Carbon Laydown Associated with Furan Hydrogenation over

Sep 25, 2003 - TPO and TPR confirmed the presence of retained species with varying surface energetics related to their chemical natures and their adso...
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Ind. Eng. Chem. Res. 2003, 42, 5489-5494

5489

Carbon Laydown Associated with Furan Hydrogenation over Palladium/Zirconia S. David Jackson,* Arran S. Canning, Elaine M. Vass, and Simon R. Watson† Department of Chemistry, The University of Glasgow, Glasgow G12 8QQ, Scotland

The hydrogenation of furan over Pd/zirconia has been studied at 373 K and 1 atm and two hydrogen/furan ratios. As expected, significant carbon laydown occurs. From FTIR spectroscopy, the primary surface species are furan and THF. Contrary to expectations, no decomposition to carbon monoxide and a C3H3 moiety was observed, i.e., no carbon monoxide was detected. The infrared assignments were confirmed by TPR over the used catalysts using hydrogen, which resulted in the desorption of THF, indicating that the fracture of the ring did not occur under hydrogenation conditions. At high temperatures, methane was formed under TPR conditions from the hydrogenolysis of adsorbed furan/THF. TPO and TPR confirmed the presence of retained species with varying surface energetics related to their chemical natures and their adsorption sites. Introduction Furan hydrogenation can be performed over most of the group VIII metals;1-3 however, research on the hydrogenation of furan is not extensive. Nickel3-5 and palladium1,6 are the metals of choice for this reaction, but even with these catalysts, little has been done in terms of investigating the actual catalytic process. Therefore, we set out to investigate the fundamentals of furan hydrogenation over a palladium/zirconia catalyst. Early studies in the literature1,7 indicated that there was a problem of catalyst deactivation, as both palladium black and Pd/C deactivated rapidly. A tendency toward decomposition (and hence deactivation) has also been seen in studies of furan adsorption on palladium.8-11 At low coverages in ultrahigh vacuum conditions over Pd single crystals, decomposition to CO and C3H3 has been observed, followed by coupling of the C3H3 units to give benzene.9,11 Under high-temperature hydrogenation reaction conditions, fracture of the furan has also been observed to give carbon monoxide and C-3 molecules.12 Therefore, we decided to study the deactivation of the system using a pulse-flow technique and to examine the surface deposit by infrared spectroscopy, isotope exchange, temperature-programmed oxidation (TPO), and temperature-programmed reaction spectrometry (TPRS). As the amount of dihydrogen present often affects the extent and nature of a carbonaceous deposit,13 changing the hydrogen/furan ratio was also examined. Experimental Section Pulsed reaction studies were performed in dynamic mode using a pulse-flow microreactor system with an on-line GC, in which the catalyst sample was placed in a glass-lined stainless steel tube (8 mm o.d.), in a vertical position, inside a furnace. Using this system, the catalyst (typically 0.27 g) could be reduced in situ * To whom all correspondence should be addressed. E-mail: [email protected]. † Current address, Johnson Matthey, PO Box 1, Billingham, Cleveland TS23 1LB, U.K.

in flowing dihydrogen (90 cm3‚min-1) by being heated to 523 K at 10 K‚min-1 and then held at this temperature for 1 h. After reduction had ceased, the catalyst was maintained at the desired temperature in either flowing 4% dihydrogen in dinitrogen or flowing dihydrogen (90 cm3‚min-1). Liquid furan was admitted by injecting pulses of known size (typically 2.5 µL) into the 4% dihydrogen/dinitrogen carrier-gas stream with resulting vaporization and hence exposure to the catalyst. After passage through the catalyst bed, the total contents of the pulse were analyzed by GC. The amount of gas reacted from any pulse was determined from the difference between the calibration peak areas and the peak areas obtained following the injection of pulses of comparable size onto the catalyst. Adsorption, desorption, and reaction were followed using a gas chromatograph fitted with a thermal conductivity detector and Porapak Q-S column and coupled to a mass spectrometer (Hiden HPR20). TPO was performed after the reaction testing by heating the sample from 293 to 773 K in 4% dioxygen/helium at a heating rate of 10 K‚min-1 and a flow rate of 40 cm3‚min-1. The effluent was continuously monitored by mass spectrometry. TPR was performed after reaction testing by heating the sample from 293 to 773 K in 4% dihydrogen/helium at a heating rate of 10 K‚min-1 and a flow rate of 40 cm3‚min-1. The effluent was continuously monitored by mass spectrometry. Reactions were performed in a continuous-flow stainless steel microreactor, specially designed for vaporphase reactions. The catalyst was place in a stainless steel reaction tube located in temperature-controlled oven with in situ thermocouple. The catalyst was reduced in situ with 100% H2 (BOC, purity ) 99.995%) at 20 cm3‚min-1 for 30 min at 523 K, with a ramping rate of 10 K‚min-1. After reduction, the catalyst was cooled to the reaction temperature, 373 K, under a 20 cm3‚min-1 H2 stream. Furan (Aldrich 99%) was placed in a glass bubbler on the reaction line, where the inlet 100% H2 was sent through at a flow rate of 20 cm3‚min-1 to collect the vapor formed in the bubbler. The reactant was then passed through the catalyst, and samples were taken via a gas sample valve attached to a HewlettPackard HP6890 series gas chromatograph with a Cp-

