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J. Phys. Chem. B 1998, 102, 6558-6565
H/D Exchange of Amines and Acetonitrile over Transition Metal Catalysts Yinyan Huang and Wolfgang M. H. Sachtler* V.N. Ipatieff Laboratory, Center for Catalysis and Surface Science, Northwestern UniVersity, EVanston, Illinois 60208 ReceiVed: March 31, 1998
H/D exchange of acetonitrile, mono-, di-, and triethylamine was carried out at 75 °C over zeolite supported transition metal catalysts in a fixed-bed microflow reactor. In order to identify the location of the D atoms, the product of this primary exchange was, subsequently, subjected to secondary exchange with liquid D2O, which affects exclusively the N-bonded hydrons. 1H-NMR and mass spectrometry were used for product analysis. The results reveal a rather dramatic difference in exchange behavior between ruthenium and the group of metals including Pt, Pd, and Ni, while Rh displays an intermediate behavior. Pt, Pd, and Ni show stepwise exchange starting with the N-bonded hydrons. In contrast, multiple exchange is found for Ru; this exchange leads to preferential formation of d3 acetonitrile. With primary, secondary, and tertiary amines, the hydrons bonded to the methylene C atom are most rapidly exchanged over Ru, followed by the hydrons in methyl groups. Surprisingly, the N-bonded hydrons are only negligibly exchanged over Ru. For instance, the d2 product of ethylamine has its two D atoms predominantly in the methylene, NOT the amine group. With diethylamine, products up to d10 are abundant, but d11 is negligible. The results are rationalized on the basis of the known high propensity of ruthenium to form CdRu double bonds. In contrast, formation of NdRu bonds appears negligible under the conditions used.
Introduction H/D exchange with paraffins has been used in the classical work of Kemball,1 Bond,2 Burwell,3 and others to shed light on the nature of the adsorption complexes of paraffins, cycloparaffins, and other molecules on the surface of transition metals. Obviously, a C-H bond is broken and a C-D bond formed when an alkane molecule exchanges one of its hydrogen atoms. Besides this “step-wise” exchange, several types of “multiple” exchange have been identified which suggest the existence of several diadsorbed complexes. R,β-Diadsorption is thought to be instrumental in the multiple exchange of cyclopentane, resulting in C5D5H5 as a primary desorption product with all hydrogen atoms at one side of the C5 ring exchanged. In contrast, R,R-diadsorption is required to form CH2D2, CHD3, or CD4 from CH4 during one residence of the methane molecule at the surface. It is also possible that the roll-over process, which, in H/D exchange of cyclopentane, brings the second set of five H atoms in contact with the surface, requires an R,Rdiadsorbed intermediate. For this type of adsorption complex, Kemball assumed formation of a multiple metal carbon bond, for instance, a MdC bond, and he showed that multiple exchange of methane involving R,R-diadsorbed intermediates can be correlated with the propensity of metals to catalyze the hydrogenolysis of butane.4 Van Broekhoven and Ponec proposed to use the d2 abundance in H/D exchange of cyclopentane as a measure for R,R-multiple bonding and the d5 concentration as a measure for R,β-diadsorption. When plotting the d5/d2 ratio against the temperature at which hydrogenolysis becomes appreciable (∼3%) over the same metal, they obtained a smooth curve, suggesting that preference of transition metals for R,Rbonding decreases in the order Ru > Ni > Co > Ir . Rh . Pt ∼ Pd.5 Little is known about the propensity of metal surface atoms to form single or multiple bonds with atoms other than carbon.
