion from the liquidto the solid phase and within the catalyst particles, themethods introduced by Carberry,28 Wheeler,29 andWeisz and Prater30 have been adopted. The Carberrynumber, Ca ) robs/k1s(6w/dpFp)C, represents the extentof the external mass-transfer limitation and ranges fromzero to unity. A Carberry number smaller than 0.05indicates that diffusion retardation by external masstransfer may be neglected. The Wheeler-Weisz group,ηæ2 ) dp

2robs/4DeffVpC, represents the extent of the porediffusion limitation and ranges from zero to infinity,where robs is the observed rate (mol/s), kls is the liquid/solid mass-transfer coefficient (m/s), w is the catalystweight (g), dp is the mean particle size (m), Fp is thecatalyst apparent density (g/cm3), C is the solubility

(mol/cm3), Deff is the diffusion coefficient (m2/s), and Vpis the catalyst volume (cm3). A value of the ηæ2 groupsmaller than 0.1 means that the pore diffusion limita-tion is negligible. These ultrafine catalysts have verysmall particle sizes (dp e 100 nm). The Carberrynumber Ca and the Wheeler-Weisz group ηæ2 aredirectly proportional to dp and dp

2, respectively. Thevalues of Ca and ηæ2 are very small (Ca < 0.05,ηæ2 <0.1). In addition, These ultrafine catalysts are nonpo-rous, as evidenced by the low BET surface area found(Table 1). Therefore the mass-transfer limitations re-lated to diffusion from the liquid to the solid phase andwithin the catalyst particles are negligible. The gas-liquid mass-transfer limitation can be eliminated by theproper stirring speed.31 This can be verified by a seriesof experiments carried out using different amounts ofcatalysts.32 Figure 6 shows the effect of catalyst loadingon the initial rate of hydrogenation of furfural at 80 °C.

Figure 5. XPS spectra of Ni-B-P powder: (I) sample (C)Ni72.5P2.0B25.5; (II) sample (D) Ni74.5P12.1B13.4.1.

Figure 6. Effect of catalyst loading on the initial rate of hydrog-enation of furfural. (reaction conditions: catalyst, Ni74.5P12.1B13.4.1;temperature, 80 °C; pressure, 250 psi; stirrer speed, 1700 rpm).

Figure 7. Conversion of furfural and selectivity for furfurylalcohol as a function of reaction time (the front symbol is forconversion; the back symbol is for selectivity): (b, O) (D)Ni74.5P12.1B13.4; (9, 0) (C) Ni72.5P2.0B25.5; ([, ]) (A) Ni71.4B28.9; (2,4) (B) Ni85.0P15.0; (/, ×) (D) Raney nickel.

2552 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999

The rate is linearly dependent on the catalyst loadingwith a zero intercept. This observation suggests thatgas-liquid mass-transfer resistance is not importantunder these conditions. Therefore, one can conclude thatthe reactions were carried out under a kineticallycontrolled regime.

Hydrogenation Reaction. Figure 7 gives the com-parison among Ni-P-B, Ni-P, Ni-B, and Raney Nicatalysts concerning the change of conversion andselectivity in the hydrogenation of furfural as a functionof time. The catalytic activity per surface area (listedin Table 2) is in the following order: sample (D) Ni74.5P12.1B13.4 > sample (C) Ni72.5 P2.0B25.5 > sample (B)Ni85.0P15.0 . sample (A) Ni71.4B28.9 > sample (E) Raneynickel. The order of the catalytic activity per gram ofnickel for the hydrogenation of furfural is sample (D)Ni74.5 P12.1B13.4 > sample (C) Ni72.5 P2.0B25.5 . sample(A) Ni71.4B28.9 > sample (B) Ni85.0P15.0 > sample (E)Raney nickel Ni72.3Al27.7. Figure 8 shows that thehydrogenation of furfural catalyzed by the ultrafine Ni-P-B, Ni-B, and Ni-P amorphous catalysts follows firstorder kinetics with respect to furfural, which is the sameas the case for the Raney nickel catalyst.