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Al2O3/Na2SO4 capillary column and an FID detector. The bubbler was cooled in an ice bath to give a H2/furan ratio of 3.7:1. TPR and TPO experiments were performed on an atmospheric glass reactor using highpurity 100% H2 (BOC, purity > 99.995%) or 2% O2/Ar (BOC, purity > 99.99%), respectively. A sample of the catalyst was taken after a hydrogenation experiment and transferred into a quartz U-type reactor mounted in a temperature-controlled furnace, where it was flushed with 100% H2 or 2% O2/Ar. The sample was heated at 10 K‚min-1 from 293 to 873 K. The gaseous products were analyzed by a Genesys quadrupole multiion mass spectrometer, where the TPR or TPO trace could be analyzed. Infrared spectra were obtained using a commercial FTIR spectrometer (Nicolet 5DXC). The studies were performed in transmission mode, with the catalyst in the form of a pressed disk, using an environmental cell. All spectra were recorded at 4 cm-1 resolution with the co-addition of 100 scans using a TGS detector. Using the environmental cell, the catalysts could be reduced in situ, and furan could be admitted with or without dihydrogen. The catalyst was reduced in flowing dihydrogen (250 cm‚min-1) for 0.25 h. The cell was then isolated, and 0.29 µmol of furan was injected into the cell with the catalyst and the 1 atm of dihydrogen. Spectra were recorded of the reaction. Subsequently, the cell was evacuated, and a spectrum was recorded. The cell was again isolated, and 0.22 mmol of [2H]dihydrogen was introduced. A spectrum was recorded; then the cell was evacuated, and a final spectrum taken. A second procedure was also adopted. In this procedure, the sole difference from that described above was the quantity of dihydrogen present at the initial reaction. In the second method, after reduction, the cell was isolated and evacuated. Immediately thereafter, 2.0 µmol of dihydrogen was injected into the cell, followed by 0.29 µmol of furan. All other aspects were as before. X-ray photoelectron spectroscopy (XPS) analyses were performed using an SSI M-probe spectrometer with Al kR X-rays for photoionization and 2 eV electrons for charge correction. Each sample was contained in a 5-mm-diameter aluminum pan and was supported directly on a sample stub. The stub itself was covered with a fine stainless steel mesh to assist in the neutralization of charge buildup. The catalyst used in this study was prepared by impregnation. Palladium chloride (PGP Industries) was dissolved in sufficient dilute hydrochloric acid to fully wet the zirconia (Degussa, S.A., 50 m2g-1). The resulting mixture was evaporated to dryness at 353 K. The weight loading obtained was 0.99% w/w Pd/zirconia. The dispersion of the catalyst was determined by carbon monoxide chemisorption and, assuming a ratio of 1:1 CO/Pd, was calculated to be 48%. Results The reduced catalyst was examined by XPS to determine the oxidation state of the palladium and to confirm the absence of chlorine. The Pd 3d5/2 peak was detected at 334.5 eV in the Pd/zirconia sample, and no residual chlorine was detected. As a reference, aliquots of THF were passed over the catalyst at the reaction temperature (373 K). No reaction or adsorption of THF was observed. The catalyst was reduced in situ and subjected to 10 pulses of furan in a 4% H2/N2 stream as described in

