Non-metal catalysts

Difficulties in desulphurising heavy feedstocks have led to attempts to use non-metallic catalyst for steam reforming. Molybdenum carbide and tungsten carbide are catalysts for steam and CO2 reforming and catal5dic partial oxidation [122] [450], However, in synthesis gas the carbides are stable only at elevated pressures (approximately 9 bar) and they are transformed into the oxides at ambient pressure. 

Molybdenum carbide will hardly be stable in a plug-flow reactor [450], because the carbide will be oxidised at the inlet. The molybdemun 

A calcium aluminate catalyst was tested in the Toyo Process [489], but ratios at 850°C were still a fraction of what is obtained on nickel at 500°C. The Toyo process is close to catalytic steam cracking discussed in the following. 

---

Turning to non-metallic catalysts, photoluminescence studies of alkaline-earth oxides in dre near-ultra-violet region show excitation of electrons corresponding to duee types of surface sites for the oxide ions which dominate the surface sUmcture. These sites can be described as having different cation co-ordination, which is normally six in the bulk, depending on the surface location. Ions on a flat surface have a co-ordination number of 5 (denoted 5c), those on the edges 4 (4c), and dre kiirk sites have co-ordination number 3 (3c). The latter can be expected to have higher chemical reactivity than 4c and 5c sites, as was postulated for dre evaporation mechanism. 

Industrially, the SR reaction is carried out at high temperatures (600-1,000°C) and high pressures (15-35 atm) in the presence of Ni-based catalysts. Cobalt and precious metals are also active for SR, but are more expensive.Other non-metallic catalysts have also been reported, but their activity is very low. ... 

Studies of catalytic asymmetric Mukaiyama aldol reactions were initiated in the early 1990s. Until recently, however, there have been few reports of direct catalytic asymmetric aldol reactions [1]. Several groups have reported metallic and non-metallic catalysts for direct aldol reactions. In general, a metallic catalysis involves a synergistic function of the Bronsted basic and the Lewis acidic moieties in the catalyst (Scheme 2). The Bronsted basic moiety abstracts an a-pro-ton of the ketone to generate an enolate (6), and the Lewis acidic moiety activates the aldehyde (3). 

Sites at the surface of non-metallic catalysts have also been investigated. The adsorption of H2 by Co304 has been studied by diffraction and has been referred to earlier.12 The adsorption of H2 by Mo and W sulphides however, has been the subject of a number of inelastic scattering investigations which have gone a long way to clarify the sites at which adsorption occurs, and probably the sites from which H2 is transferred in hydrodesulphurization reactions. 

Complications with in desulphurising heavy feedstocks have also lead to attempts to use non-metallic catalysts for steam reforming, but their activity is still inferior to that of nickel catalysts [14,15],... 

The action of adding a small amount of foreign atoms to form a solid solution in the lattice of a non-metallic catalyst is sometimes called doping. 

The activation of oxygen in oxygen transfer reactions is usually mediated by a suitable transition metal catalyst which has to be sufficiently stable under the reaction conditions needed. But also non-metal catalysts for homogeneous oxidations have recently been of broad interest and several of them have been compiled in a recent review.2 Other examples for well known alkene oxidation reactions are the ozonolysis, hydroboration reactions or all biological processes, where oxygen is activated and transferred to the substrate. Examples for these reactions might be cytochrome P450 or other oxotransferases. Of these reactions, this contribution will focus on transition-metal mediated epoxidation and dihydroxylation. 

Present knowledge of the electronic, surface, and catalytic properties of non-metals is substantially less advanced than that of metals. This is partly due to the fact that various physicochemical techniques are less rapidly applicable to non-metal catalysts. Relatively few metal oxide catalysts have been investigated by XAES techniques (Table VIII and Ref. 18). 

The pyrolysis of hydrocarbons follows the thermal cracking mechanism (4). Apart from the pressure, the conditions in the tubular steam reformer and in the preheater are not far from that of a steam cracker in an ethylene plant. With low catalyst activity, the pyrolysis route may take over. This is the situation in case of severe sulphur poisoning or in attempts to use non-metal catalysts so far showing very low activity (1). Non metal catalysts have mainly been based on alkaline oxides being active for gasification of coke precursors. However, it has been difficult to avoid the formation of olefins and other pyrolysis products (1,2,5). In fact, it was demonstrated (2,4) that co-production of syngas and light olefins was possible from heavy gas oil and naphtha over a potassium promoted zirconia catalyst. 

