Neopentane Conversion Research Articles

Modifying structure-sensitive reactions by addition of Zn to Pd

Silica-supported Pd and PdZn nanoparticles of a similar size were evaluated for neopentane hydrogenolysis/isomerization and propane hydrogenolysis/dehydrogenation. Monometallic Pd showed high neopentane hydrogenolysis selectivity. Addition of small amounts of Zn to Pd lead Pd–Zn scatters in the EXAFS spectrum and an increase in the linear bonded CO by IR. In addition, the neopentane turnover rate decreased by nearly 10 times with little change in the selectivity. Increasing amounts of Zn lead to greater Pd–Zn interactions, higher linear-to-bridging CO ratios by IR and complete loss of neopentane conversion. Pd NPs also had high selectivity for propane hydrogenolysis and thus were poorly selective for propylene. The PdZn bimetallic catalysts, however, were able to preferentially catalyze dehydrogenation, were not active for propane hydrogenolysis, and thus were highly selective for propylene formation. The decrease in hydrogenolysis selectivity was attributed to the isolation of active Pd atoms by inactive metallic Zn, demonstrating that hydrogenolysis requires a particular reactive ensemble whereas propane dehydrogenation does not.

Evidence for geometric effects in neopentane conversion on PdAu catalysts

Silica-supported Pd and Pd(shell)/Au(core) nanoparticles of a similar size were evaluated for neopentane conversion.

Correlating Heat of Adsorption of CO to Reaction Selectivity: Geometric Effects vs Electronic Effects in Neopentane Isomerization over Pt and Pd Catalysts

Silica-supported Pt and Pd nanoparticles from 1 to 10 nm in diameter were evaluated for neopentane conversion (hydrogenolysis and isomerization). Characterization of the catalysts was conducted uti...

ChemInform Abstract: Effects of Morphology and Electronic Structure on the Catalysis of Zeolite Encaged Palladium Particles

Abstract After calcination of ion exchanged Pd/Nay at 500°C, the Pd 2+ ions have lost their ligands and are located in sodalite cages, but after calcination at 250° Pd(NHY 3 ) 2+2 ions are present in supercages. Reduction leads to Pd atoms in sodalite cages or to Pd particles in supercages. An important difference between these cases is due to the co-product of hydrogen reduction: NH 4 + ions or protons. Probing these catalysts with the conversion of neopentane shows that Pd particle morphology has only a minor influence on the rate of conversion, but the presence of protons has a marked effect, increasing the activity per site by two orders of magnitude. This result is tentatively ascribed to electron deficient [Pd n -H x ] x+ clusters as superactive sites.

Regular Alumina-Supported Nanoparticles of Iridium, Rhodium and Platinum Under Hydrogen Reduction: Structure, Morphology and Activity in the Neopentane Conversion

Comparative studies of supported metal catalysts are facilitated if the catalysts consist of well-faceted metal particles whose structural changes under reaction conditions are known. We describe the use of regular noble metal crystallites (Ir, Rh and Pt), obtained by epitaxial growth on a single crystal (NaCl) substrate and subsequently contacted with the support by reactive high-vacuum deposition of the oxide, to study the inherent catalytic activity of the metals. Most of these metal particles exhibit defined zone axes parallel to the electron beam direction, which allows a characterization by selected area electron diffraction and lattice plane imaging in the electron microscope. The microstructural changes during activation processes (oxidation and reduction at temperatures up to 723 K) have been studied and correlated with corresponding changes in the conversion of neopentane with excess hydrogen. Particular emphasis was placed on the dependence of the reaction rates on hydrogen pressure. The structural changes at the metal–oxide interface upon reduction above ∼673 K may be interpreted as a prestage to alloy formation.

