Rejuvenation of Moisture-Exposed Pd/δ-Al2O3 Catalysts forIsoprene-Selective Hydrogenation: Correlation between PdDispersion and Catalytic Properties

Wei-Bin Su,† Mau-Tsu Tang,‡ and Jen-Ray Chang*,†

Center for Nano-technology Design & Prototyping, Department of Chemical Engineering, National ChungCheng University, Chia-Yi, Taiwan, R.O.C., and National Synchrotron Radiation Research Center,Hsinchu, Taiwan, R.O.C.

Exposure of commercial δ-alumina-supported Pd catalysts to moisture is unavoidable. Besideshandling the catalysts with care, development of rejuvenation procedure for moisture-exposedcatalyst is also necessary to minimize the economical loss. By use of in situ EXAFS (extendedX-ray absorption fine structure) spectroscopy and CO chemisorption, we observed (1) moistureadsorbed on the catalyst facilitates the mobility of palladium oxides, leading to formation ofbigger Pd clusters, and (2) moisture adsorbed on PdO could not be removed at temperatures of200 °C or below, whereas it can be removed significantly in flowing air at 550 °C without growthin PdO clusters. In contrast, in the flow on nitrogen at 550 °C, the PdO is decomposed to Pdmetal, which aggregates to large clusters. The impact of water exposure and the efficiency ofrejuvenation were further examined by isoprene hydrogenation at a weight hourly space velocity(WHSV) of 36 h-1 (g of feed/(h g of catalyst)), 40 °C, and 30 atm total pressures. The resultsindicate that moisture saturation caused the decrease in isoprene conversion and selectivity to2-methyl-2-butene (2M2B) by at least 20%. The losses of selectivity and activity can be recoveredto about 60% by flowing air at 550 °C to remove the adsorbed moisture. Moreover, the testresults show that isoprene conversion increases with Pd dispersion both at start of run and atnearly steady state, whereas the selectivity to 2-methyl-2-butene decreases with Pd dispersionat start of run and reaches a steady state as the time on stream is increased. The change ofselectivity to 2-methyl-2-butene can be explained on the basis of FT-IR characterizing COadsorbed on the Pd catalysts; Pd sites for CO chemisorption can be distinguished as bridgingand terminal sites, and the terminal site has a higher tendency to convert isopentene toisopentane and is more vulnerable to be poisoned by coke deposition.

Introduction

A two-stage hydrogenation process unit is used tostabilize and to reduce sulfur content of pyrolysisgasoline, a byproduct from naphtha cracker.1,2 The firststage of hydrogenation process is to saturate gumprecursors such as alkenyl aromatics and conjugateddiolefins to olefins.3 After hydrogenation, a C5 (hydro-carbon containing five carbon atoms) stream for use asthe feedstock of the tert-amyl methyl ether (TAME) isseparated from the pyrolysis gasoline in a depentanizer,and C9

+ is separated in a deoctanizer. The treatedpyrolysis gasoline is then sent to the second stagehydrogenation process to remove sulfur-containing com-pounds by use of coblt-molybdate catalysts. Since it isimportant to stabilize the gasoline without lowering theoctane number, an alumina-supported Pd catalyst isused for the first stage hydrogenation because of its highactivity, selectivity, and resistance to impurities in thefeed.2

Pd is in the form of oxide in a commercial catalyst.After being loaded into the reactor, palladium oxide is

reduced to Pd metal for the reaction. This reduction isparticularly important because the catalytic propertiesdepend to a large extent on it. It has been reported thatreduction of NiO in flowing H2 containing water needsto elevate temperature of 30-60 °C.4,5 The resultssuggest that moisture adsorbed on supported metalcatalysts may hinder hydrogen adsorption, hydrogendissociation, and nucleation of metal atoms, leading toa change of catalyst structure. Exposure of catalysts toair is unavoidable in a commercial operation. It isimportant therefore to know the effects of the moistureadsorption on the structure and catalytic properties ofthe Pd catalysts and to explore the rejuvenation condi-tions for the water-poisoned catalysts. To simulatecatalysts in a commercial plant, the catalysts werepurposely saturated with moisture prior to hydrogenreduction in this study.

Hydrogenation of isoprene at 30 atm was chosen asthe model reaction. Isoprene was chosen because it isone of the unstable species in the pyrolysis gasoline andthe catalyst life of TAME process is greatly influencedby the isoprene contained in the feed. The reaction wasrun at 30 atm because the operating pressure in acommercial run is within the range of 15-30 kg/cm2,which is a result of compromise; at too high a pressure,heat of reaction may not be removed adequately,whereas too low pressure may result in a plugging ofcatalyst bed caused by gum formation.

† National Chung Cheng University.‡ National Synchrotron Radiation Research Center.* To whom correspondence is addressed. Tel: 886-5-

2720411 ext 33455. Fax: 886-5-2721206. E-mail: chmjrc@ccunix.ccu.edu.tw.

1677Ind. Eng. Chem. Res. 2005, 44, 1677-1687

10.1021/ie049054y CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 02/23/2005

The structure of Pd catalyst samples during moistureadsorption and desorption process was monitored by useof in-situ extended X-ray absorption fine structure(EXAFS) spectroscopy, which has been shown to beparticularly successful in characterizing surface specieson nonreducible metal oxide and zeolite supports.6 Theeffects of moisture adsorption and the rejuvenationconditions on the dispersion of the catalysts wereexamined by use of CO chemisorption and EXAFSspectroscopy. The effects of moisture adsorption onthe reduction kinetics were studied by temperature-programmed reduction (TPR). The selectivity and activ-ity of the catalyst were examined in a fixed-bed reactionsystem with operation conditions (40 °C and 30 atm)similar to those of commercial plants.7 The influencesof moisture wetting on the electronic properties of Pdclusters and the acidity of the support were examinedby FT-IR characterizing CO and pyridine adsorbed onthe catalysts, respectively.

