Sulphur-resistant Catalysts

There have been a considerable number of papers reporting the properties of sulphur-resistant methanation catalysts, i.e., catalysts which can operate successfully in significant partial pressures of H2S. Most of these report work using catalysts containing vanadium, molybdenum, and such metals. However, attempts have been made to find nickel-based catalysts containing suitable additives to allow them to operate in such atmospheres. For example, Bartholomew and Uken115 have compared the deactivation behaviour of a range of nickel catalysts in 10 p.p.m. H2S. They found that nickel boride catalysts and Raney nickel materials deactivated more slowly than did unsupported nickel and alumina-supported nickel. They attributed this improvement to two factors  

Alstrup, J. R. Rostrup-Nielsen, and S. Roen, Appl. Catalysis, 1981, 1, 303. 

Rostrup-Nielsen, NATO Adv. Study Inst. Ser., Ser. E, 1982, Progress in Catalyst Deactivation , p. 209. 

Kelley and D. W. Goodman, in Chemical Physics of Solid Surfaces and Heterogeneous Catalysis , ed. D. A. King and D. P. Woodruff, Elsevier, Amsterdam, Oxford and New York, 1982, Vol. 4, p. 427. 

Bureau of Standards, Washington. Oudar has also reviewed work on sulphur poisoning of single crystal surfaces.124 

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Haldor Tops e have described a number of sulphur resistant catalyst formulations.131 132 For example, metals of Groups VB or VIB with Fe, Co, or Ni on a porous ceramic (A1203 or Ti02) as support can give gases containing large amounts of hydrocarbons in addition to methane in the... 

Sulphur is not a catalyst poison like lead but strongly competes against pollutants for space on the active catalyst surface. This limits the efficiency of catalyst systems to convert pollutants at any sulphur concentration. The effect of sulphur as a competitor on the catalyst surface may be reversible but it can cause irreversible changes to the washcoat and some of the base metal components. Sulphur resistant catalysts are not an option because that necessitates trading off catalyst performance for the removal of other pollutants. In addition when particulate removal is required the conversion of sulphur to sulphate limits the total particulate reduction and can cause net increases in particulate. 

The silica carrier of a sulphuric acid catalyst, which has a relatively low surface area, serves as an inert support for the melt. It must be chemically resistant to the very corrosive pyrosulphate melt and the pore structure of the carrier should be designed for optimum melt distribution and minimum pore diffusion restriction. Diatomaceous earth or synthetic silica may be used as the silica raw material for carrier production. The diatomaceous earth, which is also referred to as diatomite or kieselguhr, is a siliceous, sedimentary rock consisting principally of the fossilised skeletal remains of the diatom, which is a unicellular aquatic plant related to the algae. The supports made from diatomaceous earth, which may be pretreated by calcination or flux-calcination, exhibit bimodal pore size distributions due to the microstructure of the skeletons, cf. Fig. 5. 

Solid oxide fuel cellsoperateatvery high temperatures, around 1,000°C. High temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system. SOFCs are also the most sulphur-resistant fuel cell type they can tolerate several orders of magnitude more sulphur than other cell types. In addition, they are not poisoned by carbon monoxide (CO), which can even be used as fuel. This allows SOFCs to use gases made from coal. 

Model 1-6 is a new hydrodearomatization (HDA) catalyst developed by ICERP to be used in aromatics hydrogenation of gas oil blends. The catalyst has been obtained by a highly improved NiO dispersion on the promoted alumina support having a bimodale pore distribution with a total pore volume of minimum 45 cmVg. 1-6 has a good HDA activity under rather moderate hydrotreating conditions i.e. 60 bar total pressure and a remarkable sulphur resistance. 

Aromatics reduction up to 10 vol. % is much more challenging than sulphur removal. If and when such a significant reduction is required a new reactor and separation system has to be added and integrated with the deep HDS unit. A proper sulphur resistant HDA catalyst such as Model 1-6 developed by ICERP can be used to attain a 10 % level of aromatics content in desulphurised blended feed up to 40 % CKGO with SRGO. 

As in the case of homogeneous catalysis, poisons can also lead to deactivation of heterogeneous catalysts. Soluble or volatile metal or nitrogen compounds can destroy acid sites, while carbon monoxide and sulphur compounds almost invariably poison nickel and noble metal hydrogenation catalysts by bonding strongly with surface metal atoms. These considerations often lead to the selection of less active, but more poison-resistant, catalysts for industrial use. 

Ferrandon, M., Carno, J., Jaras, S., and Bjornbom, E. Total oxidation catalysts based on manganese or copper oxides and platinum or palladium, II. Activity, hydrothermal stability and sulphur resistance. Appl Catal A 1999, 180, 153-161. 

When fuel contains heavier hydrocarbons than methane, or it is biofuel, or contains alcohols, the feedstock often contains additional compounds such as sulphur and phosphorus, that is, fertiliser impurities. In the petrochemical industry, gas-borne reactive spedes (i.e., sulphur, arsenic, chlorine, mercury, zinc, etc.) or unsaturated hydrocarbons (i.e., acetylene, ethylene, propylene and butylene) may act as contaminating agents (Deshmukh et al, 2007). These impurities cause catalyst deactivation by poisoning. The effect of a poison on an active surface is seen as site blockage or atomic surface structure transformation (Babita et a/., 2011). Therefore, it is important to choose poisoning-resistant catalyst materials. For example, nickel is not the most effective MSR catalyst although it is widely used in industry due to its low market price compared to ruthenium and rhodium. Both Ru and Rh are more effective in MSR and less carbon is formed in these systems, than in the case of Ni. However, due to the cost and availability of precious metals, these are not widely used in industrial applications. 

