Libor Brabec

High nuclearity Pt carbonyls in alkali-metal X zeolites

Diffuse reflectance time-resolved UV–VIS spectroscopy has been used together with FTIR spectroscopy and 13CO–12CO isotopic exchange for the investigation of anionic Pt carbonyl complexes [Pt3(CO)3(µ-CO)3]n2– formed in alkali-metal X zeolites (alkali metal = Li+, Na+, K+ and Cs+) from [Pt(NH3)4]2+. It is shown that, compared with solutions, the zeolite matrix does not alter electronic transitions, while the vibrational frequencies of the CO ligands are appreciably changed. This latter effect is explained by the interaction of linear CO ligands with oxygen atoms of the zeolite lattice [an upward frequency shift of CO stretching vibration ν(CO)1] and the location of the bridging COs in the vicinity of alkali-metal cations [a downward frequency shift of CO stretching vibration ν(CO)b]. The effect of increasing the nuclearity of Chini complexes on the increase of ν(CO)1(at 2000–2100 cm–1) is much higher than the influence of increasing the electropositivity of the alkali-metal cations in the X matrix. Inside all the alkali-metal X zeolites and under all carbonylation conditions used, Pt6 species (n= 2) are formed. The decreasing size and electropositivity of alkali-metal cations in the sequence Cs+ > K+ > Na+ > Li+ assist in stacking of more triangular units and the appearance of the Pt9 and Pt15(n= 3 and 5, respectively) carbonyl complexes.

Pt0 in alkali faujasites. 1. Preparation by thermal decomposition of [Pt(NH3)4]2+ in vacuum

Pt2+ autoreduction during the thermal vacuum decomposition of [Pt(NH3)4]2+Me+ faujasites as well as the reaction of CO+NO were studied in dependence on the zeolite composition (effect of different alkali ions Me+: Li, Na, K, Cs, and zeolite Si/Al ratio: X and Y zeolites). Infrared spectroscopy was employed to characterize the surface species during the decomposition of the tetraamine complex. Gases evolved in this process were identified by mass spectrometry (which was also used in the CO+NO reaction), t.p.r., XPS and TEM assisted in the examination of the metal/zeolite system. It was found that at least four ammonia complexes are formed during the vacuum decomposition of [Pt(NH3)4]2+ and that two of them are more abundant in Y zeolites. Ammonia is evolved from the zeolites in two temperature regions. In the low temperature interval, the higher electropositivity and size of the alkali cations facilitate the disruption of Pt2+-N bonds in the same way in both X and Y zeolites. In the high temperature region, where the autoreduction of Pt2+ is dominant, the cations affect temperature of the NH3 release to a lesser extent. However, the amount of gases evolved from Y zeolites in this temperature region is higher compared with that from X zeolites, which corresponds with a higher fraction of more thermally stable Pt ammonia complexes found in Y zeolites. Under our experimental conditions, about 20–30% of Pt2+ does not undergo autoreduction. Pt clusters are formed inside the zeolite cavities with a size of 1–4 nm. No effect of the faujasite matrix (Si/Al ratio, nature of alkali cations) on the extent of the autoreduction or on the Pt0 size and location is observed in contrast to the reaction of CO+NO, which is substantially accelerated by the increasing electropositivity, diameter, and number (X > Y) of alkali cations.

Catalytic conversion of oxygen containing cyclic compounds. Part I. Cyclohexanol conversion over H[Al]ZSM-5 and H[B]ZSM-5