Experiment Section

Materials and Catalyst Preparation. The δ-Al2O3support was prepared by calcining γ-Al2O3 with aparticle size of about 2 mm (A2U, Osaka Yogyo) at 950°C in air under stagnant conditions for 6 h. Theresulting material had a bulk density of 0.68 g/cm3. TheBET surface area and pore volume measured with anOmnisop 360 analyzer was 82.4 m2/g and 0.57 cm3/g,respectively.

Eggshell Pd catalysts were prepared according to themethod reported by Lin and Chou.8 Palladium diacetate(0.210 g) was dissolved in 200 mL of toluene withstirring for 3 h, and 15 g of Al2O3 was then added. Thetoluene was removed by filtration and followed withevacuation. After calcinations at 550 °C in flowing airfor 3 h, the catalyst samples were stored in a desiccatorwith a N2 blanket. Since this sample was treatedwithout moisture exposure, we call it the moisture-freesample and noted as the MF sample. The Pd content ofthe catalyst sample was determined by inductivelyplasma optical emission spectroscopy (Jobin-Yvon JY-38) to be 0.43 wt %. The moisture-saturated (noted asMS) sample was prepared by contacting the moisture-free sample with moisture-saturated air (about 3.4 vol% moisture) for 48 h at room temperature with a 40mL min-1 g-1 space velocity.

Infrared Spectroscopy Characterizing CO Ad-sorbed on Pd Clusters and Pyridine Adsorbedon Support Samples. Infrared Fourier transformspectroscopy characterizing CO and pyridine adsorbedon the Pd catalyst samples was performed with aShimadzu FT-IR 8101M with a SSU-8000 second sam-pling unit. The wafer samples were load into an IR cell,which was designed to connect to a vacuum/gas-handling manifold for in situ treatment. For character-izing CO adsorbed on fresh catalyst, samples werereduced with flowing H2 (50 mL/min at 1 atm) in theIR cell, and then CO (flowing at 30 mL/min at 1 atm)was introduced into the cell and maintained for about30 min. After the CO treatment, the cell was evacu-ated to a pressure of approximately 10-3 Torr, and IRspectra having a 4 cm-1 spectral resolution were re-corded. For characterizing pyridine adsorbed on thesupport, moisture-saturated δ-alumina supports weredried by flowing N2 into the IR cell at temperatureelevated from room temperature to 500 °C. After drying,the sample was cooled to room temperature, and pyri-

dine was carried into the cell by N2 gas and maintainedfor about 20 min. After the pyridine treatment, the cellwas evacuated to a pressure of approximately 10-3 Torr,and IR spectra were recorded.

Differential Scanning Calorimeter. For exploringwater desorption temperature, MF samples were treatedwith moisture-saturated air and then characterized bya differential scanning calorimeter (DSC, Setaram, TG/DTA92/DSC121). About 50 mg catalyst was loaded intothe DSC cell, and the sample was heated at 10 °C/minfrom room temperature to a maximum temperature of550 °C with flowing nitrogen at about 1.8 L/h and 1 bar.On heating, heat flow (micro V.) was recorded.

Temperature-Programmed Reduction. The ap-paratus used for the temperature-programmed reduc-tion was described by Jones and McNicol.9 A gas streamof 10% H2 in argon was introduced and passed throughthe catalyst sample (0.05 g). When the thermal conduc-tivity detector (TCD) signal became stable, the samplein a quartz reactor was heated at 10 °C/min to 600 °Cby a temperature-programmed furnace, and the reduc-tion temperature was monitored with a K-type thermo-couple. The water produced by reduction was trappedinto a column of silica gel, and the amount of H2consumption was detected via the TCD. The outputpolarity was set so that positive TCD signals indicatedhydrogen consumption and a negative signal for hydro-gen evolved.

CO Chemisorption. A quartz tube was packed withabout 0.15 g of catalyst sample. After reduction, the tubewas connected to a three-way ball valve, and theconnection ports were carefully purged with He (200 °C,1 h), followed by cooling to room temperature. After thesystem became steady (20 mL/min He flow rate), a 60µL pulse of CO was repeatedly injected into the catalystbed with He carrier gas until none of the pulse waschemisorbed. The amount of chemisorption was thencalculated by summing up the proportions of all pulses.

Catalytic Performance Test. One MF sample andeight MS samples were tested in this study. To inves-tigate the role of moisture adsorption on the catalyticperformance, the MF sample was reduced with H2 at200 °C and 1 atm for 2 h (noted as MFH20) andcompared to the MS sample reduced at the sameconditions (MSH20). To study the effects of reductiontemperature and pressure, the MSH20 sample werecompared to the MS sample treated at 550 °C and 1atm (MSH55) and at 200 °C and 30 atm in H2 (MSH20P);note that the treatment conditions of MSH20P are thesame as those run in a commercial plant. To investigatethe effects of predrying, before reducing the MS samplewith H2 at 200 °C and 1 atm for 2 h, the sample waspredried with N2 at 200 °C (MSN20H20) and 550 °C(MSN55H20) for 2 h and with air at 200 °C (MSA20H20)and 550 °C (MSA55H20) for 2 h. In addition, thenitrogen predried sample was also reduced at 40 °C for2 h (MSN20H4) because TPR results suggested that thesample can be reduced at 40 °C, and the catalyticproperties were of interest.

Those samples were tested in a continuous-downflowfixed-bed reactor. The reactor was a vertical stainlesssteel tube with an inside diameter of 1.17 cm and aninside volume of 94 mL. It was heated by a water-bathcirculator, and the temperature was controlled with asensor at the center of the catalyst bed. The feed wasprepared by mixing 10 wt % isoprene (Merck) inn-heptane (Merck). The reactor was packed with 0.6 g

1678 Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005

of catalyst diluted with inert SiO2 (Merck) in a ratio of1:15 by volume. A gradient packing method was usedso that the catalyst bed would have a nearly uniformtemperature and the wall and bypassing effects wouldbe minimized. The ratio of bed length to catalyst particlediameter was approximately 25; the axial dispersioneffects are inferred to have been negligible.10 Theupstream part of the reactor was a preheated zone filledwith SiO2. After reduction, the hydrogenation reactionwas then carried out with a weight hourly space velocityof 36 h-1 (g of feed/(h g of catalyst)), at 40 °C, 30 kg/cm2, and a H2/oil molar ratio ) 2.26. A condensertrapped liquid products at -15 °C. Up to 98% feed wasrecovered as reaction products in the material balancetests.