For instance, amines can be thought to be chemisorbed through C-M or N-M bonds or both. Early work by Kemball and Wolf of H/D exchange between D2 and methylamine, dimethylamine, or trimethylamine over a variety of metals indicated that over Pt and Pd exchange was stepwise, but over Fe, Ni, or W a more complicated pattern was observed.6 Because these authors found two H atoms of methylamine easily exchanged over Pt and Pd, they assumed without further proof that exchange was limited to the N-bonded H atoms. However, over Ni and Fe appreciable quantities of d3 and d4 were observed, indicating exchange in the alkyl group. With trimethylamine, up to five H atoms were found exchanged over Pd and seven or eight over Fe and W, respectively, showing again that H atoms in the alkyl groups of an amine can be replaced by D atoms. No data are known on the relative rates of exchange of C-bonded and N-bonded H atoms over metals with high propensity to form multiple bonds with an adsorbate. The question arises: will a metal such as ruthenium, when exposed to an alkylamine, form double bonds with a particular C atom or with the N atom of that molecule? As ruthenium is known to be a good catalyst for ammonia synthesis, it appears conceivable that multiple Ru-N bonds might be formed with amines, but nothing is known on how the formation of such a RudN bond would compete with that for RudC bonds with carbon atoms inside the same adsorbate. For trimethylamine, formation of a RudN bond would imply fission of a N-C bond, but for methylamine only N-H bonds need to be ruptured. Another important point in this competitive situation is, of course, that the C atoms in an alkyl group can form chemical bonds with metal surface atoms only by first dissociating a C-H bond, but the N atom in an amine molecule has the possibility to form a strong bond through its lone electron pair, without the need to first break an existing bond. In this case, a
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Amines and Acetonitrile mechanism for H/D exchange is conceivable with N-H bond formation preceding N-H bond fission. Addition to the lone pair is, of course, the prevailing interaction of amines with a Brønsted acid site; but this does not justify extrapolation to surface metal atoms. The present work tries to shed some light on these fundamental questions by providing experimental data on H/D exchange of primary, secondary, and tertiary amines over transition metals, both of the weak hydrogenolysis catalysts such as Pt, Pd, or Rh and over Ru, the hydrogenolysis catalyst par excellence of this group. In order to verify that R,R chemisorption bonds are indeed formed with Ru, we decided to also include in this study the H/D exchange of acetonitrile. To identify the position inside the amine molecules where H atoms are exchanged against D atoms, we combined GC-mass spectrometry with NMR and we made use of the facile exchange of N-bonded H atoms in amines against D atoms in solution with D2O. Experimental Section Zeolite NaY supported metal catalysts Ru/NaY, Rh/NaY, Ni/ NaY, Pd/NaY, and Pt/NaY were prepared by ion exchange, as described previously.7-10 NaY (UOP, Y-54) was used for the preparation of the catalysts. Ru/NaY was prepared by room temperature ion exchange of Ru(NH3)6Cl3 with NaY for 48 h followed by filtering, washing with DDI water, and drying in air. Rh/NaY was prepared by ion exchange of [Rh(NH3)5Cl]Cl2 with NaY at 80 °C for 72 h followed by filtering, washing with warm water, and drying in air. Ni/NaY was prepared by room temperature ion exchange of Ni(NO3)2 with NaY at pH ) ∼6 for 24 h; immediately after this exchange, the pH of the zeolite/water slurry was adjusted to pH ) ∼10.0 by adding 0.5 M NaOH over 1 h under stirring. Stirring was continued for one more hour, followed by filtering, washing with DDI H2O, and drying in air. Pd/NaY was prepared by room temperature exchange of Pd(NH3)4(NO3)2 with NaY for 24 h followed by filtering, washing with DDI water, and drying in air. Pt/NaY was prepared by ion exchange of Pt(NH3)4(NO3)2 with NaY at 80 °C for 12 h followed by filtering, washing with DDI water, and drying in air. All these catalysts have about 3% metal loading. Two successive H/D exchanges were performed, a primary gas exchange with D2, and a secondary exchange with liquid D2O. The primary H/D exchange was carried out in a microflow fixed-bed reactor. Fresh catalyst (100 mg) was used for each test. Prior to reduction, all fresh catalysts were subjected to some pretreatment. Ru/NaY was heated in a helium flow of 60 mL/min at 450 °C for 20 min with a temperature ramp of 3.5 °C/min to avoid Ru loss due to the formation of