Figure 1. Plot of Σfuran inlet versus Σfuran exit showing the difference between the first and subsequent tests. Table 1. Extent of Carbon Deposition over Cycles 1-3a cycle

extent of carbon depositionb

1 2 3

3.02 × 1020 (36.4%) 1.40 × 1020 (16.9%) 1.30 × 1020 (15.7%)

a Each cycle was 10 pulses of furan passed over the catalyst at 373 K. b Amount given in number of carbon atoms. The percentage value relates to the total amount of carbon retained compared to the total amount passed through the catalyst bed.

the Experimental Section. After the 10 pulses, the catalyst was put through a TPO, rereduced, and subjected to another 10 pulses of furan. Following the second set of 10 pulses, the catalyst was put through a temperature-programmed reduction, and then a third set of 10 pulses was passed over the catalyst. After the third set of 10 pulses, the catalyst was again subjected to a TPO. When the total inlet quantity of furan (Σfuran inlet) is plotted against the total outlet quantity of furan (Σfuran exit) (Figure 1), we see that the behavior of the catalyst to the first 10 pulses is different from that of the other sets of 10. This difference is manifest with respect to carbon deposition and is shown in Table 1, where the extent of carbon deposition for the first set of pulses is three times as much as that for the second and third. Conversion over the 10 pulses was typically 50%, with a THF yield of 16%; however, the first pulse had a conversion of 92% but a THF yield of only 1.5%. No methane or other hydrocarbon fragments were detected in the gas phase. After the first set of 10 pulses, the catalyst was subjected to a TPO (Figure 2). No carbon monoxide was produced. The evolution of carbon dioxide maximized at 625 K with shoulders at approximately 550 and 660 K. The catalyst was then reduced and subjected to a second set of 10 pulses of furan in a flow of 4% H2/N2. Following these pulses, the catalyst was cooled and subjected to a TPR (Figure 3). Three maxima for THF production appear: 394, 475, and 570 K. Methane is also produced with maxima at 535, 562, and 673 K. The raised baseline seen at higher temperatures is an artifact of the experimental setup. The catalyst was then cooled to 373 K, and a further 10 pulses were passed over the catalyst in the H2/N2 flow. The catalyst was cooled, and a TPO performed (Figure 2). Maxima in carbon dioxide evolution were detected at 600 and 655 K. The above procedure was repeated but with a flow of 100% H2 during the pulses of furan. With every pulse,

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Figure 2. Temperature-programmed oxidation (TPO) results after runs 1 and 3 under low-hydrogen conditions.

Figure 5. Temperature-programmed reaction (TPR) results after run 2 under high-hydrogen conditions. Table 2. Infrared Band Positions (cm-1) during and after Furan Hydrogenation with a Low H2/Furan Ratioa

Figure 3. Temperature-programmed reaction (TPR) results after run 2 under low-hydrogen conditions.

1b

2c

3d

furan

3161 m 3142 m 3129 m 2975 vw 2871 vw 2692 vw 1578 ms 1478 s 1371 m 1267w 1193 vs 1066 vs

3161 vw 3142 vw 3129 vw 2924 m 2853 m

2375 tr

3161 m 3142 m 3129 m

2326 tr 2252 tr 2147 tr

1567 w 1092 vw 1321 vw 1250 vw 1131 w 1075 w

1180 vw 1076 vw

THF

2975 vs 2871 vs 2692 m 1578 m 1478 vs 1371 m 1267 m 1193 vs 1066 s

1464s 1364 m 1185 s 1085 s

a Ratio of H /furan ) 6.9:1, temperature ) 303 K b 1. Gas phase 2 during reaction (15 min). c 2. Surface species left after evacuation. d 3. Surface species left after system subjected to 1 atm of [2H]H 2 followed by evacuation.

Table 3. Infrared Band Positions (cm-1) during and after Furan Hydrogenation with a High H2/Furan Ratioa 1b 3163 w 3139 w 3129 w 2966 s 2868 s 2692 w 1578 m 1478 ms 1370 w

Figure 4. Temperature-programmed oxidation (TPO) results after runs 1 and 3 under high-hydrogen conditions.