Both gas and liquid samples were collected periodi-cally and analyzed by a gas chromatograph (Hewlett-Packard model 5890 A, FID model) coupled with a dataprocessor (SP4270). A capillary column (Petrocol DH150, 150 m × 0.25 mm i.d., 1.0 µm phase film) wasperformed with He flowing at 20 cm/s, starting at 30°C for 20 min and then increasing at a 10 °C/mintemperature-programming rate until it reached 150 °Cfor 0.5 h (0.9 µL split 100:1).

X-ray Absorption Spectroscopy. The X-ray ab-sorption measurements were performed on X-ray beam-line BL 12 B2 of Spring-8, Japan, with storage ringenergy of 8.0 GeV. A Si(111) double-crystal monochro-mator was used for energy selection, and it was detuned20% at E0 + 50 to suppress higher harmonic radiation;the resolution, ∆E/E, was estimated to be 2.0 × 10-4.The monochromator was scanned in energy from 200eV below the palladium K absorption edge (24 350 eV)to 1000 eV above the edge. The catalyst samples formeasurement were prepared in a fixed bed reactor withthe same operating conditions as those for catalyticperformance tests. Each sample was pressed into a self-supporting wafer in a N2-filled glovebox and then placedinto a N2-filled cell. The samples were measured in atransmission mode at room temperature. The transmis-sion measurement geometry was arranged using gas-fill ionization chambers to monitor the intensities ofincident and transmitted X-rays. To gain the properabsorption ratio for incoming X-rays, the gas composi-tion in the ionization chamber was selected at an argon-to-nitrogen mole ratio of 1/1 for the first chamber andpure argon for the second chamber.

In-situ EXAFS monitoring moisture adsorption anddesorption processes were measured on the supercon-ducting wavelength shifter (SWLS, commission in May2004) beamline 01C at the National Synchrotron Radia-tion Research Center in Hsinchu, Taiwan, with energyfrom 5 to 33 keV. The monochromator employs doubleSi(111) crystals for energy selection with a resolution(∆E/E) better than 2.5 × 10-4.

MF sample was loaded into EXAFS cell in a groovebox. The cell was connected to a vacuum/gas-handlingmanifold for in-situ treatment. During water adsorptionprocess, moisture-saturated air (about 3.4 vol % mois-ture) was introduced into the cell at room temperaturefor about 2 h, and then EXAFS measurement wasperformed. After the measurement, the temperaturewas increased at about 10 °C/min to 200 °C and to 550°C with the flow of dry air or nitrogen. After tempera-ture reaches the setting temperature, it was maintainedfor 1 h. Then the cell was allowed to cool to room temper-ature, and the EXAFS spectrum was again measured.

The EXAFS contributions for Pd-Pd were analyzedwith phase shifts and backscattering amplitudes ob-tained from EXAFS data for Pd foil. The Pd-O wereanalyzed with phase shifts and backscattering ampli-tudes calculated from FEFF.

Results

Moisture Adsorption and Desorption. DSC char-acterizing moisture-wetted catalyst samples indictedthat desorption of water begins at 60 °C to about 200°C, and TGA (thermal gravity analyzer) showed thatthe adsorption capacity of the 24 h moisture treatmentsample is about 28%. Because of sensitivity limitation,no significant peaks were observed at the desorptiontemperature higher than 200 °C. To investigate residualwater on surface at temperature higher than 200 °C,the samples were further examined by FT-IR character-izing pyridine adsorbed on the alumina support.

FT-IR Characterizing Pyridine Adsorbed onAlumina Supports. The infrared spectrum character-izing pyridine adsorbed on alumina is shown in Figure1. Two peaks, one at 1570 and another at 1445 cm-1,were observed for the sample at room temperature (25°C). After drying, the temperature was increased to 200°C and the band intensity of both peaks decreased,whereas when the drying temperature was furtherincreased, only the peak at 1570 cm-1 decreased withthe elevation of temperature. At 500 °C, almost nosignificant peaks were observed at the region between1500 and 1600 cm-1.

Hydrogen Reduction Characterized by TPR. Thetemperature reduction profiles characterizing MF andMS samples are shown in Figure 2. One positive peakat 96 °C with a shoulder at about 108 °C was observedfor the MS sample, whereas two peaks, one positivepeak at 53 °C and a negative peak at 103 °C, wereobserved for the MF sample. Since the polarity ofthermal conductivity detector has been set to be positivefor hydrogen consumption and negative for hydrogenevolved, the negative peak at 103 °C for the MF sampleis assigned as the decomposition of Pd-â hydride, andpositive peaks for both MS and MF sample are thecharacteristic peaks for the reduction of palladiumoxides. The shoulder at about 90 °C for the MS samplemay result from the combination of hydrogen consump-tion due to the reduction of palladium oxides and

Figure 1. FT-IR spectrum of pyridine on moisture-saturatedδ-alumina supports dried at elevated temperature (from bottomto top: 25, 200, 400, and 500 °C).

Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005 1679

hydrogen evolution due to the decomposition of Pd-âhydride.11-14 The TPR peaks for commercial catalyst arethe same as those for MS sample, suggesting that thecommercial catalysts might be poisoned by moisturecontamination during shipping, storage, and samplehandling.11

FT-IR Characterizing CO Adsorbed on Pd Cata-lysts. The infrared absorption bands of the CO adsorbedon the Pd catalysts are significantly affected in bothintensity and position by moisture poisoning and reduc-tion conditions (Figure 3). For the MF samples reducedat 200 °C, two CO adsorption bands located at 2060 and1910 cm-1 were observed and were assigned as theterminal and bridging CO ligand, respectively.15 The Pdsites, on which terminal CO and bridging CO areadsorbed, are noted as “sites-t” and “sites-b”, respec-tively. At the same reduction temperature (200 °C), theratio of terminal to bridging peak intensity for the MSsample (MSH20) is lower than that for the MF sample(MFH20), and an extra shoulder was observed at about1965 cm-1. As the reduction temperature was increasedto 550 °C (MSH55), the shoulder at 1965 cm-1 disap-peared, and the bridging peak shifts to lower frequency.

