Selective Dehydrogenation Catalysts Research Articles

Silica- and L-zeolite-supported Pt, Pt/Sn and Pt/Sn/K catalysts for isobutane dehydrogenation

Reaction kinetics measurements, temperature programmed oxidation (TPO), transmission electron microscopy, and chemisorption measurements were used to study silica- and L-zeolite-supported Pt catalysts for isobutane dehydrogenation at 798 K in pure isobutane and at 873 K with a 2 : 1 hydrogen : isobutane feed. Highly selective dehydrogenation catalysts were prepared by depositing Pt and Sn on K-L-zeolite, as well as by depositing Pt, Sn, and K on silica. At 873 K and in the presence of hydrogen, the Pt/Sn/K-L catalysts exhibited significantly higher rates of isobutene production compared to the Pt/Sn/K/silica catalyst. Addition of Sn and K to Pt decreased the extent of carbon deposition on the catalysts by reducing the size of the Pt surface ensembles, thereby inhibiting the formation of highly dehydrogenated surface species that lead to coke and other undesirable products (e.g. hydrogenolysis). The presence of Sn may also facilitate the transport of the carbon deposits from the active sites to the support. In the absence of hydrogen, the pores of Pt/K-L-zeolite become blocked by carbonaceous deposits during isobutane dehydrogenation at 798 K. At 873 K and in the presence of hydrogen, the Pt/Sn/K-L catalysts exhibit deactivation due to coking, enrichment of the surface with tin, and/or sintering of the metal particles.

Zeolite-NaY-supported Ir/Sn catalysts derived from single- and dual-source organometallic precursors. Preparation and characterization of highly selective dehydrogenation catalysts

Zeolite-NaY-supported Ir/Sn catalysts (1 wt% Ir, 0.6 wt% Sn; 1:1 molar ratio) were prepared by adsorption of the organometallic precursors Ir(CO)2(acac) and SnMe3OH or (COD)2Ir–SnMe3 {COD= 1,5-cyclooctadiene) from hexane solutions followed by activation in H2 up to 773 K. The Ir/Sn/NaY catalysts displayed high selectivity for the dehydrogenation of propane to propene at ca. 773 K and, in the presence of H2, maintained thermodynamic conversion levels for up to 24 h. After activation, the catalyst derived from the dual-source precursors ({Ir+Sn/NaY) appeared homogeneous in composition, whereas the catalyst derived from the single-source precursor (Ir–Sn/NaY) appeared heterogeneous with distinct regions of visible particles. Scanning transmission electron microscopy revealed that the nanoscale metal particles present were small and uniform in size (ca. 1 nm) in the Ir + Sn/NaY catalyst but ranged in size from 1 to 10 nm in the Ir–Sn/NaY catalyst. Energy-dispersive X-ray analysis showed that bimetallic particles were formed for both catalysts. Temperature-programmed reaction of chemiosorbed carbon monoxide indicated that the Ir/Sn/NaY catalysts were electronically modified by the presence of tin in comparison with analogously prepared Ir/NaY catalysts.

Fortschritte bei der Produktion von Waschmittelolefinen und von linearem Alkylbenzol / Recent Advances in the Production of Detergent Olefins and Linear Alkylbenzenes

Recent advances, listed in the order of development, have been discussed: 1982-higher conversion Pacol process; 1983-more stable dehydrogenation catalyst, DeH-7; 1984-selective hydrogenation catalyst, H-14, 1986-DeFine process; 1989-new Pacol reactor design; 1990-more selective dehydrogenation catalyst, DeH-9; 1990-new Detal fixed-bed alkylation process

Catalytic conversion of alcohols. Part 10.—Influence of pretreatment on the selectivity of MgO and CaO

With both CaO and MgO catalysts, the hydrogen pretreated material is a selective dehydrogenation catalyst while the oxygen pretreated material has about the same activity for both dehydrogenation and dehydration. Initially the air (or oxygen) pretreated MgO sample produces an alkene distribution from 2-ols that resembles that obtained with alumina; as the reaction temperature is increased a similar alkene distribution, different from the equilibrium value, is obtained with both pretreatments. The three temperatures used with CaO yield an alkene distribution, with either pretreatment, that resembles the highest temperature run with MgO. Pure cis-2- or trans-2-methylcyclohexanol undergoes extensive cis—trans isomerization; the different alkene distributions from the two alcohols suggest that a trans elimination pathway contributes to the dehydration mechanism.

Dehydrogenation and dehydrocyclization over bimetallic catalysts: Problems solved and problems generated by basic research

AbstractThe discussion is devoted to some controversial points related to the aromatization of paraffins at high temperature (around 500°C), and low hydrogen pressure (1–8 bar) over non‐acidic catalysts, in particular platinum plus a second component on a neutral(ized) support.Special attention is paid to those points which relate to the question why the second component enhances the stability of the dehydrocyclization (DHC) activity of supported platinum.We first consider the valence state of the second component. On the basis of, among other factors, the total hydrogen uptake we argue that at least part of the second component is present in the reduced state, while TPR points to alloy formation.A comparison of the catalytic and chemisorptive behaviour of bulk alloys with that of supported bimetallics indicates that modification (alloying) of the gas phase‐exposed platinum surface is an essential action of the second component. This, in turn, suggests that the superior stability of bimetallic DHC catalysts is related to the catalytic properties of the alloys.Considering the changes in selectivity observed upon alloying we will argue that the most important one is the enhanced selectivity towards dehydrogenation of paraffins (olefins) to the corresponding olefins (poly‐olefins), which leads to the question: “Are bimetallics under DHC conditions merely selective dehydrogenation catalysts?”From the invariance in TON for dehydrogenation per surface‐exposed platinum atom up to very high dilution of platinum in gold (< 1% at Pt in Au), we infer that only one platinum atom is involved in the rate‐determining step.We will extrapolate the aforementioned results to actual DHC catalysts and consider the proposition that the active site in PtSn, PtRe (S), Pt (S) and deactivated Pt (i.e. Pt (C)) DHC catalysts consists of one platinum atom embedded in an “inactive” matrix.Finally we will list some points deserving further study, amongst them the interpretation of the results of techniques such as TPR, the relative importance of different DHC pathways, the chemisorptive properties of alloys and the causes underlying the low hydrogen uptake of “overreduced” supported platinum catalysts.