THF was the sole gas-phase product; no unreacted furan was detected. The results of the first TPO are shown in Figure 4. There are three small peaks below 500 K and a large peak at 580 K with a shoulder at 630 K. After the TPO, the catalyst was cooled, rereduced, and subjected to another 10 pulses of furan at 373 K. Subsequently, the catalyst was cooled, and a TPR performed (Figure 5). Two maxima for THF production were detected: 378 and 498 K. Only a single maximum was detected for methane production at 505 K. The catalyst was finally treated with another 10 pulses at 373 K in a flow of 100% H2 and then cooled, and a TPO was initiated. The TPO results are shown in Figure 4. Again, there are three small maxima in the carbon dioxide evolution below 500 K and two large maxima at 583 and 637K.

1193 ms 1085 s

2c

3d 2350 tr

2945 m 2875 m

2945 mw 2875 mw

1558 w 1461 w 1361 w 1250 w 1187 mw 1086 w

1558 vw 1461 vw 1361 vw 1250 vw 1187 w 1086 vw

furan

THF

3161 m 3142 m 3129 m 2975 vs 2871 vs 2692 m 1578 m 1478 vs 1371 m 1267 m 1193 vs 1066 s

1464 s 1364 m 1185 s 1085 s

a H /furan ratio 3.5 × 104:1, temperature 303 K. b 1. Gas phase 2 during reaction (15 min). c 2. Surface species left after evacuation. d 3. Surface species left after system subjected to 1 atm of [2H]H 2 followed by evacuation.

Infrared spectra of the reaction and the surface residue were recorded as outlined in the Experimental Section. Furan and THF were the only species detected in the gas phase under both sets of reaction conditions. When the reaction was performed with the lower dihydrogen concentration, the major gas-phase species was furan, whereas for reaction in the presence of 1 atm of dihydrogen, THF was the major gas-phase species. Table 2 reports the band positions observed for the reaction at low dihydrogen concentration, and Table 3 reports the band positions for the reaction at high dihydrogen conditions (an example of the spectra is given in Figure 6). The major detectable surface species

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Figure 6. Example of the infrared spectra obtained after hydrogenation of furan under high-hydrogen conditions: (i) recorded after 15 min, (ii) recorded after 30 min.

Figure 7. Temperature-programmed oxidation (TPO) results after continuous-flow hydrogenation for 3 h.

was THF for the high H2/furan ratio reaction, whereas a mixture of furan and THF was observable for the low H2/furan ratio reaction. When [2H]dihydrogen was added to the system after furan hydrogenation, the main change in the spectra was the loss of surface OH and the production of OD. The CH3, CH2 bands reduced in intensity by approximately 15%, while there was a corresponding increase in the C-D band intensity for the deposit produced from the low H2/furan ratio reaction. No obvious production of CO was observed either in the gas-phase spectra or in the spectra of the surface species. The samples that had been run under continuous-flow conditions were examined after use by TPO and TPR. These samples were run for an extended period (>3 h) until no hydrogenation activity remained. The TPO spectrum (Figure 7) reveals a series of carbon dioxide evolutions at 460, 510, 555, 600, 635, 665, and 690 K. The TPR spectrum is shown in Figure 8. Here, no significant THF evolution was detected; however, trace levels of methane were desorbed at 500, 540, and 670 K. In a separate test, the catalyst was extracted with dimethyl ketone. No higher-molecular-weight species were detected in the extract, although a yellow color was observed. Discussion The carbon deposition associated with the hydrogenation of furan over Pd/zirconia was examined by TPO, TPR, and FTIR spectroscopy. Using a pulse-flow system, we were able to investigate the early stages in the catalyst life, as this period is often critical in defining

Figure 8. Temperature-programmed reaction (TPR) results after continuous-flow hydrogenation for 3 h.