For the sample predried in N2 at 200 °C and thenreduced at 40 °C (MSN20H4), the IR spectrum is similarto that for the MSH20 sample, except there was noterminal peak being observed. After catalytic perfor-

mance test, only a bridging peak was observed, and theintensity is much lower in comparing with fresh catalystsample, MSH20. These results suggest that wateradsorption and coke deposition block terminal activesites.

EXAFS Data Analysis and CO Chemisorption.Preedge and background were subtracted from rawX-ray absorption data, and the resulting spectrum wasdivided by the edge height to obtain the EXAFS func-tion.16 Prior to the detailed EXAFS data analysis,k3-weighted Pd-Pd phase- and amplitude-correctedFourier transforms were determined for the EXAFSfunctions, ø(k) (4.0 < k < 14.0). The phase- andamplitude-corrected Fourier transforms are expressedby

where φj(k) is the phase shift function and F j(k) is thebackscattering amplitude characteristic of a particulartype of neighboring atom. By the transformation, theinteratomic distance can be read off immediately fromthe transform peak position.17 In addition, since theamplitude of the Fourier transformed EXAFS function

Figure 2. TPR profiles for (a) the moisture-saturated sample and(b) the moisture-free sample.

Figure 3. FT-IR spectrum in the carbonyl stretching regioncharacterizing CO adsorbed on supported Pd catalysts of differenttreatment conditions (from bottom to top: MSN20H4, MFH20,MSH20, MSH55, and used MSH20).

Figure 4. Magnitude of Fourier transform (4.0 < k3 < 14.0, Pd-Pd phase and amplitude corrected) of the EXAFS for the moisture-free and moisture-saturated samples reduced by H2 at differenttemperature and pressure (MFH20, solid line; MSH20, dashedline; MSH55, dotted line; MSH20P, dash-dotted line).

Figure 5. Magnitude of Fourier transform (4.0 < k3 < 14.0, Pd-Pd phase and amplitude corrected) of the EXAFS for the MSsamples predried in nitrogen or air at elevated temperaturefollowed by reduction in H2 at 200 or 40 °C (MSA55H20, solid line;MSN20H20, dashed line; MSN20H4, dotted line; MSN55H20,dash-dotted line; MSA20H20, dash-double-dotted line).

θn(r) ) 1(2π)1/2∫kmin

kmaxknø(k) e-iφj(k)

Fj(k)e2ikr dk

1680 Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005

peaking at about 2.8 Å reflects the metal particle size,the Fourier transforms provide qualitative informationabout the structure of Pd clusters. As shown in Figure4 and Figure 5, the Pd-Pd phase- and amplitude-corrected Fourier transformed EXAFS functions of thenine catalyst samples show that the peaks correspond-ing to the first Pd-Pd shell (at about 2.8 Å), and themagnitude is in the order MSN55H20 > MSH55 =MSN20H4 > MSH20 = MSH20P > MSN20H20 >MSA20H20 > MSA55H20 > MFH20.

A k2-weighted Fourier transformation without cor-rection was performed on the EXAFS function over therange 3.5 < k < 14 Å-1 for nine samples. The majorcontributions were isolated by inverse Fourier trans-formation of the data in the range 1.14 < r < 3.01 Å.The coarse structural parameters characteristic of thePd-Pd contribution were determined by fitting the k2-weighted Fourier-isolated EXAFS function in the range7.0 < k < 14.0 Å-1; the metal-support and metal-adsorbate contributions in this region are small. An

Table 1. Summary of EXAFS Resultsa

shell N R, Å 1000∆σ2, Å2 ∆E0, eV EXAFS reference

MFH20Pd-O(s) 0.5 ( 0.1 2.00 ( 0.03 -1 ( 3 9 ( 4 Pd-OPd-O(l) 0.4 ( 0.1 2.58 ( 0.03 -6 ( 2 4 ( 4 Pd-OPd-Pd 6.2 ( 0.4 2.73 ( 0.01 4 ( 1 1.4 ( 0.4 Pd-Pdvariance: k0 weighted ) 0.39, k2 weighted ) 0.56

MSA55H20Pd-O(s) 0.2 ( 0.1 2.07 ( 0.06 -1 ( 6 11 ( 7 Pd-OPd-O(l) 0.5 ( 0.1 2.54 ( 0.03 -2 ( 2 8 ( 4 Pd-OPd-Pd 7.1 ( 0.3 2.73 ( 0.01 3 ( 1 -0.7 ( 0.4 Pd-Pdvariance: k0 weighted ) 0.08, k2 weighted ) 0.11

MSA20H20Pd-O(s) 0.2 ( 0.1 2.00 ( 0.03 -1 ( 3 5 ( 4 Pd-OPd-O(l) 0.3 ( 0.1 2.57 ( 0.04 -6 ( 4 -1 ( 6 Pd-OPd-Pd 7.7 ( 0.3 2.74 ( 0.01 3 ( 1 0.2 ( 0.3 Pd-Pdvariance: k0 weighted ) 0.04, k2 weighted ) 0.06

MSN20H20Pd-O(s) 0.3 ( 0.1 2.02 ( 0.02 0 ( 2 0 ( 3 Pd-OPd-O(l) 0.3 ( 0.1 2.58 ( 0.06 -2 ( 4 0 ( 6 Pd-OPd-Pd 7.7 ( 0.4 2.74 ( 0.01 2 ( 1 0.7 ( 0.6 Pd-Pdvariance: k0 weighted ) 0.43, k2 weighted ) 0.58

MSH20Pd-O(s) 0.3 ( 0.1 2.02 ( 0.03 1 ( 3 4 ( 4 Pd-OPd-O(l) 0.3 ( 0.1 2.62 ( 0.04 -3 ( 2 -5 ( 3 Pd-OPd-Pd 8.2 ( 0.5 2.74 ( 0.01 3 ( 1 -1.0 ( 0.6 Pd-Pdvariance: k0 weighted ) 0.15, k2 weighted ) 0.13