volatile Ru oxides. All other materials were calcined in an oxygen flow of 100 mL/min at 400 °C for 2 h with a ramp of 0.5 °C/min. In situ reduction was carried out in a hydrogen flow of 30 mL/min at specified temperatures for 30 min with a ramping rate of 8 °C/min. The reduction temperature was 300 °C for Ru/NaY, Rh/NaY, and Pd/NaY; 400 °C for Pt/NaY; but 500 °C for Ni/NaY. The primary H/D exchange was carried out at 75°C. After in situ reduction, the catalysts were cooled to 75 °C in a hydrogen flow, purged with D2 (20 mL/min) for 10 min, and finally switched to the feed streams for exchange reaction. The feed streams were obtained by blowing deuterium of 20 mL/ min through a saturator (0 °C for acetonitrile, -40 °C for ethylamine, and -10 °C for diethylamine, as well as 13 °C for triethylamine) and mixing this flow with another deuterium flow
J. Phys. Chem. B, Vol. 102, No. 34, 1998 6559 of 20 mL/min. Thus the total flow rate of deuterium is 40 mL/ min and the partial pressure of each reactant is about 20 torr. The primary exchange products were analyzed online by mass spectrometry, the sampling time being 20 min TOS. The products of the primary exchange were trapped by a liquid nitrogen trap and, after warming up to 0 °C in an ice bath, analyzed by 1H-NMR. Thereafter, they were subjected to the secondary exchange by adding D2O and shaking well. The mixture was analyzed again by 1H-NMR and mass spectrometry. The GC-MS (HP-G1800A GCD system) equipped with a HP-PONA column (50 m × 0.2 mm) was used for MS analysis. The natural abundance of isotopes and the fragmentation has been taken into account for each compound in the determination of the isotopic distribution. 1H-NMR analysis was preformed on a Varian Unity Plus 400 NMR system with 5 mm probe. Because ethylamine has a low boiling point (16.6 °C), NMR analysis of ethylamine containing samples was run at 5 °C. Results 1. Isotopic Effect in GC-MS Analysis. It has been known since the 1960’s that in chromatographic analysis, isotopomeric molecules have different retention times; their analysis is, therefore, possible by GC colums with an equivalent theoretical plate number of a few hundred. The molecule with the heavier isotope has the lower retention time, because of a nonnegligible isotope effect in chromatographic adsorption.11,12 This phenomenon is clearly observed in the present GC-MS analysis. Figure 1 shows the GC profile of triethylamines with different deuterium contents together with their mass spectra. Three GC peaks are well separated, the corresponding MS patterns, measured at the GC peaks, are totally different from each other. The retention time increases in the order d12 < d5 < d0. Owing to this isotopic effect, the mass spectrum of each reaction product under specific conditions can not simply be represented by the mass spectrum at one retention time: it must be averaged. Each fragment ion is extracted and integrated to obtain the total response. The final isotopic distribution is then obtained based on the integration of these fragmentation. 2. Amine/D2O Exchange. In an aqueous or nonaqueous medium, amine protons are rapidly exchangeable. Addition of a small amount of D2O to the solution of an amine leads to replacement of the amine protons by deuterium and the concomitant disappearance of its signal from the 1H-NMR spectrum.13-15 The position of the NMR signals strongly depends on the concentration, temperature, and the solvent employed. The chemical shift of protons attached to the nitrogen atom varies over a range of 0.5-5 ppm. Figure 2 shows the NMR spectra of ethylamine and an ethylamine/D2O mixture. The chemical shift of the methylene protons is higher than that of the methyl protons. The signals for both types of protons are probably broadened by the low temperature. The signal of the amino protons at 0.5 ppm, therefore, overlaps with that of the methyl protons. After adding D2O, the signal of the amino protons disappears. The relative ratio of methylene protons to methyl protons is 1:1.52. This indicates that the alkyl protons do not undergo isotopic exchange under these conditions; the same holds for diethylamine. The rapid exchange of amine protons can also be detected easily by mass spectrometry. For ethylamine and diethylamine, the main fragmentation occurs through loss of one methyl group.16 Upon replacing the amino protons by deuterium, the mass of the parent peak shifts up by 2 for the primary amine and by 1 for secondary amine, as expected. The main fragment
6560 J. Phys. Chem. B, Vol. 102, No. 34, 1998
Huang and Sachtler
Figure 2. 1H NMR spectra of (a) CH3CH2NH2 and (b) CH3CH2ND2 from exchange of CH3CH2NH2 with D2O.