Baijun et al.21 reported that the reaction of furfuralhydrogenation can be represented by Scheme 1. Fur-fural contains two kinds of reactive groups, carbonylgroup (CdO) and carbon-carbon (CdC) double bonds;hydrogenation of the former gives furfural alcohol(reaction 1), and hydrogenation of latter results in

tetrahydrofurfural (reaction 2). In this study, a signifi-cant buildup of tetrahydrofurfural was not observed; therate of reaction 2 can be negligible. Table 2 shows thatthe primary products were furfuryl alocohol and tet-rahydrofurfuryl alcohol in this study. Depending on thecatalyst, the rate of reaction 1 with respect to reaction3 is different. The differences among the catalyticactivities can be attributed to the difference of theelectron density on the nickel metal among Ni-P-B,Ni-P, and Ni-B catalysts.13,33,34 Comparing the XPSdata of Ni-P-B, Ni-P, and Ni-B catalysts reveals thatboron combined with nickel metal in Ni-B powderwould donate electrons to the nickel metal, resulting inelectron-rich nickel metal, whereas phosphorus bondedto nickel metal in Ni-P powder would accept electronsfrom the nickel metal, creating an electron deficientmetal. The different interactions between the CdO bond

Table 2. Catalytic Activities and Selectivies of Ultrafine Ni-P-B, Ni-P, and Ni-B Amorphous Alloy Catalysts

activitya selectivityb (mol %)parameterc

catalyst sample× 105 moles of

furfural‚(m2 of cat)-1‚min-1× 102 moles of

furfural‚(g of Ni)-1‚min-1furfuralalcohol

tetrahydrofurfuryl alcohol P/(P + B) ∆q

(A) Ni71.4B28.9 2.68 0.11 68 32 0 -0.07(B) Ni85.5P15.0 21.2 0.08 80 20 1 0.08(C) Ni72.5P2.0B25.5 27.3 0.67 69 31 0.07 -0.05(D) Ni74.5P12.1B13.4 73.5 1.20 82 18 0.47 0.04(E) Ni72.3Al27.7 (Raney nickel) 1.02 0.07 62 38

a Reaction conditions: PH2 ) 250 psi; T ) 80 °C; furfural/ethanol ) 2 mL/170 mL; m(cat) ) 0.3 g. b Selectivities for furfuryl alcohol aremeasured at 50% conversion of furfural. c Parameters: P/(P + B) is the atomic ratio of P to (P + B) on the catalyst surface; ∆q ) (-∆E/K)(A/Ni).

Figure 8. Plot of -ln(1 - X) versus t to test the first-orderreaction type; X is the conversion of the hydrogenation of fur-fural: ([) (D) Ni74.5P12.1B13.4; (O) (C) Ni72.5P2.0B25.5; (2) (A)Ni71.4B28.9; (0) (B) Ni85.0P15.0; (×) (E) Raney nickel.

Figure 9. Selectivity for furfuryl alcohol as a function of con-version for the disappearance of furfural, catalyzed by the cata-lyst samples (b) (D) Ni74.5P12.1B13.4, (9) (B) Ni85.0P15.0, (/) (C)Ni72.5P2.0B25.5, (O) (A) Ni71.4B28.9, and (2) (E) Raney nickel.

Scheme 1

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2553

and the modified nickel metals result in the extremelydifferent hydrogenation activity of furfural to furfurylalcohol (specific activity per surface area: Ni85.0P15.0 .Ni71.4B28.9). Furthermore, phosphorus improves theselectivity for furfuryl alcohol. The higher selectivity(shown in Figure 9) for furfuryl alcohol over theseNi74.5P12.1B13.4 (82%) and Ni85.0P15.0(80%) catalysts ver-sus Ni72.5 P2.0B25.5 (69%), Ni71.4B28.9 (68%), and Raneynickel (62%) (see Table 2) can be attributed to thechange in electron density on the nickel metal inducedby phosphorus. This interaction between the conjugateddouble bond in the furan ring and the nickel metalmodified by phosphorus will handicap the furfur ringin the furfuryl alcohol to be further hydrogenated totetrafurfuryl alcohol.

As shown in Figures 10 and 11, there is an optimalvalue of the P/B ratio for the activity and selectivity ofthe Ni-P-B catalyst in the hydrogenation of furfural.The results indicate that, by regulating the Ni/P/B ratio,one is able to optimize the electron density of nickelmetal. It should be noted that the effect of the someelectron promoter on the catalytic activity is not mono-tonically increased or decreased with the content ofelectron promoter.

