K. Seshan

Carbon dioxide reforming of methane in the presence of nickel and platinum catalysts supported on ZrO2

Publisher Summary Carbon dioxide reforming of methane to produce synthesis gas (CO+H 2 ) is currently attracting a much interest. One of the reasons for this interest is that the reaction consumes CH 4 and CO 2 , both of which are greenhouse gases. CO 2 reforming of methane is only a permanent solution to the problem of CO 2 emissions if the synthesis gas generated from the reaction is not used to produce fuels which relatively quickly end up as CO 2 once more as result of combustion. A possible solution to this is to convert the syngas to polymeric materials such as polycarbonates which will have long-term usage. Another advantage of the CO 2 /CH 4 reaction is that the syngas has a H 2 /CO ratio of 1:1 which is ideal for the manufacture of certain chemicals—for example, oxo-alcohols by the reaction of CO +H 2 plus the corresponding alcohols. Compared with the production of such a syngas mixture from the reaction of steam with coke, in which methane is also formed, the purity of the syngas formed by C q reforming is much better (>99.8%); impurities such as methane give problems during the formation of polycarbonates. The aim of this chapter is to develop catalysts which are capable of catalysing the CO 2 /CH 4 reaction under as near as possible to stoichiometric conditions (CO 2 /CH 4 = 1).

Structure-activity relations for Ni-containing zeolites during NO reduction I. Influence of acid sites

The influence of the zeolite structure on the catalytic properties for the reduction of NO with propane and propene was studied. A relation between concentration and strength of acid sites and the activity of Ni-exchanged ZSM-5, MOR, and MCM-22 was found. Ni/ZSM-5, which contains a high concentration of strong acid sites, is the most active catalyst for the NO reduction with propane. Using propene as reductant Ni/ZSM-5 and Ni/MCM-22 rapidly deactivate due to the acid site-catalyzed formation of carbonaceous deposits, while Ni/MOR is less affected by the deposits formed because of its larger pore size.

The use of niobia in oxidation catalysis

This paper summarises the background to work carried out at the University of Twente on the use of niobia as a catalyst for the oxidative dehydrogenation of propane to propylene and discusses the development of promoted niobia catalysts for this reaction. Results are also presented which illustrate the use of niobia in catalysts for other reactions such as the oxidative coupling of methane, the oxidative dehydrogenation of ethane and the oxidative dehydrogenation of methanol. It appears that niobia and niobia-modified catalysts, when used in high-temperature oxidation processes, can exhibit relatively high selectivities compared with more conventional catalysts.

Structure-activity relations for Ni-containing zeolites during NO reduction: II. Role of the chemical state of Ni

The influence of the metal in Ni-containing zeolites used as catalysts for the reduction of NO with propane and propene was studied. In the fresh catalysts, Ni is located in ion exchange positions for Ni/MOR, Ni/ZSM-5, and Ni/MCM-22. The formation of carbonaceous deposits, the removal of Al from framework positions, and the migration of Ni ions to nonaccessible positions were identified as primary reasons for the changes in the activity of the catalysts during the reduction of NO with hydrocarbons. Changes in the chemical state of Ni, i.e., isolated Ni2+ species being converted into Ni2+ on the surface of small Ni oxide clusters, affect the activity to a less significant extent compared to the changes in the location of Ni within the zeolite framework.

The influence of acidity on zeolite H-BEA catalyzed isobutane/n-butene alkylation

The influence of the concentration of acid sites for isobutane/n-butene alkylation on zeolite BEA with varying degrees of Na+ ion exchange is reported. All catalysts studied showed complete n-butene conversion over a significant time-on-stream. Isooctanes were the dominating products over H-BEA, while the importance of di- and multiple alkylation increased with increasing Na+ concentration in the zeolite. In parallel, the integral number of turnovers per catalytic site markedly decreased. The acid site deactivation is affiliated with blocking by hexadecane species. The higher rate of catalyst deactivation with increasing degree of Na+ exchange is thus attributed to the increase in the probability of having a further butene molecule added to a C8 alkoxy group in place of undergoing hydride transfer from isobutane to desorb as isooctane. This is a direct result of the decreasing concentration of alkoxy groups that can react with n-butene while maintaining the same rate of olefin transport through the pores. Addition of a strong hydride donor/acceptor molecule (adamantane) indicates that the product composition of the isooctane isomers is determined by their lifetime as alkoxy groups. The longer the lifetime before experiencing hydride transfer, the closer the product composition approaches the chemical equilibrium.