Figure 1. Analysis of H/D exchanged triethylamines. (a) GC spectrum. Mass spectra of (b) d12, (c) d5, and (d) d0.
peaks shift by the same values, as our data shows. Figure 3 shows the mass spectra of ethylamine and D2O exchanged ethylamine. With D2O exchanged ethylamine, the parent and the main fragment peak are at 47 and 32, respectively. These values exceed those of ethylamine-d0 by 2. A peak at m/e ) 46 and a fragmental peak at m/e ) 33 indicate the presence of CH3CH2NHD in addition to CH3CH2ND2. The smooth exchange of D2O with amine protons provides an effective way to decide whether in the H/D exchange over metal catalysts the amino or the alkyl protons have been
Figure 3. Mass spectra of (a) CH3CH2NH2 and of (b) CH3CH2ND2 from exchange of CH3CH2NH2 with D2O.
exchanged. Collecting a particular product from the catalytic exchange and subjecting it to a secondary exchange with D2O will result in a secondary change of the mass spectrum only if H atoms bonded to N atoms were present in the primary product of the catalytic exchange.
Amines and Acetonitrile
Figure 4. Isotopic distribution of acetonitrile d1-d3 after primary exchange with D2.
3. H/D Exchange of Acetonitrile. Passing acetonitrile + D2 over a metal catalyst leads to both H/D exchange and amine formation. In this paragraph only the results of H/D exchange are presented. In general, the exchange ratio is low in comparison to the deuteration of the nitrile to amines. Less than 4.5% of the acetonitrile molecules which undergo any reaction have exchanged some of their H atoms against D. After averaging and corrections, the isotopic distributions of CH3CN from H/D exchange shown in Figure 4 are obtained. Clearly, high concentrations of d1 and d3 are observed over Ru/NaY, but over the other catalysts only d1 is abundant and d3 is very low. The abundance of d3 decreases in the order of Ru, Rh, Ni, Pd, and Pt. No H/D exchange is observed over NaY under the same conditions. The multiplicity of the isotope exchange has been defined by van Broekhoven and Ponec5 as Σ(idi)/100 (i ) D number in each molecule; di ) abundance of molecule having i D atoms). The calculated multiplicities in CH3CN/D2 exchange are Ru/NaY (1.79), Ni/NaY (1.28), Rh/NaY (1.21), Pd/NaY (1.06), and Pt/NaY (1.10). It is clear that Ru shows the highest multiplicity, while Pt and Pd only catalyze stepwise exchange. 4. H/D Exchange of Ethylamine. Isotopic distributions of ethylamine from ethylamine exchange with D2 are shown in Figure 5. Clearly, Ru/NaY is totally different from all other catalysts. Only over Ru/NaY the most abundant species is d2; for all other metals d1 prevails and the concentration of molecules with higher D content appears to follow the binomial law. The abundance of d2 over Ru could, in principle, indicate a preferential multiple exchange in either the amine or the methylene group. A distinction between these possibilities is achieved by the secondary exchange with D2O. Reactor effluents have been collected in a liquid nitrogen trap and subjected to secondary exchange with D2O. The NMR spectra of the product of the primary exchange with D2 over Ru/NaY and that after secondary exchange with D2O are shown in Figure 6. In these, the NMR signal of amino protons is not clearly separated from that of the methyl protons. However, the relative ratio of the proton numbers at about 2.55 ppm and about 0.9 ppm drastically changes by the secondary exchange. After the primary exchange, it is 1:4.3, and after the secondary exchange, it is 1:3.3. It follows that the primary product must contain a large number of amino protons that are exchanged against D in the secondary exchange. Thus, the high abundance
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Figure 5. Isotopic distribution of ethylamines d1-d7 after primary exchange with D2.