Regarding the relationship between electronic proper-ties and activities in hydrogenation reactions, a param-eter, ∆q ) (-∆E/K)(A/Ni), had been introduced34,35 onthe basis of the chemical shifts in the XPS bindingenergies of the additive elements, where ∆E is the XPSchemical shift for the component element A (B, P, etc.)in the metal (Ni) catalyst compared to that for pureelement A, the constant values of K were calculated fromthe published results (B1s ) 2.32, P2p ) 1),35,36 and (A/Ni) is the atomic ratio of a component element to metal

in the catalyst surface. ∆q is used to represent theextent of change of the electron density of Ni metalinduced by charge transfers from or to the additives.Okamoto et al.6,7 further correlate ∆q with the catalyticproperties of the nickel catalysts. The specific hydroge-nation activity per surface area of nickel metal forstyrene, cycloolefins, and acetone, the resistivity againstpoisoning, and the characteristic selectivity were sum-marized in terms of the parameter ∆q. They concludedthat ∆q is a useful parameter to reflect the electronicproperties of the nickel catalysts (Ni-M; M ) B, P, etc.).It seems intriguing to apply ∆q to other hydrogenationreactions in order to examine the electronic effects onthe reactions.

Although the definition of ∆q is theoretically imper-fect, it has been demonstrated that ∆q is a goodparameter to predict some catalytic properties of Nicatalysts: specific activities and selectivity in thehydrogenation. These results suggest the parametermay be useful in the characterization of modifiedcatalysts in terms of the electronic properties of Nimetal. It would be valuable to shown how these activi-ties and selectivities correlate with the electron densityof Ni metal modified by the additives in the Ni-P-Bcatalysts. In the study, by assuming the action ofdifferent electron transferences (boron donates electronsto nickel and phosphorus draws electrons from nickel),the modification of the electron density on the nickelmetal of Ni-P-B can be evaluated with ∆q (listed inTable 2). As show in Figures 12 and 13, the activity andselectivity can be correlated with ∆q. The results of thisstudy indeed show that a better catalyst can be obtainedby regulating the Ni/P/B ratio.

The fact that the electron transference between nickel

Figure 10. Selectivity of furfural alcohol versus P/(P + B) atomicratio.

Figure 11. Specific activity versus P/(P + B) atomic ratio.

Figure 12. Selectivity of furfural alcohol versus ∆q.

Figure 13. Specific activity versus ∆q.

2554 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999

and metalloids in the Ni-P-B catalyst is complexreflects why the mechanism is still not completelyunderstood. However, the modification of the hydroge-nation activities of Ni-P-B catalysts can be ascribedto the modification of the electron density on the nickelmetal. By regulating a suitable P/B ratio, the Ni-P-Bcatalyst dramatically reveals an extremely higher hy-drogenation activity for furfural than Ni-P and Ni-B.All the ultrafine Ni-Px-By amorphous alloy catalysts,by combining the effect of metalloid elements and thefeature of ultrafine amorphous structure, reveal amarkedly higher specific activity per surface area of thecatalyst than Raney nickel. The nature of ultrafineamorphous structure and the P/B ratio may be the keysto manipulate the catalytic properties of the ultrafineNi-Px-By amorphous alloy catalysts.

Conclusion

A series of ultrafine Ni-B, Ni-P, and Ni-P-Bamorphous alloy catalysts with various Ni/P/B ratioswere prepared by chemically reacting nickel acetate,sodium hypophosphite, and sodium borohydride inaqueous solution. On the basis of the results presentedherein, we could conclude the following:

The initial Ni/P/B molar ratio of starting materialsaffected the concentration of boron and phosphorusbonded to the nickel metal, resulting in the change ofthe surface areas, the amorphous structures, and thehydrogenation activities of the catalysts.

The modification of the hydrogenation activity andselectivity of these ultrafine Ni-B, Ni-P, and Ni-P-Bcatalysts can be ascribed to the modification of theelectron density on the nickel metal. The differentelectron transfers between nickel metal and the met-alloid elements (P and B) (phosphorus accepted elec-trons from the nickel metal, and boron donate electronsto the nickel metal) result in the extremely differenthydrogenation activities of furfural (specific activity persurface area: Ni85.0P15.0 . Ni71.4B28.9). By regulating asuitable P/B ratio, the ultrafine Ni-P-B catalystdramatically reveals an extremely higher hydrogenationactivity for furfural than Ni-P and Ni-B. The specificactivities per surface area of catalyst are in the followingorder: Ni74.5P12.1B13.4 > Ni72.5P2.0B25.5 > Ni85.0P15.0 .Ni71.4B28.9 > Raney nickel. The phosphorus is the activecomponent of the catalyst to improve the selectivity forfurfuryl alcohol. The nature of the ultrafine amorphousstructure and the P/B ratio may the keys to manipulatethe catalytic properties of the ultrafine Ni-Px-Byamorphous alloy catalysts.