Nature of the Metal–Support Interaction in Supported Pt Catalysts: Shift in Pt Valence Orbital Energy and Charge Rearrangement

Conversion of neopentane (hydrogenolysis and isomerization on Pt/LTL, Pt/SiO2–Al2O3, Pt/MgO–Al2O3) and tetralin (hydrogenation on Pt/Y) catalytic data, combined with spectroscopic Pt atomic XAFS (AXAFS) data, and theoretical calculations are utilized to elucidate the nature of the metal–support interaction for Pt-supported catalysts. The turnover frequency (TOF) of both the neopentane and tetralin conversion strongly depends on the composition of the support. The TOF increases with increasing acidity, polarization power of the charge-compensating cations (Na+, H+, K+, La3+), Si/Al ratio, and the presence of extra-framework Al. The intensity of the experimental Pt atomic XAFS correlates with the TOF. Ab initio scattered wave cluster calculations on a Pt4O3 cluster were performed using the FEFF7 code. The electron charge on the three “support” oxygens was changed from +0.05 to −0.01 electron to mimic changes in the support Madelung potential, which for the cluster is dominated by the nearest neighbor oxygen charge. The trends found in these theoretical AXAFS results are in excellent agreement with the experimental Pt AXAFS data and suggest that a metal cluster–support potential model is adequate for describing the changes seen in the experimental AXAFS. The experimental AXAFS results can also be understood using a molecular orbital scheme. This molecular orbital scheme further indicates that the metal–support interaction not only changes the ionization potential of the Pt valence orbitals but also induces a charge rearrangement from the Pt 6s orbitals within the particle to the oxygens of the Pt-support interface and vice versa. This charge rearrangement is also indicated by the AXAFS through the shift, ΔR, in AXAFS peak position. Both effects influence the electronic s tructure of the Pt particles. The changes in the electronic structure alter the catalytic properties of the Pt surface atoms by varying the bond strength and bond order (single or bridged) to the catalytic intermediates. The consequences of the metal–support interaction for tailor-made supported metal catalysts will be discussed.

The metal–support interaction in Pt/Y zeolite: evidence for a shift in energy of metal d-valence orbitals by Pt–H shape resonance and atomic XAFS spectroscopy

The turnover frequency (TOF) for conversion of neo-pentane was determined for Pt in Y zeolite with different numbers of protons and La +3 ions, different Si/Al ratios and with non-framework Al being present. Comparing Pt/NaY to Pt/H-NaY and Pt/K-USY with Pt/H-USY, respectively, shows an increase in the ln (TOF) which is proportional to the number of protons. Compared to NaY, the TOF of Pt in non-acidic NaLaY zeolite is about 25 times higher, which indicates also a strong influence on the charge of the cations on the TOF of Pt. The 20 times increase in the Pt TOF for K-USY compared to NaY is attributed to the effect of a higher Si/Al ratio and non-framework Al in the K-USY. EXAFS data collected on Pt/NaY and Pt/H-USY showed platinum particles consisting of 14–20 atoms on an average. These results were confirmed by HRTEM, which also showed that the Pt particles were dispersed inside the zeolite. The EXAFS data indicate that the metal particles are in contact with the oxygen ions of the support. The peak in the Fourier transform of the atomic XAFS (AXAFS) spectrum of the Pt/H-USY is larger in intensity than the corresponding peak of the Pt/Na-Y data. A detailed analysis of the L 2 and the L 3 X-ray absorption near edge structure revealed a shape resonance due to the Pt–H anti-bonding state (AS) induced by chemisorption of hydrogen on the surface of the platinum metal particles. The difference in energy ( E res) between the AS and the Fermi-level ( E F) is 4.7 eV larger for Pt/H-USY than for Pt/NaY. Both the AXAFS spectra and the shape resonances of the Pt-NaY and the Pt/H-USY catalysts provide direct experimental evidence of how the support properties determine the electronic structure of the platinum metal particles. Previous AXAFS and shape resonance work lead to a model in which the position in energy of the Pt valence orbitals is directly influenced by changes in the potential (i.e. electron charge) of the oxygen ions of the support and how the proton density affects this oxygen charge. This work shows that the potential of the oxygen ions is also a function of the Si/Al ratio of the support and the polarisation power of the charge compensating cations (H +, Na +, La 3+ and extra-framework Al); the metal particles experience an interaction which is determined by several properties of the support. The data further reveal how the change in the Pt electronic structure directly influences the catalytic properties of the catalyst. While the TOF is dependent on the metal–support interaction, the hydrogenolysis selectivity is determined by the Pt particle size, and increases linearly with increasing dispersion, or decreasing particle size.