Figure 6. 1H NMR spectra of primary exchange product of (a) CH3CH2NH2 + D2 on Ru/NaY and (b) its secondary exchanage product with D2O.
of d2 is presumably caused by preferential exchange of H atoms in the methylene, not the amine group. At this point, the conclusion appears only to be of qualitative value since the reaction mixture after the primary exchange contains both exchanged and unexchanged ethylamine. However, the NMR data also provide direct evidence for preferential exchange in the methylene group. Because the unexchanged molecule has 2H atoms in the methylene and 3H atoms in the methyl group, the theoretical ratio would be 2:3 ) 1:1.5. This strongly deviates from the observed ratio after secondary exchange (when the contribution of amino H atoms is negligible), viz., 1:3.3. After NMR analysis, the products of the primary and the secondary exchange have been analyzed by mass spectrometry. The mass spectra are shown in Figure 7. The most abundant molecular ion and the main fragmentation ion are at m/e ) 47
6562 J. Phys. Chem. B, Vol. 102, No. 34, 1998
Figure 7. Mass spectra of ethylamine exchange products: (a) after primary exchange with D2 on Ru/NaY, (b) after secondary exchange with D2O.
and 32 for the primary exchange product. After secondary exchange with D2O, these peaks shift by 2 m/e units to m/e ) 49 and 34, respectively. This unambiguously shows that most amino hydrons in the primary product are H, not D. In the MS spectrum of the primary exchange product the most abundant peak of a parent ion at 47 corresponds, of course, to the d2 product. Higher mass satellites at m/e ) 48, 49, and 50 are also detected corresponding to d3, d4, and d5. After secondary exchange with D2O, peaks at m/e ) 51 and 52 are clearly seen. This evidently indicates that all D atoms are located in an alkyl group in the primary product. The exceedingly low peaks at m/e ) 51 and 52 in the primary product are presumably caused by the natural 13C abundance. The presence of d1 is indicated in Figure 7a by the CHDNH+ 3 fragment ion, being the most probable cause of the peak with m/e ) 31. The results unambiguously show that H/D exchange with ethylamine over Ru/NaY takes preferentially place in the methylene group. The MS spectra of the ethylamine H/D exchange products over Pt/NaY are shown in Figure 8, both for the primary exchange and after secondary exchange with D2O. Afrer primary exchange, the parent ions at m/e ) 45 and 46 and fragments at m/e ) 31 and 32 are prominent with the peak at 31 being the strongest. After secondary exchange with D2O, features at m/e ) 47 and 32 have increased in a spectacular manner. The results clearly show that the strong d1 peak observed after primary exchange over Pt/NaY, and shown in Figure 5, is predominantly due to a molecule containing one D atom bonded to the N atom: CH3CH2NHD. H/D exchange on Ru/NaY and on Pt/NaY are thus two extreme situations. The H/D exchange over zeolite supported Ni and Pd closely resembles that over Pt/NaY. 5. H/D Exchange of Diethylamine. Isotopic distributions from diethylamine exchange with D2 over M/NaY are shown in Figure 9. Clearly, for Ni, Pd, and Pt, d1 prevails, followed by a steep decrease in concentration of higher exchanged
Huang and Sachtler
Figure 8. Mass spectra of ethylamine exchange products: (a) after primary exchange with D2 on Pt/NaY, (b) after secondary exchange with D2O.
