Ferromolybdenum in Catalysis: Hydrotreating, Oxidation and Purity Limits

Ferromolybdenum as the Upstream Source of Catalytic Molybdenum

Ferromolybdenum is an alloy of molybdenum and iron, normally produced by aluminothermic reduction of technical molybdic oxide together with iron oxide and scrap, and supplied in crushed lump form. Its primary consumption is in steelmaking, but a second and technically demanding outlet exists in catalysis: molybdenum chemicals, molybdenum oxides and molybdenum-bearing catalyst precursors are all derived from the same oxidation route that starts with molybdenum concentrate and passes through roasted oxide before any ferroalloy is tapped. Understanding the catalytic applications of ferromolybdenum therefore means understanding where the alloy, and the oxide stream behind it, enters the catalyst supply chain, what impurity ceilings apply, and which catalytic duties genuinely tolerate an iron-bearing molybdenum source.

Molybdenum is one of the few elements that combine a variable oxidation state, a layered sulphide structure and a high melting point. Those three features explain almost every catalytic use: partially reduced molybdenum oxides exchange oxygen with hydrocarbon molecules, molybdenum disulphide provides edge sites for hydrogen activation, and molybdenum oxide dissolved in a mixed-oxide lattice modifies acidity and redox behaviour of the host. Ferromolybdenum supplies molybdenum in a form that is easy to oxidise, easy to dissolve in ammoniacal liquor and cheap to ship in drums, which is why chemical converters buy it alongside technical molybdic oxide.

How Molybdenum Works in Hydroprocessing Catalysts

The largest single catalytic outlet is hydrotreating and hydrocracking, where molybdenum sulphide supplies the hydrogenation function. A typical catalyst consists of gamma-alumina extrudates or spheres carrying 8-15 wt% MoO3 with 2-5 wt% CoO for hydrodesulphurisation duty, or 1-4 wt% NiO for hydrodenitrogenation and higher aromatic saturation. The molybdenum is applied by pore-volume impregnation from ammonium heptamolybdate or ammonium dimolybdate solution, dried and calcined at roughly 400-550 degrees C so that the molybdate decomposes into a dispersed surface oxide monolayer.

Before the catalyst is put on stream it must be sulphided, usually with a spiked feedstock or with dimethyl disulphide, because the active phase is not the oxide but nanostructured MoS2 slabs that are one or two layers thick. Cobalt or nickel atoms decorate the edges of those slabs and create the so-called promoted sites. The practical consequence for a molybdenum buyer is that the finished catalyst performance depends on molybdenum dispersion, and dispersion depends on the purity of the molybdenum salt: sodium, potassium, chloride, silica and phosphorus all interfere with monolayer formation or poison the finished surface.

Because of that sensitivity, most catalyst plants do not dissolve ferromolybdenum directly. They buy technical molybdic oxide or ammonium dimolybdate that has already been purified by sublimation or by ammonia leaching and crystallisation. The ferromolybdenum route is used where an integrated plant roasts the alloy to oxide and then leaches it, capturing the iron as an insoluble residue, or where the catalyst is a lower-value product such as a guard bed, sulphur recovery catalyst or a catalyst for a non-selective oxidation duty.

Where Ferromolybdenum Enters the Catalyst Supply Chain

The conventional chain runs from molybdenite concentrate to roasted technical oxide, then to either ferromolybdenum or to purified molybdenum chemicals. In an integrated flowsheet, ferromolybdenum can be oxidised in a multiple-hearth furnace or rotary kiln at 550-650 degrees C in air, producing a mixture of MoO3 and iron molybdate phases. The calcine is then leached with ammonia liquor at pH 8-9, in which molybdenum dissolves as ammonium molybdate while iron, copper and most of the silica remain in the residue. Filtration, sulphide precipitation of heavy metals and crystallisation of ammonium dimolybdate complete the purification.

Two process details determine whether this route is economic. First, roasting temperature must stay below the point at which molybdenum trioxide begins to volatilise appreciably, otherwise molybdenum reports to the off-gas and recovery falls. Second, the ammonia leach must be followed by an efficient solid-liquid separation, because residual iron hydroxide carries molybdenum by adsorption and every percentage point of molybdenum in the leach residue is lost revenue. Plants that run this route normally target a molybdenum recovery in the mid-nineties across roast and leach, and they monitor molybdenum in the residue as a routine control assay.