Abstract Cyclohexanol (CHL) conversion was measured under low-pressure on-stream conditions on HZSM-5 zeolites and on H-boralite using small amounts of catalysts. Catalytic runs were followed by TPD of the surface species, which also were studied using FTIR spectroscopy. It was found that the individual reaction steps, low-temperature dehydration (yielding cyclohexene), skeletal isomerization (ring contraction to methylcyclopentene) and reactions involving H-transfer (formation of methylcyclopentane, substituted aromatics and simple olefins) were conditioned by the presence of strong acid sites whose number needed for these reactions increased from the dehydration to the H-transfer reactions. The CHL reaction was compared with the reactions of cyclohexene, and the effect of Lewis acid sites present in some cases together with the Bronsted sites was discussed. Ammonia was found to block the active centers, reduce the isomerization and especially the reactions involving H-transfer. Contrary to ammonia, methanol reacted with the CHL surface complexes yielding more substituted aromatics and more olefins than CHL or methanol alone (and/or simple sum of CHL and methanol products).

Effect of zeolitic water on the carbonylation route of platinum(II) in NaX to [Pt3(CO)6]22− Chini complexes embedded in cavities of the zeolite

Abstract The effect of water content in [Pt(NH 3 ) 4 ] 2+ NaX on the direct synthesis of Pt Chini complexes [Pt 3 (CO) 6 ] 2 2− embedded in zeolitic cavities as well as the carbonylation process of PtNaX to the same anionic complex were studied using in situ FTIR and UV–Vis spectroscopies. It was found that the presence of water affects carbonylation route of the Pt tetrammine complexes; low water content makes this carbonylation similar to that of PtNaX. The interplay of water content and active sites needed for its decomposition via WGS reaction is assumed to play the decisive role in the reaction route. The rate of carbonylation is affected by the amount of zeolitic water, which supports migration of Pt species and supplies protons to the charge-compensating species, accelerating thus the carbonylation process. IR wavenumbers of both bridge and on-top bonded CO in platinum Chini complexes depend on the amount of water ligands.

Ship-in-bottle synthesis of anionic Rh carbonyls in faujasites

Abstract Direct carbonylation of [Rh(NH3)5Cl]2+ ion exchanged in sodium and potassium faujasites yields carbonyls, the composition of which depends on the extent of dehydration before carbonylation, and on the carbonylation temperature. Polynuclear anionic Rh carbonyls are formed at 25–100°C, if the water amount is sufficient. Greenish color of the species created at 75–100°C in NaX, KX and KY points to the [Rh6(CO)15]2− complex; at 50°C a white carbonyl is formed in NaX which structure seems to differ from the green complex in CO configuration. The anionic character of this species also follows from the lower wavenumber of terminal CO ligands in X zeolites and in KY compared to polynuclear neutral Rh carbonyl formed in less basic NaY. Carbonylation at 70–100°C after dehydration or carbonylation at 160–200°C lead to the formation of [Rh(CO)2]+. This carbonyl reacts to the polynuclear complexes at 25–100°C after readsorption of water vapors. Individual carbonyl types once formed can be interchanged by varying the temperature and water amount. In addition to the FTIR spectroscopy, UV/VIS measurements and mass spectrometric analysis of ammonia released during the thermal decomposition of carbonyls were also employed.

Morphology and structure of silicalite-1 crystals. Evidence of twinning by X-ray and electron diffraction

Silicalite-1 was grown hydrothermally in two batches with crystals 2–3 and 200–250 μm in length. Small crystals are penetrating overgrowths with a cruciform cross section, large crystals are typical columns known from the literature. X-ray and electron diffraction show the material to be orthorhombic, with the Pnma space group and unit cell a =20.0801(2) , b =19.9239(2) , c =13.4167(1) . Silicalite-1 is ubiquitously twinned on (110), and thus all four prominent vertical crystal faces on the large columnar crystals belong to the form {100}. In early stages of their growth, the untwinned crystals are tabular on (010) and elongated along [001], with prominent crystal forms {010}, {100}, and {101}. Etching of large crystals with diluted HF and optical examination of the material in polarized light offer and explanation of the hourglass structure and provide new clues about the growth process.