carbon laydown. The literature indicates that, with supported and unsupported palladium, significant deactivation occurs,1,7 and we have shown that, with a supported palladium system, there is indeed significant carbonaceous deposition. In the following discussion, we will examine the form, nature, and reactivity of the carbonaceous deposit laid down under low- and highdihydrogen conditions. The XPS analysis of the catalyst confirmed that there was no detectable chloride residue and that the palladium was in the metallic state. The absence of chloride was important to establish, as any residue could potentially alter the catalytic properties and cause deactivation of the catalyst. At the low H2/furan ratio, significant amounts of carbon laydown occurred. From the first pulse, the catalyst retained 91% of the carbon. Over the first 10 pulses, the amount retained was 36% of the total carbon passed over the catalyst; over the next two sets of 10 pulses, the carbon laydown was reduced to 16%. This type of behavior, where the catalyst on its first exposure to reactant has a unique response, is common in heterogeneous catalysis.14 The infrared spectroscopic analysis (Table 2) indicates that the species detectable on the surface is a mixture of furan and THF. No evidence was found for adsorbed carbon monoxide. Therefore, much of the carbon that combusted in the TPO must be strongly adsorbed furan and dissociatively adsorbed THF. The carbon burnoff in the TPO also shows a difference between the first TPO and the second. The first gives two poorly defined peaks superimposed on a general carbon dioxide evolution over a wide temperature range, whereas in the second TPO, two peaks are clearly identifiable. A low-temperature peak (800 K) can be assigned to carbon associated with the support.16 The TPO from the first run gives a broad poorly defined peak shape, suggesting a range of surface species all of which can be attributed to species adsorbed on the metal function. The infrared spectroscopic analysis indicated that the surface species were furan and dissociatively adsorbed THF. Therefore, in the TPR experiment, we would expect hydrogenation and hydrogenolysis of the adsorbed furan and THF. Three identifiable evolutions of THF occur: 394, 475, and 570 K. The low-temperature

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peak can be assigned to dissociatively adsorbed THF. The peak maximum is close to the reaction temperature (373 K) and represents THF that is strongly adsorbed and requires reaction with hydrogen to be able to desorb into the gas phase. It is worth remembering that TPR is, in effect, a high-hydrogen-concentration experiment so that we should expect further hydrogenation of adsorbed species that would be unreactive under lowhydrogen conditions. Some of this low-temperature THF peak might also be made up of adsorbed furan being hydrogenated from the surface. Using Chan’s method,17-19 the activation energy of the THF desorption was calculated to be ∼29 kJ‚mol-1. The second peak can be assigned to the hydrogenation of a more dehydrogenated form of furan/THF that still has its carbonoxygen skeleton intact. This species had an activation energy of ∼138 kJ‚mol-1. Note that, as this peak decays, the first methane peak is observed. Therefore, at around 500 K, hydrogenolysis of furan/THF can be initiated. Thus, the first methane evolution is assigned to the hydrogenolysis of furan/THF with an activation energy of ∼255 kJ‚mol-1. A small THF peak accompanies the second methane peak, indicating that, even at this temperature (562 K), some species that have retained their cyclic backbone still remain on the surface. As the temperature is increased, the extent of dehydrogenation and C-C bond fracture will increase. Hence, the final methane peak can be assigned to dehydrogenated carbonaceous fragments associated with palladium atoms with a low coordination number.15,16 The second TPO also indicates a background evolution of carbon dioxide across a wide range (Figure 2), but two peaks and a shoulder can be discerned. The shoulder sits on the leading edge of the first peak at approximately 490 K. From the literature,15,16 this feature would be assigned to carbon deposited on palladium atoms with high coordination number, e.g., the (111) plane. The small size of the peak is to be expected from a well-dispersed catalyst. However, although there are undoubtedly other species present on the surface that infrared spectroscopy cannot detect, we should remember that furan and THF are the major surface species. Therefore, a significant part of the TPO profile represents the combustion of furan and THF, not carbon. Hence, we assign the peak at 600 K to the combustion of adsorbed THF and the peak at 655 K to the combustion of furan. The TPO results of the catalyst that had been subjected to pulses of furan in 100% H2 are shown in Figure 4. In these spectra, there are clearly two welldefined peaks at ∼583 and ∼635 K. There is little evidence of a continuous evolution of carbon dioxide, but there are three small peaks below 500 K. When we compare these peak temperatures with those from the low-hydrogen experiment, we find that the peak temperatures are decreased by approximately 20 K. The reduction in combustion temperature can be interpreted simply by considering the effect of having an enhanced hydrogen content in the deposit. Carbon deposition in a more hydrogen-rich environment generally leads to a residue that has a high H/C ratio. Such species are usually easier to combust. Therefore, we propose that the peak at 583 K is related to the combustion of the adsorbed THF species while the peak at ∼635 K can be assigned to the combustion of the adsorbed furan species. Both of these species will be less dehydrogenated than their equivalents under low-hydrogen ratio