The alternative is that ferromolybdenum is not oxidised at all. Instead it is used as a master alloy where molybdenum is required in the metallic state, and molybdenum-bearing components of the catalyst system rather than the catalyst itself are produced: reactor internals, distributor trays, thermowells, catalyst support grids and packings in molybdenum-containing stainless steels and nickel alloys. This is a real and often overlooked catalytic application, because hydroprocessing reactors operate at 300-420 degrees C and 50-200 bar hydrogen partial pressure, and the internals must resist both hydrogen attack and sulphidic corrosion.

Selective Oxidation and Ammoxidation Catalysts

The second major family of molybdenum catalysts is based on mixed oxides rather than sulphides, and it uses molybdenum in its highest oxidation state inside a crystal lattice. Iron molybdate, nominally Fe2(MoO4)3 with excess MoO3, catalyses the oxidation of methanol to formaldehyde in the 250-400 degrees C window; the molybdenum-to-iron ratio is deliberately kept above the stoichiometric value because surface molybdenum enrichment suppresses over-oxidation to carbon monoxide and maintains selectivity above 90%. Catalyst life is limited by molybdenum loss as a volatile oxide-hydroxide species, so the make-up of molybdenum is a recurring operating cost.

Bismuth molybdate and multicomponent Mo-Bi-Fe-Ni-O phases catalyse the ammoxidation of propylene to acrylonitrile and the oxidation of propylene to acrolein. Here molybdenum provides the lattice oxygen that is inserted into the allylic intermediate, and the redox cycle between Mo6+ and Mo5+ is the reason the reaction can run with gas-phase oxygen rather than a stoichiometric oxidant. Molybdenum-vanadium mixed oxides are used for oxidation of acrolein to acrylic acid and for oxidative dehydrogenation duties. In each case the specification is dominated by the same variables: phase purity, surface area, alkali content and the absence of elements that stabilise the wrong crystallographic phase.

Ferromolybdenum is relevant to these duties through the precursor, not through the alloy itself. Molybdenum oxide obtained by roasting ferromolybdenum can be converted into the ammonium molybdate solution from which iron molybdate or bismuth molybdate is precipitated, provided iron is either removed in the leach residue or deliberately added as a separately dosed high-purity iron salt. Adding iron as a stray impurity from an incompletely leached alloy is unacceptable, because the Fe:Mo ratio governs selectivity and cannot be corrected downstream except by re-precipitation.

Environmental, Sulphur Recovery and Hydrogen Catalysis

Molybdenum also appears in environmental catalysis. Cobalt-molybdenum catalysts on titania support are used for the hydrolysis of carbonyl sulphide and for tail-gas hydrotreating in sulphur recovery units, converting residual sulphur species back to hydrogen sulphide before the amine absorber. Molybdenum oxide is added as a co-promoter to vanadia-titania selective catalytic reduction catalysts, where it suppresses the transformation of anatase to rutile, limits vanadium mobility and reduces sulphur trioxide formation at operating temperature. Molybdenum carbide and molybdenum sulphide have been extensively studied as platinum-free hydrogen evolution electrocatalysts, and molybdenum-nickel alloys are used as electrode materials in alkaline water electrolysis.

In all these duties the functional requirement is a high-surface-area, well-dispersed molybdenum phase with controlled alkali and silica content. Alkali metals are strong promoters of sintering in molybdenum oxide systems and are also a poison in hydrotreating; silica forms an unreactive molybdenum-silica glass that removes molybdenum from the catalytically available surface. This is why catalyst-grade molybdenum chemicals carry ceilings on sodium, potassium, silica and chloride that are one to two orders of magnitude tighter than those in metallurgical-grade ferromolybdenum.

Grade and Impurity Control: Ferromolybdenum Compared with Catalyst Chemicals

The table below sets out typical ferromolybdenum grades under GB/T 3649 and shows why catalyst producers usually convert to a purified chemical before use. Values are typical specification ceilings for the grade families named; the purchase contract and the supplier certificate of analysis govern the actual lot.