Neopentane Conversion over Zeolite-Supported Platinum and Palladium Catalysts

Neopentane conversion has been studied over platinum supported on L- and Y-zeolites, and palladium supported on KL-zeolite. The Pd/KL-zeolite catalysts exhibit the enhanced activation energies previously reported in the literature. The preexponential factors and activation energies show a compensation effect for both the palladium and the platinum catalysts. The trend in activation energies is compared with hydrogen TPD studies. Pressure dependence studies (in excess hydrogen) show reaction orders of approximately −2 and 0.8 for hydrogen and neopentane, respectively, except for hydrogenolysis over Pd/KL which exhibits reaction orders of ∼−4 and 0.64, respectively. At lower hydrogen pressures (<70 Torr) the reaction order in hydrogen becomes less negative, which may signify a change in the rate-limiting step of the reaction. The nature of the adsorbed intermediates, probable reaction pathways, and the compensation effect are discussed.

An X-Ray Absorption Spectroscopy Determination of the Morphology of Palladium Particles in K L-Zeolite

Particle morphologies for 2 and 3 wt% Pd/K L-zeolite have been determined using X-ray absorption spectroscopy (XAS). X-ray absorption near edge structure–temperature programmed reduction (XANES-TPR) was used to check for the presence of unreduced palladium cations in the locked cages after reduction. A series of calculations were performed to determine coordination numbers for the first three coordination shells for different ideal particle morphologies (spherical, hemispherical, cylindrical, and disc-like). The X-ray absorption fine structure (XAFS) results suggest the formation of ∼10 and 16 atom disc-like particles, for 2 and 3 wt% Pd/K L-zeolite, respectively. The lattice expansion for the 3 wt% Pd/K L-zeolite is consistent with chemisorption and TPD of hydrogen and indicates a bulk hydride formation. The effects of morphology and metal support interaction on neopentane conversion over these catalysts are briefly discussed.

Single file diffusion in mordenite channels: neopentane conversion and H/D exchange as catalytic probes

When small molecules inside one-dimensional micropores of zeolites are unable to pass along each other, transport is limited by single file diffusion. Product molecules generated inside such micropores are unable to escape to the gas phase if the pores are plugged with physisorbed molecules. As a consequence, the apparent activation energy, E app, is higher than the true activation energy. This is experimentally confirmed by comparing neopentane reactions (H/D exchange, isomerization and hydrogenolysis) over Pt/H-mordenite and Pt/SiO 2 catalysts. At 150°C the Arrhenius line for Pt/H-mordenite shows a break, indicating the switch from the single file diffusion controlled to the chemically controlled regime. The isosteric heat of physisorption of neopentane is −11.5 kcal/mol on Na-mordenite, whereas physisorption on SiO 2 is negligible. In the regime of significant physisorption of neopentane, the increase in E app caused by single file diffusion overcompensates the decrease in E app caused by pre-equilibration between gaseous and physisorbed neopentane.

Pd-Au/SiO2: Characterization and Catalytic Activity

A series of Pd-Au/SiO2 catalysts was prepared by incipient wetness coimpregnation of silica with respective chloride solutions, followed by calcination in air and reduction in hydrogen at 380°C for 15 h. A variety of techniques, chemisorption of hydrogen and carbon monoxide, temperature-programmed reduction, X-ray diffraction, X-ray photoelectron spectroscopy, and neopentane conversion as a catalytic probe, were employed to characterize metal dispersion and interaction between two metal components and the support. It has been discovered that the prepared materials are not uniform: a palladium-rich phase of very high metal dispersion (particle size ∼ 1.3 nm) coexists with much less dispersed gold-rich phase (particle size ∼ 15 nm). X-ray photoelectron spectroscopy revealed some presence of Si<IV species (as reduced silicon or ex-palladium silicide) in reduced silica-supported Pd-Au catalysts. Because the amount of Si<IV positively correlates with metal dispersion it is suggested that these species are in a close vicinity to the highly dispersed Pd (Pd-rich) material, forming a kind of "chemical glue" between metal and support. In contrast to other Pd-based systems (e.g., Pd-Cu and Pd-Zn), coimpregnation of silica with aqueous solutions of palladium and gold chlorides does not produce well-homogenized bimetallic material.