MSN20H4Pd-O(s) 0.5 ( 0.1 2.16 ( 0.04 -1 ( 3 11 ( 2 Pd-OPd-Pd 8.1 ( 0.2 2.74 ( 0.02 2 ( 1 2.0 ( 0.2 Pd-Pdvariance: k0 weighted ) 0.09, k2 weighted ) 0.13

MSH55Pd-Pd 8.7 ( 0.1 2.74 ( 0.01 2.0 ( 0.2 0.4 ( 0.1 Pd-Pdvariance: k0 weighted ) 0.25, k2 weighted ) 0.30

MSN55H20Pd-Pd 10.5 ( 0.4 2.75 ( 0.01 1.0 ( 0.1 0.8 ( 0.1 Pd-Pdvariance: k0 weighted ) 0.13, k2 weighted ) 0.15

MSH20PPd-O(s) 0.3 ( 0.1 2.00 ( 0.03 -1 ( 2 9 ( 4 Pd-OPd-O(l) 0.3 ( 0.1 2.60 ( 0.04 -3 ( 3 4 ( 4 Pd-OPd-Pd 8.0 ( 0.6 2.74 ( 0.01 3 ( 1 0.4 ( 0.5 Pd-Pdvariance: k0 weighted ) 0.11, k2 weighted ) 0.15

a N, the coordination number for the absorber-backscattering pair; R, the average absorber-backscattering distance; ∆σ2, the differencein Debye-Waller factors between sample and standard; ∆E0, the inner potential correction; variance ) 100 × (∫(kn(Xmodel(k) - Xexp(k))2/∫(kn(Xexp(k))2).

Table 2. Summary of Catalyst Preparation, Pd Dispersion, and Catalytic Propertiesa

Pd dispersion and catalytic propertiessample name(sample, treatment conditions) CO/Pd conv (SOR) conv (EOR) sel (SOR)

MFH20 (MF, H2 200 °C) 0.46 0.882 0.819 0.070MSA55H20 (MS, air 550 °C, H2 450 °C) 0.37 0.838 0.741 0.102MSA20H20 (MS, air 200 °C, H2 450 °C) 0.27 0.760 0.670 0.120MSN20H20 (MS, N2 200 °C, H2 450 °C) 0.26 0.761 0.664 0.129MSH20 (MS, H2 200 °C) 0.25 0.762 0.679 0.112MSN20H4 (MS, N2 200 °C, H2 40 °C) 0.23 0.737 0.661 0.201MSH55 (MS, H2 550 °C) 0.20 0.692 0.585 0.270MSN55H20 (MS, N2 550 °C, H2 200 °C) 0.18 0.699 0.568 0.302

a MF, moisture-free sample; MS, moisture-saturated sample; SOR, start of run; EOR, end of run (nearly steady state); Pd dispersion(CO/Pd): mol of CO uptake/mol of Pd.; conv, isoprene conversion; sel, selectivity to isopentene.

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EXAFS function calculated from these parameters wasthen subtracted from the raw data (Fourier-isolatedEXAFS function). Such a residual spectrum was ex-pected to represent the metal-support and metal-adsorbate interactions. EXAFS spectroscopy character-izing a metal-support interface in highly dispersedmetal catalysts has been reported by Koningsberger andGates.18 Normally, when a supported-metal catalyst isreduced in hydrogen at a relatively low temperature(<350 °C), M-O distances of 2.0-2.2 Å (hereafterreferred to as M-O(s), where subscript s refers to short)and 2.5-2.7 Å (hereafter referred to as M-O(l), wheresubscript s refers to long) are observed. The structuralparameters characteristic of the contributions of themetal-support interface were then estimated by fittingthe resulting spectrum obtained from the subtractionof the Pd-Pd contributions from the raw data. ThePd-O contributions were subtracted from the raw data,and better parameters for Pd-Pd contribution wereestimated. The structural parameters obtained from therecursive iteration method were then used as initialguess for nonlinear least-squares multiple-shell fittingroutine.19

Criteria of Fit. The criterion of a satisfactory numer-ical fit requires good agreement between experimentaland calculated results with k0- and k2-weighted EXAFSfunctions. Based on XDAP user guide,19 a reasonablevalue for a good fit is to have the variance, which isdefined in Table 1, for the fit down to around 1 (a 1%difference between model and data). For the sampleswith coordination number of the first Pd-Pd shell lessthan 8.0, the variance is higher than 1.0, and asignificant peak appeared at about 2.6 Å for the residualspectrum, Xexp(k) - Xmodel(k), when the contribution ofPd-O(l) was not considered. Hence, for those samples,both Pd-O(l) and Pd-O(s) have to be considered for goodfitting.

The structural parameters obtained from the fittingwith the CO chemisorption results (CO/Pd, mol of COuptake/mol of Pd) are summarized in Tables 1 and 2.The amount of CO adsorbed on Pd clusters is inverselyproportional to the coordination number of Pd-Pd,indicating that the Pd dispersion characterized by thesetwo techniques is consistent.

In-Situ EXAFS Experiments. Results of in-situEXAFS monitoring water adsorption and desorption areshown in Figure 6a. To magnify the EXAFS changeresulting from moisture adsorption and desorption, theEXAFS spectrum characterizing the MF sample wassubtracted from each spectrum. After Pd-O phase-corrected Fourier transform was performed on theresidue spectrum, a peak at about 3 Å was appearedfor the MF sample treated with moisture-saturated air(Figure 6b). For an X-Y absorber-backscattering pair,a peak, which has a positive imaginary part of thephase-corrected EXAFS function, is due to neighbors oftype Y.20,21 The results suggested that water wasadsorbed on palladium oxide. The characteristic peakof water adsorption did not have any change when thesample was heated from room temperature to 200 °C.After the treatment temperature was increased to 550°C in air, the peak decrease significantly. In contrast,after the MS sample was treated in N2 at 550 °C,palladium oxides were decomposed, and a Pd-Pdcontribution was observed (Figure 6c).