NaX-encaged Pt carbonyls: reversible substitution of CO ligands by oxygen and ammonia. Evidence for a conservation of the polynuclear Pt skeleton

The carbonylation of platinum tetrammine ions in NaX zeolite to platinum anionic (Chini) carbonyl complexes ([Pt3(CO)6]n2−, n=2), oxidation of these complexes and their recarbonylation were studied using in situ FTIR and UV/Vis spectroscopy. The near-surface layers as well as the Pt state were examined by XPS, and normalized absorbance of white line of the Pt L3-edge, evaluated from EXAFS experiments, was monitored in the course of the formation and decomposition of anionic Pt carbonyl complexes. It follows that the positive charge of platinum of the starting complex [Pt(NH3)4]2+ in NaX is largely maintained during carbonylation as well as oxidation and recarbonylation. From the kinetics of recarbonylation of the oxidized complex, which is faster than the primary carbonylation by more than one order of magnitude, a preservation of the Pt skeleton of the primary carbonyl is deduced. Mass spectrometric analysis of the gases evolved during decomposition of the complexes formed by carbonylation, oxidation and recarbonylation, enabled two suggestions to be made: (i) the composition of the oxidized complex; and (ii) stoichiometries for the conversions in agreement with all experimental data.

Subnanometer platinum clusters in zeolite NaEMT via stoichiometric carbonyl clusters

Abstract The direct carbonylation of [Pt(NH 3 ) 4 ] 2+ -exchanged zeolite NaEMT generates [Pt 3 (CO) 3 (μ-CO) 3 ] 2− 3 complexes nearly exclusively. The preferential formation of triplane complexes is attributed to the perfect matching of the complex with a nearly threefold symmetry axis to the hosting hypercage exhibiting a threefold symmetry as well. The complexes are formed by a reductive carbonylation, where (i) the reducing hydrogen is produced via the low-temperature water–gas shift reaction and (ii) the formed protons are stored in charge compensating ammonium ions. Small subnanometer platinum clusters of zero (Pt 0 x ) and partial positive (Pt δ + x ) charge are found during the decarbonylation of the anionic platinum carbonyl clusters, generated by reoxidation with the aid of the stored protons. The smallest clusters are assumed to exhibit a size-quantization (metal–insulator transition) effect. All transient states of the processes are monitored by UV–vis and IR spectroscopy in situ.

Ship-in-bottle synthesis of Pt–Rh carbonyls in NaX and NaY: FTIR study

Abstract [Pt(NH 3 ) 4 ] 2+ and [Rh(NH 3 ) 5 Cl] 2+ were ion exchanged into NaX or NaY either individually or simultaneously in Pt/Rh molar ratios 1:1 and 1:5. Carbonylation of the Pt ammine in NaX at 75–100°C leads to the orange [Pt 3 (CO) 6 ] 2 2− carbonyl and that of Rh ammine to the greenish [Rh 6 (CO) 15 ] 2− . Under the same reaction conditions, the mixture of both ammines (1:5) reacts to an almost white or pale beige bimetallic carbonyl, assigned to [PtRh 5 (CO) 15 ] − . This is also formed from the (1:1) mixture with the excess of Pt ammine carbonylated to [Pt 3 (CO) 6 ] 2 2− . The same bimetallic monoanionic carbonyl is formed in less basic NaY, although Rh ammine alone reacts in this matrix very slowly only to the neutral Rh 6 (CO) 16 complex. Oxidation of [PtRh 5 (CO) 15 ] − at 100°C extracts some Rh(CO) 2 + from the bimetallic carbonyl, but fast recarbonylation to the original species occurs.

Incorporation of zeolites in polyimide matrices

3-Aminopropyltriethoxysilane terminated polyimide precursors (polyamic acids) and Silicalite-1 were used to prepare zeolite-filled polyimide films. The accessibility of the pores of the zeolite built in both non-treated and treated polyimide matrices was studied using sorption of iodine from the vapour phase. Transport properties of the filled films were investigated using small probe molecules of hydrogen and methane.