ChemInform Abstract: The Structure and Activity of Silica‐Supported Palladium‐Cobalt Alloys. Part 1. Alloy Homogeneity, Surface Composition, and Activity for Neopentane Conversion.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The Structure and Activity of Silica-Supported Palladium Cobalt Alloys .II. Skeletal Reactions of N-Hexane and Methylcyclopentane over Pd-Co/SiO 2 Catalysts

n-Hexane and methylcyclopentane conversions appeared to be very useful tools for probing surfaces of bimetallic Pd-Co composites supported on silica. Very large changes in activity and selectivity constitute a good basis for diagnosing the extent of interaction between two metal components. The overall activity of cobalt is much higher than that of palladium; in particular, Co predominantly promotes hydrogenolysis of hydrocarbons, whereas other, nondestructive reactions are preferred over Pd and the Pd-richest alloy. A variety of information offers several "fingerprint-like" parameters which enable one to characterize additional properties of these composition regions (i.e., the range of 25-75 at% Co) as compared to neopentane conversion [Part I (W. Juszczyk, Z. Karpinski. D. łomot, J. Pielaszek. Z. Paál, and A. Yu. Stakheev, J. Catal. 142, 617 (1993)] This work confirms our earlier conclusions that the employed preparation method (incipient wetness coimpregnation technique) produces well-mixed Pd-Co alloy catalysts. Individual Pd-like properties promoting C 5-cyclic skeletal reactions (ring closure, isomerization) arc observed up to 25% Co content only.

ChemInform Abstract: Neopentane Conversion Catalyzed by Pd in L Zeolite: Effects of Protons, Ions, and Zeolite Structure

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The Structure and Activity of Silica-Supported Palladium-Cobalt Alloys I. Alloy Homogeneity, Surface Composition, and Activity for Neopentane Conversion

Abstract Chemisorption of H 2 and CO, temperature-programmed reduction (TPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and neopentane conversion show that differences in preparation method and overall metal loading lead to the formation of different Pd-Co bimetallic materials (changes in reducibility, metal particle size, and lateral homogeneity). Well-homogenized Pd-Co particles exhibit a very pronounced minimum in the plot of catalytic activity versus Pd-Co alloy composition. At the same time, a maximum exists in the isomerization selectivity vs the alloy composition. These deviations from the catalytic behavior of physical mixtures of the monometallic (Pd and Co) catalysts provide a basis for the estimation of how two metals interact with each other in a supported catalyst. Incipient wetness impregnation of silica by considerable amounts (10 wt%) of metal salts leads to the formation of catalyst precursors which are fully reduced at 380°C yielding good dispersion and lateral homogeneity. Chemisorption and in situ XRD studies are compatible with the kinetic investigations, but information on metal interaction by these methods is less clear. For high metal-loaded Pd-Co/SiO, catalysts, comparison of H 2 chemisorption and XRD data suggests surface enrichment in palladium, which is confirmed by XPS.

The Effects of Protons on the Hydrogenolysis of Neopentane over Rhodium Catalysts: Rh/Hy, Rh/Nahy, and Rh/SiO2

The hydrogenolysis of neopentane (2,2-dimethylpropane) has been studied over the rhodium catalysts Rh/HY, Rh/NaHY, and Rh/SiO2. The catalytic activity follows the sequence: Rh/HY > Rh/NaHY ≈ Rh/SiO2. For all catalysts, the conversion of neopentane was 100% selective to hydrogenolysis products, methane, and isobutane; they are formed in a ratio of 1 : 1. In an attempt to determine the true activation energies from the apparent activation energies derived from the Arrhenius curves, corrections were made for the preequilibrium between gaseous and physisorbed neopentane. The adsorption of neopentane on the metal-free supports was determined: isosteric heats of adsorption were calculated from these data. For zeolites NaY and HY they are 8.2 and 7.0 kcal/mol, respectively. After correcting for the preequilibrium, the true activation energies are 33.5 and 38.8 kcal/mol for Rh/HY and Rh/NaHY, respectively. The results lend support to the concept that the superior activity of Rh/HY is due to the "electron deficiency" of the rhodium particles.