Catalytic Performance Tests by Isoprene Selec-tive Hydrogenation. Hydrogenation of isoprene is a

consecutive reaction. By 1,2 addition, isopentenes, 3-methyl-1-butene (3M1B), and 2-methyl-1-butene (2M1B)are produced. The isomer 2-methyl-2-butene (2M2B) isformed by 1,4 addition and the double-bond migrationof 2M1B and 3M1B. Further hydrogenation of iso-pentenes, including 2M1B, 3M1B, and 2M2B, yields theundesired product isopentane.22

Figure 6. (a) In-situ EXAFS monitoring moisture adsorption anddesorption process: MF sample (solid line), MS sample (dottedline), MS sample dried in air at 200 °C for 1 h (dashed line), andMS sample dried in air at 550 °C for 1 h (dash-dotted line). (b)Magnitude and imaginary part of the EXAFS transform (3.8 < k1

< 11.5, Pd-O phase corrected) for EXAFS of MS sample minusEXAFS of MF sample (solid line), EXAFS of the MS sample driedin air at 200 °C for 1 h minus EXAFS of MF sample (dotted line),and EXAFS of the MS sample dried in air at 550 °C for 1 h minusEXAFS of MF sample (dash-dotted line). (c) Magnitude andimaginary of Fourier transform (4.0 < k2 < 12.0, Pd-Pd phaseand amplitude corrected) for the MS sample dried in nitrogen at550 °C for 1 h.

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The total isoprene conversion is shown as a functionof time on stream in the flow reactor for the ninesamples (Figure 7a,b). The order of the conversion foreach sample is the same as that for Pd dispersioncharacterized by EXAFS and CO chemisorption. Theconversion decreased with time on stream and becamenearly time invariant after 100 h on stream. In contrast,selectivity to isopentenes [isopentenes/(isopentenes +isopentane)], which has about 2% deviation caused bythe deviation of gas chromatography measurement]increased with time on stream and became time invari-ant after about 100 h (Figure 8a,b). Except for MSH55,the ratio of 2M2B to the sum of 2M1B and 3M1B(2M2B)/(2M1B + 3M1B), increased with time on streamto a maximum at about 20-50 h and then decreased(Figure 9a,b).

Discussion

Impact of Moisture. Since moisture contaminationis unavoidable, the principal objective of this researchis to investigate rejuvenation conditions, thereby mini-mizing the economical loss due to the degrading of thecatalysts.

Effect of Moisture on Dispersion. To understand theeffects of moisture adsorption on catalyst structure, MF(moisture-free) and MS (moisture-saturated) sampleswere reduced at a typical commercial condition of 200

°C and characterized by EXAFS, FT-IR, and CO adsorp-tion. In comparing Fourier transformed EXAFS func-tions, the peak for the moisture-free catalyst sample,MFH20, is much lower than that of MSH20 (Figure 4).The Pd dispersion of the catalysts was determined bydetailed EXAFS analysis and CO chemisorption: forMFH20, the Pd-Pd coordination number and CO-to-Pd mole ratio (CO/Pd) are 6.2 and 0.46, respectively,and those for MSH20 are 8.2 and 0.25, respectively.These results indicted a loss of metal dispersion causedby water adsorption and are consistent with the experi-mental results reported in the literature.23,24 The roleof water adsorption in enhancing Pd aggregation hasnot been fully understood. However, on the basis of thesintering mechanism proposed by Geus,25 we suggestthat the enhancement in sintering might be attributedto the decrease the metal-support interactions causedby the adsorption of water on PdO.

Thermal Stability of Pd Clusters in Reducing Atmo-sphere. To investigate the thermal stability of Pdclusters on the water-adsorbed alumina, the reductiontemperature for the MS sample was further elevatedto 550 °C; CO chemisorption and EXAFS results indi-cated that sintering of Pd clusters was pronounced.

Figure 7. (a) Isoprene conversion at temperature ) 40 °C, P )30 kg/cm2, H2/oil ) 2.26, LHSV ) 36 h-1 catalyzed by the MFsample treated by H2 at 200 °C, and the MS sample treated by H2at elevated temperature and pressure (O, MFH20; ], MSH20; 4,MSH20P; ×, MSH55). (b) Isoprene conversion at temperature )40 °C, P ) 30 kg/cm2, H2/oil ) 2.26, LHSV ) 36 h-1 catalyzed bythe MF sample treated by H2 at 200 °C, and MS samples predriedin nitrogen or air at elevated temperature followed by reductionin H2 at 200 or 40 °C (O, MFH20; ], MSA55H20; 4, MSN20H20;×, MSA20H20; 3, MSN20H4; +, MSN55H20).

Figure 8. (a) Isopentenes selectivity at temperature ) 40 °C, P) 30 kg/cm2, H2/oil ) 2.26, LHSV ) 36 h-1 catalyzed by the MFsample treated by H2 at 200 °C, and the MS sample treated by H2at elevated temperature and pressure (O, MFH20; ], MSH20; 4,MSH20P; ×, MSH55). (b) Isopentenes selectivity at temperature) 40 °C, P ) 30 kg/cm2, H2/oil ) 2.26, LHSV ) 36 h-1 catalyzedby the MF sample treated by H2 at 200 °C, and MS samplespredried in nitrogen or air at elevated temperature followed byreduction in H2 at 200 or 40 °C (O, MFH20; ], MSA55H20; 4,MSN20H20; ×, MSA20H20; 3, MSN20H4; +, MSN55H20).

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Unlike the supported metals, such as Pt, Rh, Ir, andRu, which are thermally stable in reducing and inertatmospheres, Pd clusters significantly aggregate inhydrogen.23

Effects of Pressure. Pd catalysts were often reducedwith H2 at 200 °C and 30 atm in a commercial run.7 Asshown in Figure 4 and Table 1, the structure ofMSH20P is very close to that of MSH20. The dataindicate that severe reduction conditions will not im-prove the Pd dispersion of the catalysts.