Apart from the hydrolysis step, the SCR-urea process is equivalent to that of stationary sources, and in fact the key idea behind the development of SCR-urea for diesel powered cars was the necessity to have a catalyst (1) active in the presence of 02, (2) active at very high space velocities ( 500.000 per hour based on the washcoat of a monolith) and low reaction temperatures (the temperature of the emissions in the typical diesel cycles used in testing are in the range of 120-200°C for over half of the time of the testing cycle), and (3) resistant to sulphur and phosphorus deactivation. V-Ti02-based catalysts for SCR-NH3 have these characteristics and for this reason their applications have also been developed for mobile sources. 

There are also different hypotheses on the reaction mechanism, as will be discussed in the following chapters. This is still an open area of research and a further understanding will certainly lead to the development of improved catalysts. There are, in particular, three main areas in which further development is necessary (1) improve the low-temperature activity, e.g. below 250°C, (2) improve resistance by deactivation by sulphur and (3) improve the hydrothermal stability. Hydrotalcite-based materials [3la,97] offer interesting opportunities in this direction. 

Serra, J.M., Chica, A. and Corma, A. (2003) Development of a low temperature light paraffin isomerization catalysts with improved resistance to water and sulphur by combinatorial methods. Appl. Catal. A Gen., 239, 35. 

New, low-cost catalyst materials (reducing or possibly eliminating precious metals) that achieve useful power densities and are resistant to damage from CO or sulphur compounds would benefit both fuel cell and fuel processor technologies. 

Another difference between Co and Fe is their sensitivity towards impurities in the gas feed, such as H2S. In this respect, Fe-based catalysts have been shown to be more sulfur-resistance than their Co-based counterparts. This is also the reason why for Co F-T catalysts it is recommended to use a sulphur-free gas feed. For this purpose, a zinc oxide bed is included prior to the fixed bed reactor in the Shell plant in Malaysia to guarantee effective sulphur removal. Co and Fe F-T catalysts also differ in their stability. For instance, Co-based F-T systems are known to be more resistant towards oxidation and more stable against deactivation by water, an important by-product of the FTS reaction (reaction (1)). Nevertheless, the oxidation of cobalt with the product water has been postulated to be a major cause for deactivation of supported cobalt catalysts. Although, the oxidation of bulk metallic cobalt is (under realistic F-T conditions) not feasible, small cobalt nanoparticles could be prone to such reoxidation processes. 

Transition metal sulphides are able to catalyze a very large number of reactions. The most important utilization concerns catalytic hydrotreating, but many others can be foreseen due to the resistance of these catalysts towards sulphur. For example, recent studies have demonstrated the interest of such catalysts for the selective conversion of carbon monoxide into hydrocarbons [1] or alcohols [2]. Until now, only few papers and patents report on the utilization of sulphides for fine chemical applications [3-6]. Nevertheless, this type of solids fits well to catalyze the reactions dealing with sulphur containing molecules. 

In 1949, the development of a catalyst based on a combination of platinum and an acidic component (e.g. A1203, A1C13) allowed the use of lower reaction temperatures than with the early catalysts.6 However, problems were still encountered with chlorine corrosion. In the 1960s, Universal Oil discovered that the addition of rhenium to a bifunctional Pt/Al203 catalyst resulted in slower deactivation by carbon deposition, and other dopants have since been found to modify the catalyst acidity and resistance to poisons, e.g. Cl, Sn, Ir. More recently, catalysts based on zeolites and noble metals have been shown to be more resistant to nitrogen and sulphur compounds, while giving a high activity and selectivity to branched alkanes. 

Slaugh has claimed that a catalyst consisting of MoSi2 has resistance to sulphur.130 This material gave 20% conversion of CO + H2 mixture (1 3) at 500-600 °C and the conversion was >2% after 90 min of H2S treatment. Other transition metal silicides (e.g., ZrSi, WSi2) were tested for methanation but had no activity. 

It is clear that methanol is less stable than many possible by-products, such as methane. The catalyst has to be selective. The selectivity of modern catalysts is above 99%. The original catalysts were active only at high temperature (300-400°C). The pressure applied was 25-35 MPa. Until the end of the 1960s basically the original catalyst was used. More active catalysts were known, but they were not resistant to impurities such as sulphur. In modern plants the synthesis gas is very pure and very active catalysts can be used. This has led to low pressure plants (260°C, 50-100 bar). The temperature is critical. A low temperature is favourable from a thermodynamic point of view. 

One of the main problems is the desulfurization of resistant molecules such as the substituted dibenzothiophenes (DBT), especially the 4,6-disubstituted DBTs (Figure 6). It is believed that the first step of HDS involves the coordination of the sulfur or of the C-S bond of the substituted thiophene to the catalyst, and the steric hindrance around the sulphur in DBTs impedes coordination. One possibility is to develop classical HDS catalysts on supports other than alumina. For example on acidic supports such as zeolites, the first reaction could be an isomerization of the alkyl groups to other positions of the aromatic ring, making the subsequent HDS easier. Some results are promising, but it is difficult to prepare the correct CoMoS phase on the microporous support. 

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