Neopentane Conversion Catalyzed by Pd in L-Zeolite: Effects of Protons, Ions, and Zeolite Structure

The conversion of 2,2-dimethylpropane (neopentane) has been studied over zeolite-L-supported Pd catalysts; the results are compared with data for Pd on zeolite Y. Variation of the charge compensating ion (Li+, K+, or Ca2+) in L has only a moderate effect on the catalytic parameters, due either to variations in the heat of physisorption or to "electronic" effects on Pd. The reduction temperature, TR, of Pd/L samples that have previously been calcined at 250°C, strongly affects the catalytic performance: after TR = 250°C, sufficient NH3 ligands of the ion exchanged Pd(NH3)2+4 ions survive to neutralize the protons that are formed during reduction; catalytic activity then increases during reaction while NH+4 ions decompose and Pd-proton adducts of high catalytic activity are formed. At TR = 400°C, the NH3 ligands are destroyed and this catalyst is stable and very active. Activity, selectivity, and kinetic parameters depend on the structure of the support; apparent activation energies, Ea, are in the range of 45-55 kcal/mol for Pd/Y, but 75-90 kcal/mol for Pd/L. A linear relation between Ea and log of the preexponential factor is obtained. While classical pore-diffusion operates in Y, single-file diffusion appears more likely in L; i.e., channels tend to be plugged with physisorbed molecules at low temperature. The difficult of primary reaction products to escape from the pores correlates with a higher fraction of secondary products observed for Pd/L vis-á-vis Pd/SiO2. It is conceivable that different diffusion types also contribute to the different Ea values for zeolites with three-dimensional or unidimensional channel systems.

Probing palladium-cobalt/NaY catalysts by neopentane conversion

Neopentane conversion in hydrogen was used as a catalytic probe for zeolite encaged PdCo particles. Activity and selectivity strongly depend on the pretreatment conditions. PdCo/NaY catalysts exhibit higher isomerization selectivity than Pd/NaY in conformity with previous results on SiO2 supported Pd and PdCo. This is remarkable, because reduced Co/SiO2 displays 100% selectivity for hydrogenolysis. The isomerization selectivity thus appears to be a useful measure for the extent of alloying between Pd and Co.

Effect of zeolite protons on palladium-catalyzed hydrocarbon reactions

Catalytic superactivity of electron-deficient palladium for neopentane conversion, previously reported for Pd/Al 2O 3, has been verified for Pd/NaHY. The reaction rate, per accessible Pd atom, correlates with the proton content of the catalysts; for samples that contain all the protons generated during H 2 reduction of Pd, it is two orders of magnitude higher than that for Pd/SiO 2. Samples prepared by reduction of Pd(NH 3) 2 2+NaY display an intermediate activity. It is suggested that Pd-proton adducts are highly active sites in neopentane conversion. With methylcyclopentane as a catalytic probe, all Pd/NaHY samples deactivate rapidly and coke is deposited. Temperature-programmed oxidation reveals two types of coke, one of which correlates with the proton concentration in the catalyst.

Chemisorption and catalysis of zeolite‐entrapped palladium

AbstractThe chemisorption of H2 and of CO and the catalysis of neopentane conversion have been studied on two groups of Y‐zeolite‐supported Pd samples. One group was prepared under conditions which result in the formation of “grape‐shaped” Pd particles filling adjacent supercages. In the second group, pretreatment conditions were chosen which yield isolated Pd particles of the size of the supercages. Both groups of Pd/NaY samples differ in adsorption of H2 and CO; the COads/Hads ratios are in agreement with the different particle morphologies. The selectivity of neopentane conversion does not detectable depend on Pd morphology; however, the activation energy for reaction is significantly higher for the samples of the first group.