Efficacy of Rejuvenation Conditions. The natureof the surface species was characterized by in-situ FT-IR and EXAFS. The infrared spectrum characterizationof pyridine-adsorbed alumina support provides theinformation on water desorption and surface acidity atelevated temperature. EXAFS provided the structureinformation on the Pd catalysts during moisture ad-sorption and desorption. Thus, combined analyses of FT-IR and EXAFS guide us to the optimum rejuvenationconditions.

In-Situ EXAFS Monitoring Drying Process. Normally,nitrogen or air is used in industry to dry moisture-wetted catalysts. Thus, the MS sample was treated byflowing air and nitrogen at elevated temperatures to

desorb the adsorbed water. As shown in Figure 6b, theintensity of the EXAFS peak characterizing adsorbedwater does not decrease at all at 200 °C, indicatingmoisture adsorbed on PdO is not removed at dryingtemperature at 200 °C or below. In flowing air at 550°C, however, the peak is decreased significantly, indi-cating that the adsorbed water can be removed at thattemperature. In contrast, in flowing nitrogen, the PdOis decomposed and the resulting Pd aggregates to largePd clusters (Figure 6c).

Effects of Rejuvenation Conditions on Dispersion. Asshown in Figure 5 and Tables 1 and 2, the Pd dispersionis increased with increasing drying temperature whenthe MS sample was treated in air. On the contrary, thedispersion of the catalyst is decreased with increasingdrying temperature when the MS sample was dried innitrogen. When the sample was treated in nitrogen, PdOis decomposed concomitant with the removal of wateron the catalysts. Since the Pd cluster on alumina ismobile, subsequent treatment in hydrogen further ag-gregates the Pd cluster.

As for temperature effects, we expected that thedispersion of the Pd catalysts would be inverselyproportional to reduction temperature; the lower thereduction temperature is, the higher the dispersion.Hence, the MSN20H4 sample is expected to have betterdispersion than do MSN20H20. Surprisingly, the op-posite was observed from experimental results. Theresults remind us that the effects of water, which formedduring the reduction, on sintering cannot be overlooked.The water adsorbed on Pd clusters lessens metal-support interactions, leading to aggregation of Pdclusters.

Correlations of Pd Dispersion with CatalyticPerformance. To test a catalyst sample for commercialrun is very time-consuming. As shown in Figure 7, thecatalyst properties are not stable before 150 h onstream. Therefore, catalytic properties were correlatedwith Pd dispersion, and the correlations can hopefullybe used in industry to predict catalyst performance,thereby accelerating the efficiency in exploring optimalrejuvenation conditions.

Correlation of Pd Dispersion with Catalytic Propertiesat Nearly Steady State. The purposes of selectivehydrogenation process for pyrolysis gasoline are tostabilize gasoline without octane number loss and toprovide feed for TAME process by converting diolefinsto olefins. Among the isomers of isopentenes, only 2M2Band 2M1B are valuable for the TAME process, and2M2B is the most desirable product of the hydrogena-tion of isoprene. Hence, the desirable Pd catalysts wouldgive high isoprene conversion and high selectivity tototal isopentenes, particularly high 2M2B to (2M1B +3M1B) ratio.

From an industrial application point of view, thecatalyst performance at the start of run is less importantthan that at nearly steady state (about 200 h on stream).As shown in Figure 9, at nearly steady state, the 2M2Bto (2M1B + 3M1B) ratio has a tendency to increase withPd dispersion. Since 3M1B and2M1B are formed by 1,2addition of isoprene, whereas 2M2B is formed by 1,4addition and the double-bond migration (isomerization)of 3M1B and 2M1B,22 apparently, isomerization of3M1B and 2M1B to 2M2B is promoted by high-dispersed Pd catalysts. It has been reported by Germainthat isomerization of alkenes at low temperature isgoverned by free hydrogen.26 Since the hydrogen dis-

Figure 9. (a) Ratio of 2M2B to (2M1B + 3M1B) at temperature) 40 °C, P ) 30 kg/cm2, H2/oil ) 2.26, LHSV ) 36 h-1 catalyzedby the MF sample treated by H2 at 200 °C, and the MS sampletreated by H2 at elevated temperature and pressure (O, MFH20;], MSH20; 4, MSH20P; ×, MSH55). (b) Ratio of 2M2B to (2M1B+ 3M1B) at temperature ) 40 °C, P ) 30 kg/cm2, H2/oil ) 2.26,LHSV ) 36 h-1 catalyzed by the MF sample treated by H2 at 200°C, and MS samples predried in nitrogen or air at elevatedtemperature followed by reduction in H2 at 200 or 40 °C (O,MFH20; ], MSA55H20; 4, MSN20H20; ×, MSA20H20; 3,MSN20H4; +, MSN55H20).

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sociation rate is proportional to metal dispersion, cata-lysts of higher dispersion are expected to have higher2M2B selectivity as observed in the present study.However, the selectivity to isopentenes is insensitive tocatalyst dispersion and maintained at about 90% selec-tivity at nearly steady state.

The correlations clearly indicate that the benefit ofthe pyrolysis-gasoline-hydrogenation process is de-teriorated by the loss of Pd dispersion due to thecatalysts exposed to moisture. To minimize the eco-nomical loss, we suggest a rejuvenation procedure basedon EXAFS, FT-IR, and chemical reaction results. Theprocedure is to dry moisture-contaminated Pd catalystsin air with temperature high enough to remove adsorbedmoisture and then to reduce the dried catalyst inhydrogen with temperature high enough to remove thewater formed during reduction.

In this study, catalytic performance tests indicate thatmoisture saturation causes the decrease of isopreneconversion and selectivity to 2-methyl-2-butene (2M2B)by 20% at nearly steady state of 200 h on stream. Basedon the operation temperature vs time on stream dataof a commercial plant reported by Lin and Chou27 andthe activation energy reported by Cheng et al.,11 theactivity loss in exposing to moisture can be compensatedby the raising reaction temperature of about 15 °C,which is about 25% of the workable temperature range;i.e., the operation cycle is shortened by about a quarterdue to water exposure of the catalysts. Rejuvenating theMS catalyst with flowing air at 550 °C to remove theadsorbed water and then reducing the catalyst with H2at 200 °C can recover 60% selectivity and activity loss.