Figure 9. Isotopic distribution of diethylamine after primary exchange with D2.
products. Obviously, H/D exchange is stepwise over these catalysts. In contrast, Ru/NaY and Rh/NaY show the typical behavior of multiple exchange with a maximum near d4 for Rh/ NaY and a broad maximum near d7 and d8 for Ru/NaY. The extremely low abundance of d1-d3 and the precipitous decrease from d10 to d11 over Ru/NaY are characteristic for this catalyst. The condensed effluent over Ru/NaY was subsequently subjected to secondary exchange with D2O; 1H-NMR and mass spectrometry were used to determine the exchange positions. The NMR results are similar to those described above for ethylamine: after primary exchange the signal of the amino proton is very pronounced; it disappears only after the secondary exchange with D2O. GC separation of the diethylamine molecules with different D numbers is quite good. Three
Amines and Acetonitrile
Figure 10. Mass spectra of the product of diethylaminel/D2 exchange on Ru/NaY, only d10 is presented here. (a) Fresh, (b) After adding D2O.
isotopomeric diethylamines (i.e., d0, d4, and d10 in the primary exchange product) are well separated. For illustration, the MS spectrum of the d10 fraction is shown in Figure 10, both after primary exchange with D2 over Ru/NaY and after secondary exchange with D2O. The shift by one m/e unit is evident, see for instance m/e ) 83, 65, and 33. The same holds for the d0 fraction and the d4 fraction. It is evident that H/D exchange of the alkyl protons is strongly preferred over that of the amino protons with Ru/NaY. A totally different exchange pattern is obtained over Pt/NaY. In this case the exchange patterns after primary and secondary exchange are virtually identical, as shown by the MS data in Figure 11. Obviously, the H atom of the amino group is readily exchanged with D2 over Pt. The same conclusion holds for Ni and Pd; only amino hydrons are exchangeable. With Rh, both amino and alkyl hydrons are readily exchangeable. 6. H/D Exchange of Triethylamine. The product patterns of the H/D exchange of triethylamine over NaY supported tension metals are shown in Figure 12. Over Ru/NaY, two maxima are observed at d6 and d15, while d1 is very low. In contrast, d1 is the most abundant product over Pd and Pt, the concentration of other products decreasing monotonously with increasing D atom number. The concentration of dn products with n > 6 is very low. Since there is no N-bonded hydron in triethylamine, the exchange only takes place with alky protons. Discussion 1. Adsorption and H/D Exchange of Acetonitrile. The adsorption of acetonitrile on well-defined metal crystal surfaces (e.g. Ru(001),17 Ni(111),18 and Pt(111)19), films of nickel or palladium,20 and Raney Ni21 has been studied by a number of researchers. A semiempirical theoretical study was conducted for the adsorption of acetonitrile on Ni(111), Ni(100), and Ni(110).22 The main point of these results is that, on most metal surfaces, CH3CN is adsorbed with the CtN group parallel to
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Figure 11. Mass spectra of the product of diethylamine/D2 exchange on Pt/NaY. (a) Fresh, (b) after adding D2O.
Figure 12. Isotopic distribution of triethylamine after primary exchange with D2.