conditions and hence shifted to a lower temperature. There is little evidence of a continuous evolution of carbon dioxide underlying the other peaks. This is due to the absence of highly dehydrogenated fragments. The TPR profile from the high H2/furan ratio reaction (Figure 5) has significant similarities to that observed with the lower hydrogen/furan ratio experiments. The infrared spectroscopic analysis indicated that the major surface species, after a reaction with a high hydrogen/ furan ratio, was THF, rather than the mixture of furan and THF, as found with the low hydrogen/furan ratio. Therefore, the low-temperature THF peak at 378 K was found to have an activation energy of ∼29 kJ‚mol-1 and can be assigned to hydrogenation from the surface of dissociatively adsorbed THF. This conclusion is in keeping with the low hydrogen/furan ratio experiments and would be expected from the infrared analysis. The second evolution of THF at 498 K with a desorption activation energy of ∼138 kJ‚mol-1 can be assigned to hydrogenation of a more dehydrogenated form of adsorbed THF, again similar to that found with the low hydrogen/furan ratio. The evolution of methane is observed as a single well-defined peak at 505 K. This is noticeably different from the low hydrogen/furan ratio TPR, where three peaks were observed. The production of methane at 505 K reinforces the proposal that hydrogenolysis of furan/THF is initiated only at around 500 K. This desorption has an activation energy of ∼258 kJ‚mol-1. Therefore, we can assign this methane peak to the hydrogenolysis of THF. The absence of highertemperature peaks indicates that no highly dehydrogenated carbonaceous fragments are formed during hydrogenation of furan under conditions of hydrogen excess. The results from the continuous-flow experiments confirm the results of the pulse experiments. In the early stages of the catalyst life, the retained species are easily identifiable as strongly adsorbed furan and THF moieties. These species cover a range of adsorbed states, indicating varying degrees of dehydrogenation, and hence, they display multiple peaks in the TPO and TPR spectra. In the continuous-flow experiment, the catalyst was heavily deactivated. At this stage, no furan or THF was detected on the surface of the catalyst. As the carbonaceous deposit on the catalyst ages, it converts from having an identifiable signature of furan or THF to a nonvolatile coke-type species. Extraction of such catalysts removed no identifiable organic residues, indicating a very low H/C ratio. The TPO revealed a highly featured spectrum, indicating a range of species with different energetics. The TPR spectrum is almost featureless and shows the resistance of the aged carbon deposit to removal by hydrogen. Conclusions We have shown that significant carbon laydown is associated with furan hydrogenation over Pd/zirconia catalyst. In the early stages of the catalyst life, the primary surface species as detected by FTIR spectroscopy are furan and THF. Contrary to what has been found in high-vacuum studies,8-11 where decomposition to carbon monoxide and a C3H3 moiety was observed, no carbon monoxide was detected. TPR resulted in the desorption of THF, confirming the infrared assignments and indicating that the fracture of the ring did not occur under hydrogenation conditions. At high temperatures, methane was formed under TPR conditions from the