An important consideration in catalyst, reagent or substrate recovery is measuring and verifying how effective such recovery actually is. While we have modeled such recovery using dye-modified polymers, analyses of catalysts typically requires additional analytical work. For example, ICP analysis for residual metal can be used as a quantitative and sensitive assay. Such assays are however more problematic with non-metallic catalysts. In this paper, we show that bifunctional polymers where both a catalyst and a colorimetric label are included in the same polyacrylamide polymer provide a simple way to monitor separability and catalyst recovery for non-metallic polymer-bound catalysts. 

Vedrine, J.C. (1982) Physical Methods for the Characterization of Non-Metal Catalysts. In Surface Properties and Catalysis by Non-Metals. NATO ASI Series, 123. C.D. Reidel PuW. Comp., Dordrecht. 

Non-metallic catalysts — MgO, Li/MgO and La203 — known to produce methyl radicals during the methane oxidative coupling reaction have been shown to be active for NO reduction by CH4. Li-promoted MgO in the absence of O2 produces N2 and N2O with a (N2/N2O) selectivity below 2 at low temperature but which increases to 3 or more at higher temperatures. Unpromoted MgO is less active but produces almost 100% N2 at high temperatures. La203 is more active and selective for NO reduction to N2 by CH4 than MgO and Li/MgO catalysts. The activity of La203 continuously increases with temperature, at least up to 973 K, and selectivity for N2 rather than N2O... 

Matter PH, Ozkan U (2006) Non-metal catalysts for dioxygen reduction in an acid electrolyte. Catal Lett 109 115-123... 

As with PEMFCs, efforts have been done to replace metal catalysts completely with non-metal catalysts or at least by non-precious metal catalysts. Amongst these are, as for PEMFCs, macrocyclic complexes of porphyrins and phthalocyanines, and materials derived from these via heat-treatment steps. Very promising results, however, have recently been obtained using nitrogen-modified CNTs and carbon nanofibers [58, 59]. Here, activities close or similar to that of Pt were observed. However, the stability of these materials in a long-term durability study has still to be demonstrated. 

With low catalyst activity, the thermal cracking route (pyrolysis) may also take over in the reformer tube [389]. This is the situation in case of severe sulphur poisoning or in attempts to use non-metal catalysts with low activity. The risk of carbon formation depends on the type of hydrocarbon with the contents of aromatics being critical. Ethylene formed by pyrolysis results in rapid carbon formation on nickel (refer to Section 5. 2). Ethylene may also be formed by oxidative coupling if air or oxygen is added to the feed - or by dehydration of ethanol. 

An article is exclusively focused on outer sphere catalysts. In outer-sphere hydrogen catalysis, substrates such as ketone, imine, or A-heterocycle remain in the outer sphere. A hydride and a proton are transferred to these substrates by either a concerted or a stepwise path from catalysts such as Bullock s ionic hydrogenation catalysts, bifunctional catalysts in the tradition of Shvo and Noyori, and Stephan s frustrated Lewis pair catalysts. The outer-sphere pathways can use inexpensive metals and even non-metal catalysts, and lead to useful selectivity properties, particularly Noyori s asymmetric catalysis. ... 

Ceria is one of the best known oxygen storage materials and is, for example, used in three-way catalysts. Several recent works describe ceria as an oxidizer for the conversion of CH4 to syngas. Perovskite-type oxides (ABO3) represent another class of reducible oxides with the potential to be partial oxidation catalysts." In general, however, non-metal catalysts are found to have too low an activity for the CPO reaction to be commercially interesting. Therefore, studies of these classes of materials for CPO have focused on high temperature applications (above 800°C). Most recent works combine such materials with metals for use in the CPO process. 

In summary, there are readily available and easy to use XRD and chemisorption techniques that are applicable to metal catalysts, in particular, and to other catalysts, in general. If available, TEM can add additional information. Chemisorption is the most sensitive method, and all kinetic studies of metal catalysts should be accompanied by a measurement of the metal surface area and dispersion via a standard adsorption procedure. For non-metallic catalysts, adsorption sites can still be counted in many situations by finding the appropriate region of temperature and pressure to measure adsorption of one of the reactants, and some (or all) of these sites would be expected to be active sites under reaction conditions. Examples of such efforts have been reported for N2O, NO and O2 adsorption on Mn203 and... 

Intrinsic Activity. The steam reforming catalyst is normally based on nickel (Rostrup-Nielsen, 1984a). Cobalt and the noble metals are also active, but more expensive. Attempts to use non-metallic catalysts (Rostrup-Nielsen, 1984a) have not had commercial success, because of low activity and thereby the impact of pyrolysis (reaction (9)). 

---