Material	Mo content	C max	S max	P max	Si max	Typical catalytic role
FeMo55 (GB/T 3649)	55.0% min	0.15%	0.15%	0.05%	1.5%	Oxidation feed to ammonium molybdate route
FeMo60 (GB/T 3649)	60.0% min	0.10%	0.10%	0.05%	1.0%	Standard alloying; intermediate calcine feed
FeMo65 (GB/T 3649)	65.0% min	0.10%	0.10%	0.04%	1.0%	Lower iron load per unit Mo in calcine
Technical molybdic oxide	57-61% Mo as MoO3	0.05-0.10%	0.05-0.10%	0.02%	0.5-1.0%	Direct impregnation after ammonia leach
Ammonium dimolybdate	56.0-56.5% Mo	0.01%	0.01%	0.005%	0.002%	Pore-volume impregnation of hydrotreating catalyst
Catalyst-grade MoO3	66% Mo nominal	0.01%	0.02%	0.005%	0.01%	Mixed-oxide oxidation catalyst synthesis

Beyond the five elements in the table, catalyst buyers screen for arsenic, antimony, tin, lead, bismuth, selenium and tellurium in the low parts-per-million range, plus chloride and total alkali. Arsenic is a cumulative poison for hydrotreating catalyst and behaves similarly to phosphorus in blocking active sites. Chloride causes corrosion in the impregnation plant and residual chloride in the finished catalyst promotes metal agglomeration during sulphiding. Sampling follows GB/T 3650 and GB/T 4010 for ferroalloys, with lot size and the number of increments defined by the contract; chemical analysis is normally by ICP-OES after acid digestion, with carbon and sulphur by combustion infrared and chloride by ion chromatography.

Purchasing and Acceptance Practice for Catalyst-Bound Molybdenum

A catalyst plant buying ferromolybdenum for oxidation wants to know four things before the lot is accepted: contained molybdenum, the size fraction and the fraction of fines, the iron-to-molybdenum ratio, and the level of volatile matter and moisture. Because ferromolybdenum is priced on contained molybdenum, an assay dispute of one percentage point is commercially significant. The robust practice is to sample by lot using a falling-stream or riffle splitter, to retain a sealed reference sample, and to require that the certificate of analysis be issued on the same composite that is shipped rather than on a cast sample.

Size specification matters more than it first appears. Catalyst converters roasting in a multiple-hearth furnace need a narrow lump size, typically 10-50 mm, so that the bed is permeable and the residence time is uniform; excessive fines cause elutriation and dust losses, while oversize lumps remain unoxidised in the core and report molybdenum to the leach residue. Standard practice is to screen at the loading port and to specify a maximum percentage passing 5 mm. Moisture should be determined on the as-received sample, and the packing should be moisture-barrier lined, because damp ferromolybdenum oxidises slowly in storage and generates hydrogen on contact with water over long periods.

Finally, buyers should compute the delivered cost per kilogram of contained molybdenum rather than per tonne of alloy. That calculation combines the molybdenum assay, the recovery achieved in roast and leach, the energy and reagent cost of the conversion, and the freight and duty on the alloy. A grade with two points more molybdenum may be cheaper per contained unit even at a higher alloy price, and a lower-iron grade reduces the leach residue load and the molybdenum loss adsorbed on iron hydroxide.

Frequently Asked Questions

Q: Is ferromolybdenum itself a catalyst?
No. Ferromolybdenum is an alloy used as a master alloy or as a molybdenum source. The catalytically active phases are molybdenum sulphide, molybdenum oxide dispersed on a carrier, or mixed oxides such as iron molybdate and bismuth molybdate.

Q: What molybdenum loading is typical in a hydrotreating catalyst?
Most hydrodesulphurisation catalysts carry roughly 8-15 wt% MoO3 with 2-5 wt% CoO on gamma-alumina; nickel-promoted hydrodenitrogenation grades typically carry 2-4 wt% NiO with similar molybdenum levels.

Q: Why must hydrotreating catalyst be sulphided before use?
The active phase is nanostructured MoS2 with promoted edge sites, not the calcined oxide. Sulphiding with a spiked feed or a sulphiding agent converts the oxide to sulphide and creates the Co-Mo-S or Ni-Mo-S edge structures that activate hydrogen.

Q: Which impurities matter most when ferromolybdenum is oxidised for catalyst use?
Chloride, total alkali metals, silica, phosphorus and arsenic. Chloride and alkali cause corrosion and sintering, silica forms an inert molybdenum-silica phase, and phosphorus and arsenic block active sites.

Q: Can iron from ferromolybdenum be tolerated in a molybdenum catalyst?
Only where iron is part of the intended formulation, as in iron molybdate methanol oxidation catalysts, and then the Fe:Mo ratio is dosed deliberately from high-purity salts rather than left to alloy residues.

Q: What size fraction is normally specified for roasting-grade ferromolybdenum?
A 10-50 mm lump with a capped percentage of fines is the common request, because a permeable, uniform bed oxidises evenly and limits molybdenum loss to the leach residue.