Zongchao Zhang

Zeolite-Supported Transition Metal Catalysts

Publisher Summary This chapter discusses zeolite-supported transit ion metal catalysts. In catalytic applications, zeolites are predominantly used in their acidic form. The most important process in this category is fluidized catalytic cracking, based on rare-earth-exchanged zeolites, mainly X and Y of the faujasite structure with small admixtures of ZSM-5. Another industrial process in this group is catalytic dewaxing using mordenite and ZSM-5. Within the vast group of zeolite catalysts, the focus in the chapter is on one subgroup: materials that contain reduced particles of a transition metal or several transition metals dispersed inside zeolite cavities. A majority of transition metal/zeolite catalysts are bifunctional, i.e., strong acid sites are present in the same zeolite. A process based on a zeolite-encaged metal in the absence of acid sites is the dehydrocyclization of small linear alkanes (such as n -hexane) to aromatics. NO x abatement by zeolite-supported Cu is mentioned briefly in the chapter that illustrates the potential for environmental catalysis; it also opens prospectives for stabilizing elements in unusual valence states, in addition to unusual states of aggregation and complexation.

The effect of Ca2+ and Mg2+ ions on the formation of electron-deficient palladium-proton adducts in zeolite Y

FTIR spectra of adsorbed CO on Pd particles in supercages of zeolite Y show that positively charged Pdn clusters are formed when the concentration of protons in the supercages is high and thereby confirm our hypothesis that [PdnH]+ adducts are the “electron-deficient” palladium clusters that are responsible for catalytic superactivity. The formation of such adducts is favored by a high concentration of divalent charge-compensating ions, for example, Mg2+ or Ca2+. Carbon monoxide displaces the protons from the [PdnH]+ adducts; this leads to an increase of the IR band due to supercage OZH groups. Simultaneously the Pdn(CO)x entities migrate through the supercage channels and coalesce with each other. Temperature-programmed reduction (TPR) shows that substituting Na+ by Mg2+ ions results in a downward shift by 70°C of the TPR peak for Pd2+ reduction. This suggests that the divalent ions preferentially populate sodalite cages and hexagonal prisms, thus preventing the migration of Pd2+ ions into these positions. During reduction, both Pdn clusters and protons are formed in supercages, setting the stage for the formation of the [PdnH]+ adducts.

EXAFS evidence of carbon monoxide-induced growth of Pd particles encaged in zeolite Y

Abstract EXAFS data of Pd in zeolite NaY show that after calcination at 500°C and reduction at 350°C small Pd clusters are formed in the supercages of the zeolite with a compressed PdPd distance of 2.68 A and an average coordination number of CN = 4, corresponding to nuclearity of 6 Pd atoms. Adsorption of CO results in a significantly increased cluster size and a relaxed PdPd distance of 2.70 A. The average coordination number of Pd (CN = 6) is consistent with Pd clusters of 13 atoms which are confined in the supercages by the dimension of the windows. The enhanced rate of coalescence due to CO indicates that CO adsorption weakens not only PdPd bonds in the cluster, but also the interaction of the cluster with the zeolite. However, no Pd cluster disintegration by CO adsorption is observed.

Identification by diffuse reflectance and EXAFS of the changes in coordination of NaY entrapped Pd(NH3)x2+ ion during calcination

Abstract The coordination chemistry of Pd(NH 3 ) x 2+ ions in zeolite NaY after calcination has been identified by u.v.-VIS diffuse reflectance (d.r.s.) and EXAFS spectroscopies. Pd(NH 3 ) 4 2+ ions remain intact in NaY supercages after calcination in 1 atm of O 2 at T c = 150°C; they show a characteristic absorption maximum at 33.5 × 10 3 cm −1 . Roughly 50% of the ammine ligands are known to be destroyed by calcination at 250°C; EXAFS and d.r.s. both show that Pd ions are coordinated to oxygen and nitrogen. Calcination at 500°C leads to bare Pd 2+ ions coordinated to four O z ions; the EXAFS data indicate coexistence of Pd 2+ in two locations; sodalite cages and hexagonal prisms.

Migration and coalescence of Pd carbonyl clusters in zeolite Y

Admission of carbon monoxide to reduced Pd/NaY at room temperature induces substantial migration and coalescence of primary Pd clusters, as evidenced by EXAFS. In samples calcined at TC= 500 °C and reduced at TR= 200 °C, isolated Pd atoms in sodalite cages prevail initially; Pd clusters in supercages with Pd coordination numbers of 2.4 or 4 are generated by the pretreatment programs TC= 250 °C/TR= 200 °C or TC= 500 °C/TR= 350 °C, respectively. In all these cases Pd13 carbonyl clusters prevail after admission of CO at room temperature. The growth of Pd nuclei is rationalized by assuming that, before CO is admitted, bare Pd interacts with the zeolite matrix, e.g. via proton or Na+ bridges linking Pd with cage-wall oxygens. Adsorption of CO weakens this interaction as primary Pd carbonyl clusters are formed. The mobile Pd carbonyl clusters will coalesce in the supercages until further transport is sterically restricted. This process leads to prevailing Pd13 carbonyl clusters at room temperature as the Pd core of 8.2 A is unable to traverse the 7.5 A window. At elevated temperatures, e.g. 200 °C, further growth is possible as the supercage windows expand. As a result, Pd40 clusters are formed and completely fill their supercage.

