A reaction cell for in situ studies of hydrothermal titration

Abstract

A reaction cell for in situ studies of both ion-exchange and hydrothermal titration reactions has been designed and built. This cell allows the injection of reagents into a reaction mixture over time during X-ray data collection. A demonstration experiment followed the injection of Na2SO4(aq) into BaClz(aq) to yield a time-resolved X-ray powder diffraction pattern of the precipitation of BaSO4(s). Time-resolved synchrotron X-ray powder diffraction has provided information on the crystallization kinetics and the identity of intermediate phases during hydrothermal synthesis experiments. In particular, the formation of zeolites, microporous phosphates, mesoporous silicates, mieroporous sulfides and Sorel cements have been investigated extensively (Barnes et al., 1996; Francis et al., 1996; Gualtieri et aL, 1997; Norby, 1996, 1997a,b; N~rlund Christensen et aL, 1997). We have previously developed a reaction cell for these in situ studies (Norby, 1996) in which reactions take place in a 0.5-1 mm quartz glass capillary mounted in a Swagelock fitting and held in place using a ferrule. Pressure applied from a nitrogen cylinder and heating using a hot air stream allow hydrothermal conditions to be achieved. This method permits the study of a reaction mixture under static or noninteractive conditions. Some syntheses however require the addition of reactants over time. Such hydrothermal titration techniques have been used in syntheses of microporous sulfides (Parise et aL, 1996) as well as to study the formation of iron sulfide phases via precipitation/transformation reactions (Schoonen & Barnes, 1991). These studies were performed using steel autoclaves where injection of reactants without loss of pressure could be performed. Typically, a reaction mixture is kept at temperature and pressure for some time to allow formation of precursor species. Later, additional reactants are injected into the solution to induce precipitation of the materials of interest. These investigations are important for gaining mechanistic information, but are restricted in that reactions must be quenched before analysis. By performing time-resolved in situ experiments, it is possible to obtain detailed information about the system under actual reaction conditions. We describe here the development of a microreaction cell for in situ powder diffraction studies which can be used for hydrothermal titration, ion exchange, hydrothermal precipitation reactions and to investigate solid/gas phase interactions in real time. The design of the reaction cell is shown in Fig. 1. Reactions take place in a 1 mm quartz glass capillary (A) which is mounted in a Swagelock tee (B) using a Vespel ferrule. Through the connected tube (C), a pressure can be applied to the surface of the reaction mixture in the capillary. Injection takes place through a 0.3 mm quartz glass capillary (D), which goes through the tee and into the 1 mm capillary. This 0.3 mm capillary is mounted between a Swagelock elbow and the tee with Vespel ferrules, all of which are mounted on a modified goniometer head (F). Injection under pressure through the elbow (E) is possible via a gas chromatography syringe (not shown) mounted in an aluminium holder. A screw connected to the piston of the syringe ensures pressurization. By turning the screw, the piston is depressed and a controlled volume can be injected into the 1 mm capillary through the 0.3 mm capillary. Alternatively, the position of the sample capillary (A) can be plugged and the assembly extended to expose the injection capillary location to the X-ray beam. Ports (G) and (C) then become supply and exhaust lines, respectively, for flowthrough ion-exchange experiments or solid/gas phase reac