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hydrogenolysis of adsorbed furan/THF. After significant deactivation, no furan or THF were detectable on the surface, indicating that the initial carbonaceous deposit, derived from strongly bound dissociatively adsorbed furan and THF, had aged to give a nonvolatile surface species that was resistant to hydrogenation. TPO and TPR confirmed the presence of retained species with varying surface energetics related to their chemical natures and their adsorption sites. Acknowledgment The authors thank I. J. Huntingdon and N. A. Hussain for their assistance in obtaining some of the experimental data. Literature Cited (1) Godawa, G.; Gastet, A.; Kalck, P.; Maire, Y. Mise en Oeuvre d’un Catalyseur Actif pour L’Hydrogenation Selective du Furanne en Tetrahydrofuranne. J. Mol. Catal. 1986, 34, 199. (2) Phillips Petroleum Co.U.S. Patent 3,828,077, 1971. (3) Ikushima, Y.; Arai, M.; Nishiyama, Y. A Temperature Programmed Desorption Study on Supported Nickel Catalysts: Empirical Relationship between the Adsorbability of Hydrogen and the Catalytic Activities. Appl. Catal. 1984, 11, 305. (4) Zdzislaw, D.; Gasiorek, M. Polish Patent 104070, 1979. (5) Ikushima, Y.; Arai, M.; Nishiyama, Y. Pretreatments of Porous Silica for Improving the Activity of a Nickel-Loaded Catalyst. Bull. Chem. Soc. Jpn. 1986, 59, 347. (6) Beloslyudova, T. M. Russia Patent 417150, 1974. (7) Starr, D.; Hixon, R. M. Reduction of Furan and the Preparation of Tetramethylene Derivatives. J. Am. Chem. Soc. 1934, 56, 1595. (8) Caldwell, T. E.; Abdelreheim, I. M.; Land, D. P. Furan Decomposes on Pd(111) at 300 K to form H and CO plus C3H3, Which Can Dimerize to Benzene at 350 K. J. Am. Chem. Soc. 1996, 118, 907. (9) Ormerod, R. M.; Baddeley, C. J.; Hardacre, C.; Lambert, R. M. Chemisorption and Reactivity of Furan on Pd(111). Surf. Sci. 1996, 360, 1. (10) Caldwell T. E.; Land, D. P. Desulfurization, Deoxygenation and Denitrogenation of Heterocycles by a Palladium Surface: A

Mechanistic Study of Thiophene, Furan and Pyrrole on Pd(111) Using Laser-Induced Thermal Desorption with Fourier Transform Mass Spectrometry. Polyhedron 1997, 16, 3197. (11) Caldwell, T. E.; Land, D. P. In Situ Kinetics and Mechanism of Furan Decomposition and Desorption with CO formation on Pd(111). J. Phys. Chem. B 1999, 103, 7869. (12) Pratt, K. C.; Christoverson, V. Hydrogenolysis of Furan over Nickel-Molybdenum Catalysts. Fuel Process. Technol. 1983, 8, 43. (13) See, e.g.: (a) Jackson S. D.; Casey, N. J. The Hydrogenation of Propyne over Palladium Catalysts. J. Chem. Soc., Faraday Trans. 1 1995, 91, 3269. (b) Jackson, S. D.; Kennedy, D. R.; Lennon, D.; Webb, G. Deactivation and Selectivity: The Effect of Hydrogen Concentration in Propyne Hydrogenation over a SilicaSupported Palladium Catalyst. In Catalyst Deactivation 1999, Studies in Surface Science and Catalysis; Froment, G. F., Waugh, K. C., Eds.; Elsevier: Amsterdam, 1999; Vol. 126, p 341 and references therein. (14) (a) Al-Ammar, A. S.; Webb, G. J. Chem. Soc., Faraday Trans. 1 1978, 74, 175. (b) Jackson, S. D.; Kelly, G. J. The Hydrogenation of Propyne to Propene over Platinum/Silica. J. Mol. Catal. 1994, 87, 275. (c) Jackson, S. D.; Grenfell, J.; Matheson, I. M.; Munro, S.; Raval, R.; Webb, G. Deactivation and Regeneration of Alkane Dehydrogenation Catalysts. Stud. Surf. Sci. Catal. 1997, 111, 167. (15) Marecot, P.; Akhachane, A.; Barbier, J. Coke Deposition on Supported Palladium Catalysts. Catal. Lett. 1996, 36, 17. (16) Marecot, P.; Akhachane, A.; Micheaud, C.; Barbier, J. Deactivation by Coking of Supported Palladium Catalysts. Effect of Time and Temperature. Appl. Catal. A 1998, 169, 189. (17) Chan, C. M.; Aris, R.; Weinberg, W. H. An Analysis of Thermal Desorption Mass Spectra. I. Appl. Surf. Sci. 1978, 1, 360. (18) Chan, C. M.; Weinberg, W. H. An Analysis of Thermal Desorption Mass Spectra. II. Appl. Surf. Sci. 1978, 1, 377. (19) Falconer, J. L.; Schwarz, J. A. Temperature Programmed Desorption and Reaction: Application to Supported Catalysts. Catal. Rev.-Sci. Eng. 1983, 25, 141.

Received for review February 18, 2003 Revised manuscript received August 4, 2003 Accepted August 25, 2003 IE030154Y