On the surface reactions during NO reduction with propene and propane on Ni-exchanged mordenite

The reduction of NO with propene and propane on Ni-exchanged mordenite was investigated by in situ infrared spectroscopy. Nitrates and organic nitro compounds were formed on the catalyst upon exposure to NO, O2, propene and/or propane. In the presence of propene, carbonaceous deposits consisting of oxygenated carbon species, condensed aromatic compounds and polyenes were formed. The concentration of deposits on the catalyst was considerably lower for propane. Isocyanates associated with Ni were identified as reactive intermediates during the reduction of NO to N2. Oxygenated carbon species contribute to the formation of CO2 and might be involved in the formation of isocyanates.

The influence of extraframework aluminum on H-FAU catalyzed cracking of light alkanes

The conversion of light linear and branched alkanes on two faujasite samples containing different concentrations of free Bronsted acid sites and extraframework alumina (EFAL) was studied between 733 K and 813 K. Protolytic cracking and bimolecular hydride transfer proceeded solely on Bronsted acid sites. For cracking of n-alkanes, the variation of the concentration of extraframework aluminum did not affect the catalytic activity per accessible Bronsted acid site. The activity to dehydrogenation is enhanced in the presence of EFAL and, unlike protolytic cracking, it decreased with time on stream. At high conversions relatively high concentrations of olefins change the selectivity and decrease the turnover frequencies. Compared to n-alkanes, the catalytic activity to convert iso-alkanes is enhanced in the presence of extralattice alumina.

Reduction of nitric oxide by propene and propane on Ni-exchanged mordenite

The reduction of NO over Ni-exchanged mordenite in the presence of excess oxygen using propane and propene as reducing agents was studied. During ion exchange of the zeolite with Ni2+, Bronsted acid sites were formed resulting from the hydrolysis of divalent Ni2+ ions. The chemical state of the Ni ions and the density and strength of the acid sites strongly determine the activity and selectivity of the catalysts for the NO reduction. Non-reduced catalysts showed a high selectivity to N2, while after partial reduction of the Ni ions significant concentrations of NO2 and N2O were formed. In addition, low concentrations of NO2 were formed during reduction of NO with propane and related to the more demanding activation of the alkane. An optimum concentration of acid sites, which in turn depends on the nickel loading, is required to obtain high NO conversion with propane. Deactivation due to deposition of carbonaceous species was observed during the NO reduction with propene for samples with high concentration of acid sites.

Presence of Lithium Ions in MgO Lattice: Surface Characterization by Infrared Spectroscopy and Reactivity towards Oxidative Conversion of Propane

The surface morphology of Li-promoted MgO catalysts prepared using the sol-gel method (sg) and wet impregnation procedure (imp), respectively, has been studied by low-temperature infrared spectroscopy of adsorbed CO molecules. The results show that step sites, as unselective catalytic centers, are the major features existing on the surface of pure MgO, and those are active toward the oxidative conversion of propane. However, the concentration of these sites is drastically reduced by the incorporation of lithium ions in the MgO lattice. In fact, the incorporated Li (+) ions tend to move into the surface region and occupy sites associated with lower coordination number (e.g., step sites). Li/MgO-sg catalysts are characterized by a higher concentration of incorporation of lithium compared to Li/MgO-imp. In the case of oxidative dehydrogenation/cracking of propane, Li/MgO-sg catalysts show higher activity and selectivity to olefins compared to materials prepared using wet impregnation. Catalytic performance differs strongly regarding (i) the amount of olefins formed, and (ii) the ratio of C(3)H(6)/C(2)H(4). It is shown that high density of active sites is essential for further oxidative dehydrogenation of propyl radicals to propylene and suppression of cracking reactions pathway.

Oxidative dehydrogenation and cracking of ethane and propane over LiDy Mg mixed oxides

The oxidative dehydrogenation and cracking of ethane and propane over LiDyMg mixed oxides is reported. High yields of olefins and only moderate formation of carbon oxides was observed. Both are primary products that hardly interconvert under the reaction conditions used. Addition of chloride increases the rate of reaction, while slightly decreasing the selectivity to olefins. The addition of carbon dioxide strongly decreases the rate of reaction, the negative order of 0.5 indicating that two active Li+sites are blocked by the adsorption of one CO2molecule. The reaction proceeds at low oxygen pressure primarily via elimination of dihydrogen, while at higher oxygen partial pressure the hydrogen elimination occurs via water formation. It is speculated that dehydrogenation and cracking involve Li+and a rather nucleophilic oxygen site.