the metal surface. The H/D exchange of acetonitrile indicates a different type of adsorption complex. This might be a minority species, because the fraction of acetonitrile molecule participating in H/D exchange is very low compared with the fraction that undergoes hydrogenation. On Pt and Pd the H/D exchange is stepwise, which suggests a short residence time of a monoadsorbed CH2CN fragment, possibly coexisting with another complex with parallel CtN group and leading to hydrogenation. Ru/NaY, however, displays a maximum abundance of d3, indicating R,R-multiadsorption through the C atom of the methyl group, possibly initiated by collision of the molecules with the CH3 groups hitting the surface. The results are in good conformity with the known pattern of transition metals in exchange and hydrogenolysis, as pointed out by Kemball, Bond, Burwell, and other authors.1-5 Ruthenium consistently shows
6564 J. Phys. Chem. B, Vol. 102, No. 34, 1998 the highest propensity for multiple exchange through R,Rmultiadsorption and the highest selectivity for hydrogenolysis. Pd/NaY and Pt/NaY are at the opposite extreme, because they catalyze stepwise exchange and prefer chemisorption through single C-M bonds. The present data thus fit well with the universal curve proposed by van Broekhoven and Ponec,5 showing that the trend to form multiple metal-carbon bonds follows the order of Ru, Ni, Co, Rh, Pd, and Pt. A subtle difference is, however, that in the present work Ni seems to be closer to Pt and Pd, the sequence being Ru . Rh . Ni > Pd, Pt, whereas in hydrogenolysis Ni is usually closer to Ru. 2. Adsorption and H/D Exchange of Amines. Adsorption of methylamine and ethylamine was investigated on well-defined surfaces Ru(001),23 Rh(111),24 Ni(111), Ni(100),25 and Pt(111).26 The adsorption of cyclohexylamine over Ni(111)27 and other amines on iron powder,28 or evaporated films of Rh, Ni, and other metals29 was also studied. The results indicate that primary amines are adsorbed on metal surfaces through the nitrogen lone pair. This remains valid for temperatures that are high enough for activating the C-H, N-H, C-N, or C-C bonds. In 1955 Kembal and Wolf reported on H/D exchange of methylamine, dimethylamine and trimethylamine over Pt, Pd, Ni films; they found stepwise exchange for all three metals, with Pd and Pt readily catalyzing the exchange of N-hydrons while both N-bonded and C-bonded hydrons were exchanged over Ni.6 No exchange of the alkyl hydrogens in C2H5ND2, CH3ND2, and (CH3)2ND had been observed by Roberts et al. in 1939 over reduced nickel catalyst at 20-195 °C.30 Hagiwara and Echigoya studied H/D exchange with aniline over Pt and Ni; their results indicate that H atoms in the amino group exchange faster than those at the benzene ring.31 None of these previous studies indicated a marked qualitative difference in the exchange patterns of ethylamine on Ru as opposed to Pt, Pd, or Ni. The present results show for the first time that the marked propensity of Ru to form multiple RudC chemisorption bonds leads to preferential exchange on methylene C atoms. Apparently, formation of RudN bonds is negligible. In contrast, Pd and Pt form single M-C and M-N bonds, leading to stepwise exchange with preference for the hydrons bonded to the N atom. Whereas the present results over Ni/NaY, Pd/NaY, and Pt/ NaY with ethylamine are in full conformity with earlier literature results, indicating preferential exchange of N-hydrons, the findings with Ru indicate a different chemistry. It appears that the strong propensity of Ru for forming MdC multiple bonds will lead to the formation of an R,R-diadsorbed CH3(CdRu)NH2 complex. Its hydrogenolytic desorption with deuterium will result in a CH3CD2NH2 molecule. Indeed, the observed maximum abundance is d2 for the H/D exchange with ethylamine over Ru. NMR and mass spectrometric fragmentation analysis of the product indicates that indeed both D atoms are located in the same -CD2- group. Likewise, the H/D exchange with diethylamine confirms the dramatic difference in the catalytic signature of Ru, as contrasted to the signatures of Pd, Pt, or Ni. With the latter metals, N-H activation occurs presumably by formation of an N-M adsorption bond and the exchange mechanism is stepwise, d1 being the main product at low exchange. The lower abundances of higher deuterated products follow the binomial law. Mass spectroscopic fragmentation analysis confirms preferential exchange at the N atom. The exchange pattern over Ru is entirely different; exchange is multiple and shows a broad distribution from d4 up to d10. Already, the primary data show