Active Sites Responsible for Isopentene Conversion.The total isoprene conversion is shown as a function oftime on stream (Figure 7). The conversion decreasedwith time on stream and became nearly time invariantafter 100-200 h. Concomitant with the catalyst de-activation, the selectivity to isopentenes increases withtime on stream. Since gum buildup from the polymer-ization of diolefins has been reported as the main reasonfor catalyst deactivation,27-29 the disappearance ofterminal CO adsorption peaks after test reaction (Figure3) suggests coke could preferentially block active “site-t”. Selectivity to isopentenes is enhanced by cokedeposition, suggesting that the “site-t” is relativelyelectron deficient and has a higher tendency to convertisopentenes to isopentanes.

Correlation of Dispersion with Catalytic Properties atStart of Run. From an academic point of view, it is veryimportant to correlate a catalyst structure with itsinitial catalytic properties in order to understand thefundamental of catalysis. The correlation has beenstudied by many researchers.30-32 Hydrogenation reac-tions of unsaturated hydrocarbon on palladium havebeen found to be sensitive for highly dispersed Pdcatalysts.31 Ryndin et al. have attributed the low specificactivity (conversion of unsaturated hydrocarbon peractive sites) of highly dispersed Pd catalyst to be causedby a stronger chemisorption of dienes on Pd clusters.31,32

Bazin et al. have reported a strong correlation existedbetween the electronic density and the metal cluster sizeof a nanometer scale through ab initio calculation.33

Recently, the model proposed by Mojet et al. indicatesthat when metal-support interactions decreases withincreasing Pd cluster size, the binding energy of exposedPd atoms decreases as well.34 Although the structuralparameters of metal-support interactions, Pd-O, are

less precise than those of Pd-Pd because of the rathersmall EXAFS contributions, we do observe that thecoordination number of metal-support interactions,NPdO (NPdO(s) + NPdO(l)), is roughly proportional to Pddispersion in this study. This result suggests that Pdatoms in contact with support increases with Pd disper-sion. Hence, the decrease in isopentene selectivity withPd dispersion (Figure 10) is consistent with the decreasein electronic density with increasing catalyst dispersionand metal-support interactions.

When CO is adsorbed on Pd clusters, electron transferoccurs from the d orbital of Pd to the π* (antibonding)orbital of CO.35 A catalyst sample with greater electrondensity on the Pd clusters causes more back-electron-donation from Pd to the CO π* orbital. This back-electron-donation effect strengthens the Pd-C bond andweakens the C-O bonding. As shown in Figure 3, theratio of terminal CO magnitude to bridging CO magni-tude decreases with decreasing catalyst dispersion, andthe frequency of bridging CO has a tendency to shift tolower frequency. The combined FT-IR (Figure 3), COchemisorption, and EXAFS results (Tables 1 and 2) thussuggest a Pd catalyst with higher metal dispersion hashigher metal-support interactions and lower electronicdensity of Pd clusters. The decrease of specific activityand isopentene selectivity with increasing Pd dispersionmight be caused by the increase in overlap betweend-orbital of surface Pd atom and the π-orbital of iso-prene. Increasing the affinity between Pd clusters and

Figure 10. (a) Isoprene conversion catalyzed by δ-alumina-supported Pd catalysts as a function of Pd dispersion (CO/Pd): O(solid line), at start of run; ] (dashed line), at nearly steady state.(b) Isopentene selectivity catalyzed by δ-alumina-supported Pdcatalysts as a function of Pd dispersion (CO/Pd): O (solid line), atstart of run; ] (dash line), at nearly steady state.

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isoprene hinders isoprene conversion while promotingthe conversion of isopentene to isopentane.

It is interesting to observe that the residual moistureon the fresh catalyst decreases the selectivity for iso-pentenes. As shown in Figure 10b, the selectivity ofMSH20, MSA20H20, and NSN20H20 was lower thanthat predicted by the correlation curve, which is ob-tained by the correlation of MF samples and the MSsamples reduced at 550 °C. The lower selectivity couldbe caused by the interaction of Bronsted acid formedfrom residual water with the Pd clusters, evidenced bythe observation of a small peak observed for the FT-IRcharacterizing MSH20 sample.

Conclusion

Our experimental results indicate that the activityand selectivity of the Pd catalyst are deteriorated bymoisture due to Pd agglomeration. Now, we found afeasible procedure to rejuvenate the moisture-exposedcatalyst. In-situ EXAFS studies indicate that wateradsorbed on Pd/δ-Al2O3 catalysts can significantly beremoved at 550 °C in flowing air without agglomerationof Pd clusters. In contrast, flowing nitrogen at the sametemperature led to PdO decomposition and formationof big Pd clusters. This result combined with catalyticperformance tests for the MS samples rejuvenated atdifferent conditions suggests that the water-poisonedcatalyst can be rejuvenated by flowing air at tempera-ture high enough to remove water and then reducingthe dried catalyst in hydrogen at temperature highenough to remove the water formed in the reduction.By drying in flowing air at 550 °C, followed withhydrogen reduction at 200 °C, about 60% of activity andselectivity loss in moisture poisoning is recovered.Correlation between catalytic properties at nearly steadystate and Pd dispersions indicates that the conversionof isoprene and 2M2B/(1M1B + 3M1B) increases withincreasing catalyst dispersion while selectivity to totalisopentenes is insensitive with Pd dispersion. Thiscorrelation results also suggest that the rejuvenationprocedure can further be tuned to accommodate com-mercial needs.

Acknowledgment

The EXAFS data were analyzed using the XDAP DataAnalysis Program, developed by M. Vaarkam, J. C.Linders, and D. C. Koningsberger. The support of theNational Science Council of the Republic of China(Contract NSC-92-2120-M194-002), Spring-8, Japan,NSRRC, Taiwan, and the Refining & ManufacturingResearch Center of the Chinese Petroleum Corp. isacknowledged.

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Received for review September 28, 2004Revised manuscript received December 30, 2004

Accepted January 14, 2005

IE049054Y

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