Effect of zeolite protons on Pd particle size and H2 chemisorption

Zeolite protons enhance metal dispersion by chemically anchoring small reduced Pd particles; they also markedly reduce the propensity of Pd to chemisorb H2. Whereas the true metal dispersion, derived from Pd–Pd coordination numbers determined by EXAFS is higher for Pd/HY than for Pd/NaHY or Pd/NaNH4Y, conventional H2 chemisorption gives lower H : Pd ratios for the samples with higher dispersion. This apparent discrepancy is confirmed by static and dynamic adsorption measurements and by rapid isotope exchange between gaseous D2 and zeolite protons. Remarkably, the mononuclear Pd proton adduct [Pd1—Hx]x+ does not catalyse D–H isotope exchange at 300 K, because it apparently does not chemisorb D2 or H2.

Location, ligancy and reducibility of metal ions in zeolite cages: Co and Pd in NaY

UV-VIS diffuse reflectance spectroscopy (DRS), temperature-programmed reduction (TPR) and temperature-programmed oxidation (TPO) have been used to study ion migration and reducibility in Co/NaY and (Co + Pd)/NaY, after ion exchange of Co(NH3)3+6 and Pd(NH3)2+4. It has been found that the behaviour of the Co(NH3)3+6 ion under calcination conditions differs markedly from that of the Co(H2O)2+6 ion, studied previously. While the latter ion loses all ligands rapidly and migrates swiftly into sodalite cages and hexagonal prisms, the Co(NH3)3+6 ion loses five of its six ammine ligands easily while being reduced, even in flowing O2, to Co(NH3)2+. A stable tetrahedral configuration is attained with three cage-wall oxygens and one strongly bonded ammine group, detectable by strong absorption bands at 16.6, 17.3 and 18.5 kK. Probing with ethylenediamine shows that this complex, most remarkably, is still located in the supercages. At 500°C, the last ammine ligand can be oxidized. Its oxidation is, however, promoted in the bimetal (Pd + Co)/NaY system by the simultaneous oxidation of the ammine ligands of Pd at 300°C. While Pd ions migrate into sodalite cages at calcination temperature Tc 300°C, the monoamminecobalt(II) ions remain in supercages. The migration of Co ions to hexagonal prisms and sodalite cages is, however, observed at higher temperature. The reducibility of Co is markedly enhanced by the presence of Pd, but the locus of the enhanced reduction depends on the history of the sample.

Proximity requirement for Pd enhanced reducibility of Co2+ in NaY

The temperature which is required to reduce Co2+ ions in NaY can be markedly lowered by the presence of Pd in the same zeolite. Temperature programmed reduction (TPR) in combination with diffuse reflectance spectroscopy shows, however, that the reducibility of Co is enhanced only if Pd2+ and Co2+ ions share the same zeolite cages. No hydrogen spillover effect is detected, if this proximity requirement is not fulfilled.

Oxidative leaching of Cu atoms from PdCu particles in zeolite Y

Abstract NaY-supported PdCu samples of various Cu/Pd ratios were prepared by ion exchange with Pd(NH 3 ) 4 2+ and Cu(NH 3 ) 4 2+ precursors. Reduction of Cu is enhanced by Pd; however, after calcination at 500°C the bare ions have migrated into sodalite cages, and reduction is only enhanced for those Cu 2+ ions which happen to share their cage with a Pd 2+ ion. Reduction with HZ results in bimetallic PdCu particles and protons of high Bronsted acidity. At 280°C, and in an inert atmosphere, these protons selectively reoxidize surface Cu atoms to Cu+ ions, which are still attached to the surface of the bimetal particles; they can be reduced at 210°C. Complete oxidation of the Cu component takes place at 500°C; the Cu 2+ ions leached from PdCu particles migrate to small zeolite cages, as indicated by EXAFS and a peak at 290°C in the profile of subsequent TPR. After complete leaching of Cu, the monometallic Pd particles are discerned from the original particles in PdCu/NaY and PdCu 2 /NaY by their propensity to form hydrides that are detectable by TPD. No hydride is formed with bimetal particles with Cu/Pd > 0.5; particles in Pd 2 Cu/NaY do form a hydride which is, however, distinguished from Pd hydride by its TPD peak position.

Coordination, atom reorganization, and catalysis of palladium in zeolite cages

Zeolite encaged palladium clusters undergo thorough atomic reorganizations when exposed to an adsorptive such as carbon monoxide or when used as catalysts e.g. in CO hydrogenation. Exposure to carbon monoxide of metal particles, which are initially anchored to zeolite walls via proton bridges, transforms the metal clusters into small, highly mobile carbonyl clusters. They coalesce and form larger clusters. At low temperature, this process is limited by the geometric constraints of the cage windows. At higher temperatures, further growth of metal particles occurs, conceivably via partial destruction of the zeolite matrix. The interaction of the metal particles with zeolite protons gives rise to electrondeficient metal clusters, which catalyze neopentane at a much higher rate than neutral metal particles. Such clusters might also act as collapsed bifunctional sites in bifunctional catalysis.