Acknowledgment

The authors would like to thank the National ScienceCouncil of the Republic of China for financially sup-porting this research under Contract Number NSC 88-2214-E-008-008.

Literature Cited

(1) Wonterghem, J. V.; Morup, S.; Christion, J. W.; Charles,S.; Wells, W. S. Formation of Ultra-Fine Amorphous Alloy Particlesreduction in Aqueous Solution. Nature 1986, 322, 622.

(2) Corrias, A.; Ennas, G.; Licheri, G.; Marongiu, G.; Musinu,A.; Paschina, G.; Piccaluga, G.; Pinna, G.; Magini, M. Fe-Co-BAmorphous Alloy Powder by Chemical Reduction. J. Mater. Sci.Lett. 1988, 7, 407

(3) Linderoth, S.; Morup, S.; Meagher, A.; Larsen, J.; Bentzon,M. D.; Clausen, B. S.; Koch, C. J. W.; Wells, S.; Charles, S. W.

Amorphous to Crystalline Transformation of Ultrafine Fe62B38Particles. J. Magn. Magn. Mater. 1989, 81, 138.

(4) Linderoth, S.; Morup, S. Chemically Prepared AmorphousFe-B Particles: Influence of pH on the composition. J. Appl. Phys.1990, 67, 4472.

(5) Xue, D. S.; Li, F. S.; Zhou, R. J. Control of the Boron Contentin Amorphous Powder Prepared by Borohydried Reduction. J.Mater. Sci. Lett. 1990, 9, 506.

(6) Jiang, J.; Dezsi, U.; Gonser; Lin, X. A study of the Prepara-tion Conditions of Fe-B Amorphous Alloy Powders Produced byChemical Reduction. J. Non-Cryst. Solids 1990, 124, 139.

(7) Shen, J.; Hu, Y. Z.; Zhang, L. F.; Li, Y. Z.; Chen, Y. ThePreparation of Ni-P Ultrafine Amorphous Alloy Particles Pro-duced by Chemical Reduction. Appl. Phys. Lett. 1991, 59, 3545.

(8) Shen, J. Y.; Hu, Z.; Hsia, Y. F.; Chen, Y. Fe-P-B UltrafineAmorphous Particles Produced by Chemical Reduction. Appl. Phys.Lett. 1991, 59, 2510.

(9) Saida, J.; Inoue, A.; Masumoto, T. Preparation of Ultra-fineAmorphous Powders by the Chemical Reduction Method and theProperties of Their Sintered Products. Mater. Sci. Eng. 1991, A133,771.

(10) Deng, J. F.; Chen, H. Y. A Novel Amorphous Ni-W-PAlloy Powder and Its Hydrogenation Activity. J. Mater. Sci. Lett.1993, 12, 1508.

(11) Hu, Z.; Shen, J. Y.; Fan, Y. N.; Hsia, Y. F.; Chen, Y.Formation of Ultrafine Amorphous Alloy Particles with Uni-form Sized by Autocatalytic Method. J. Mater. Sci. Lett. 1993, 12,1020.

(12) Yamashita, H.; Yoshikawa, H.; Funabiki, T.; Yoshida, S.Catalysis by Amorphous Metal Alloys. J. Chem. Soc., FaradayTrans. 1 1986, 82, 1771.

(13) Okamoto, Y.; Nitta, Y.; Imanaka, T.; Teranishi, S. SurfaceCharacterisation of Nickel Boride and Nickel Phosphide Catalystsby X-ray Photoelectron Spectroscopy. J. Chem. Soc., FaradayTrans. 1 1979, 75, 2027.

(14) Deng, J.; Zhang, X.; Min, E. Amorphous Nickel-Phospho-rus Alloy Deposited on a Support and its Hydrogenation Activity.Appl. Catal. 1988, 37, 339.

(15) Deng, J.; Yang, J.; Sheng, S.; Chen, H.; Xiong, G. The Studyof Ultrafine Ni-B and Ni-P Amorphous Alloy Powders asCatalysts. J. Catal. 1994, 150, 434.

(16) Shen, J. Y.; Hu, Z.; Zhang, Q.; Zhang, L. F.; Chen, Y.Investiagtion of Ni-P-B Ultrafine Amorphous Alloy ParticlesProduced by Chemical Reduction. J. Appl. Phys. 1992, 71, 5217.

(17) McEvoy, J.; Shalit, H. Copper Chromite HydrogenationCatalyst. U.S. Patent 3,374,184, 1968.

