Speier's Catalyst (H₂PtCl₆)

Overview

Speier's catalyst — hexachloroplatinic(IV) acid hexahydrate, H₂PtCl₆·6H₂O, CAS 18497-13-7 — is the historical hydrosilylation catalyst, first reported by Speier and colleagues at Dow Corning in 1957 and widely used as a lower-cost alternative to Karstedt where chloride residue is tolerated. Molecular weight 517.9 g/mol (hexahydrate), platinum content 37.6% by weight in the pure crystalline form. Commercial supply is typically as ready-to-dose 0.1%, 1.0% or 10% Pt solutions in isopropanol.

Speier is a Pt(IV) precatalyst that requires in-situ reduction to the catalytically active Pt(0) state before hydrosilylation can begin. The reducing agent is the Si–H functionality of the crosslinker itself: Si–H reduces H₂PtCl₆ to colloidal Pt(0) clusters with formation of Si–Cl byproduct and HCl gas. The induction period — typically 1–5 minutes at 80°C — is the rate-limiting step in early cure and the source of much batch-to-batch variability in Speier-cured silicone consistency.

Once reduced to Pt(0), Speier-derived catalyst proceeds through the same Chalk-Harrod hydrosilylation cycle as Karstedt, but with reduced selectivity — colloidal Pt clusters are more prone to side reactions (β-hydride elimination, Si–H homologation, isomerisation of α-olefin substrates to internal olefins). The chloride ligand also inhibits some β-substituted vinyl substrates, requiring higher Pt loading to achieve equivalent cure rates. Speier remains preferred over Karstedt for: silane manufacture where chloride byproduct is anyway present from chlorosilane chemistry; industrial RTV-2 mould rubber where part appearance is not critical; hydrosilylation R&D where cost-per-Pt-mole drives experimental design.

The chloride byproduct is the key operational distinguishing feature versus Karstedt. It produces minor part discolouration during cure (yellow to brown depending on Pt loading), induces minor corrosion in cured silicone in contact with copper or silver, and is the reason Karstedt has displaced Speier in modern medical, food-contact, and optical applications.

Technical Specifications

Applications

Industrial RTV-2 Mould Rubber

Non-medical applications — concrete-form release moulds, low-cost prototype tooling, candle and wax-art moulds — use Speier 1% IPA solution at 20–40 ppm Pt loading. The slight cure-yellowing is acceptable when the mould rubber is itself not the visible final part. Cure cycle is 30 minutes to 2 hours at 80°C, or overnight at room temperature.

Silane Synthesis and Silicone Polymer Manufacture

Hydrosilylation of methyl-hydrogen silicone fluid with α-olefins to make alkyl-modified silicones for cosmetic and lubricant applications, or addition of Si–H to vinyl-trichlorosilane to make functional silane intermediates. Uses Speier at 50–200 ppm Pt loading because the substrates are inherently chloride-bearing and the reactor is designed to handle HCl byproduct. The 10% Pt IPA solution is the working stock for these applications.

Laboratory Hydrosilylation R&D

Model substrate development, mechanism studies, novel ligand screening — typically uses 0.1% or 1% Speier solutions at 100–500 ppm Pt loading because reaction-rate and conversion data drive experimental design rather than finished-part appearance.

Fixed-Bed Continuous Hydrosilylation

Chloroplatinic acid-impregnated alumina catalyst preparations are used in fixed-bed continuous hydrosilylation reactors in commodity silicone manufacturing.

Selection Guide vs Karstedt

Choose Speier when: substrate or product is anyway chloride-bearing (silane synthesis from chlorosilanes); cost per Pt mole drives the decision and slight cure colour is acceptable (industrial RTV-2 mould rubber); R&D mechanism work or hydrosilylation substrate screening. Choose Karstedt for: medical, dental, food-contact, optical, electronic encapsulation, and any application where part discolouration or chloride residue would fail downstream qualification.

Equivalent Grades

• Heraeus Speier — primary commercial supply
• Umicore H₂PtCl₆·6H₂O — alternative supplier
• Johnson Matthey Pt-Cl — alternative supplier
• Generic Chinese H₂PtCl₆ IPA solution — commodity grade, ICP-MS verified