Huang and Sachtler an abrupt decrease between d10 and d11, indicating a much lower exchange rate of the N-bonded hydrons in comparison to the C-bonded ones. The propensity of Ru to form C)Ru bonds appears to be the cause of this remarkable result that almost no exchange takes place of the N-bonded H atom. Again, the low abundances of d1, d2, and d3 indicate more rapid exchange of the H atoms in the two -CH2- groups, making d4 a predominant product. Interestingly, Rh displays a position intermediate between Ru and the other metals, both with respect to the exchange of ethylamine and diethylamine. For Rh, the hermaphrodite in CO hydrogenation, hydrons are exchanged both in the amino group and the alky group. Triethylamine does not have a N-H hydrogen atom. H/D exchange takes place only in the alkyl groups, though adsorption can occur through the nitrogen lone pair. Very low abundance at d1 in the exchange product of triethylamine over Ru is evidence for multiple exchange. Again, RudC bond formation is the most probable cause for the low abundance of d1 and the maxima at d6 (all methylene hydrons) and d15 (all alkyl hydrons). Again, the behavior of Ru is in striking contrast with that of Ni, Pd, and Pt, where C-M single bond formation accounts for the stepwise exchange. And again, the signature of Rh is intermediate between those of Ru at one hand and Ni, Pd, and Pt on the other. Adsorption of amines on transition metals is similar to the adsorption of ammonia. The NH3 molecule is adsorbed on metal surfaces through the nitrogen lone pair. Adsorption on Ru(001) is reversible, with only negligible dissociation.32 These studies might suggest that also the amines are weakly adsorbed on a ruthenium surface through the lone pair of nitrogen. The present results indicate, however, that this adsorption type does not result in H/D exchange. Apparently, the formation of Rud C bonds with methylene groups is much stronger; hydrogenolytic fission of this adsorption complex results in -CD2groups. Acknowledgment. The authors thank the management of Air Products and Chemicals for funding this research and kindly permitting the publication of this report. They thank Dr. J. N. Armor for many elucidating discussions. They also thank Ms. Dr. Q. Ning of Northwestern University Analytical Lab for help with the interpretation of the 1H-NMR data. References and Notes (1) Kemball, C. AdV. Catal. 1959, 11, 223. (2) Bond, G. C. Catalysis by Metals; Academic Press: London, 1962; p 183. (3) (a) Burwell, R. L., Jr. Acc. Chem. Res. 1969, 2, 289. (b) Burwell, R. L., Jr. Catal. ReV. 1972, 7, 25. (4) Kemball, C. Catal. ReV. 1971, 5, 33. (5) van Broekhoven, E. H.; Ponec, V. J. Mol. Catal. 1984, 25, 109. (6) Kemball, C.; Wolf, F. J. Trans. Faraday Soc. 1955, 51, 1111. (7) McCarthy, T. J.; Marques, C. M. P.; Trevino˜, H.; Sachtler, W. M. H. Catal. Lett. 1997, 43, 11. (8) Trevin˜o, H.; Sachtler, W. M. H. Catal. Lett. 1994, 27, 251. (9) Feeley, J.; Sachtler, W. M. H. J. Catal. 1991, 131, 573. (10) Jiang, H. J.; Zhou, M. S.; Sachtler, W. M. H. Catal. Lett. 1988, 1, 99. (11) Klein, P. D. AdV. Chromatog. 1966, 3, 3. (12) Ozaki, A. Isotopic Studies of Heterogenous Catalysis; Kodansha and Academic Press: New York, 1976; p 172. (13) Ionin, B. I.; Ershov, B. A. NMR Spectroscopy in Organic Chemisty; Plenum Press: New York, 1970; p 298. (14) Gunther, H. NMR Spectroscopy, An Introduction; John Wiley and Sons: New York 1980; p 22. (15) Sanders, J. K. M.; Hunter, B. K. Modern NMR Spectroscopy: A Guide for Chemists, 2nd ed.; Oxford University Press: Oxford, 1993; p 221.
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