(18) Seo, G.; Chon, H. Hydrogenation of Furfural over Copper-containning catalysts. J. Catal. 1981, 67, 424.

(19) Rao, R.; Dandekar, A.; Baker, R. T. K.; Vannice, M. A.Properties of Copper chromite catalysts in hydrogenation reaction.J. Catal. 1997, 171, 406.

(20) Schreifels, J. A.; Maybury, P. C.; Swartz, W. E., Jr. X-rayPhotoelectron Spectroscopy of Nickel Boride Catalysts: Correlationof Surface States with Reaction Products in the Hydrogenation ofAcrylonitrile. J. Catal. 1980, 65, 195.

(21) Baijun, L.; Lianhai, L.; Bingchun, W.; Tianxi, C.; Iwatani,K. Liquid phase Selective Hydrogenation of Furfural on Raneynickel Modified by Impregnation of Salts of Heteropolyacids. Appl.Catal. A 1998, 171, 117.

(22) Thomas, D. Catalyst Comprising Raney Nickel with Ad-sorbed Molybdenum Compound. U.S. Patent No. 4,153,578,1979.

(23) Izum, Y.; Tanaka, Y.; Urabe, K. Selective Catalytic Hy-drogenation of Propargyl Alcohol with Heteropoly Acid-ModifiedPalladium. Chem. Lett. 1982, 679.

(24) Tamaki, J.; Takagaki, H.; Imanaka, T. Surface Charac-terization of Amorphous Ni-B Films Treated with Oxygen. J.Catal. 1987, 108, 256.

(25) Honda, F.; Hirokawa, K. A Photoelectron SpectroscopicObservation of Iron Surfaces Exposed to N2, N2O, NO, NO2 andAir at 200 Torr or 1 atm. J. Electron. Spectrosc. Relat. Phenom.1976, 8, 199.

(26) Pelavin, M.; Hendrickson, D. N.; Hollander, J. M.; Jolly,W. L. Phosphorus 2p Electron Binding Energies Correlation withExtended Huckel Charges. J. Phys. Chem. 1970, 74, 1116.

(27) Ramachandran, P. A.; Chaudhari, R. V. Three-PhaseCatalytic Reactors; Gorden and Breach Science Publishers: NewYork, 1983; p 15.

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2555

(28) Carberry, J. J. Physico-Chemical Aspects of Mass and HeatTransfer in Heterogeneous Catalysis. In Catalysis, Science andTechnology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag:Berlin, 1987; Vol. 8, p 131.

(29) Wheeler, A. Reaction Rates and Selectivity in CatalystPores. In Advances in Catalysis; Academic Press: New York, 1951;Vol. 3, p 249.

(30) Weisz, P. B.; Prater, D. C. Interpretation of Measurementsin Experimental Catalysis. In Advances in Catalysis; AcademicPress: New York, 1954; Vol. 6, p 143.

(31) Folger, H. S. Elements of Chemical Reaction Engineering;Prentice Hall: Englewood Cliffs, NJ, 1992; p 670.

(32) Roberts, G. W. The Influence of Mass and Heat Transferon the Performance of Heterogeneous Catalysis in Gas/Liquid/SolidSystems. In Catalysis in Organic Synthesis; Rylander, P. N.,Greenfield, H., Eds.; Academic Press: New York, 1976.

(33) Okamoto, Y.; Nitta, Y.; Imanaka, T.; Teranishi, S. SurfaceState, Catalytic Activity and Selectivity of Nickel Catalysts inHydrogenation Reactions. J. Chem. Soc., Faraday Trans. 1 1980,76, 998.

(34) Okamoto, Y.; Nitta, Y.; Imanaka, T.; Teranishi, S. SurfaceState and Catalytic Activity and Selectivity of Nickel Catalystsin Hydrogenation Reactions III. Electronic and Catalytic Porper-ties of Nickel Catalysts. J. Catal. 1980, 64, 397.

(35) Hendrickson, D. N.; Hollander, J. M.; Jolly, W. L. Core-Electron Binding Energies for Compounds of Boron, Carbon andChromium. Inorg. Chem. 1970, 9, 612.

(36) Pelavin, M.; Hendrickson, D. N.; Hellander, J. M.; Jolly,W. L. Phosphours 2P, Electron Binding Energies, Correlation withExtended Huckel Charges. J. Phys. Chem. 1970, 74, 1116.

Received for review January 28, 1999Revised manuscript received April 28, 1999

Accepted May 5, 1999

IE990071A

2556 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999