Reactions of Hydrogen Chloride with Carbonaceous Materials and the Formation of Surface Chlorine Species

Evolution of Hydrogen Chloride (HCl) in industrial processes produces hazardous chlorinated organic compounds through secondary reactions with unburned carbon downstream of initial combustion sites. According to Stieglitz et al. (Chemosphere 1991, 23, 1255−1264), metallic chlorides present in unburned carbon works as source of chlorine as well as catalyst for formation of dioxins.

The role of unburned carbon formation of organochlorides is still unclear, however O₂ gasification of carbonaceous substances such as CO or CO₂ upon heating yields carbon active sites which works for formation of organic chlorides since they are more reactive than basal plane carbon.

In a recent article by Tsubouchi et al. 2016, and published in Energy & Fuels, the objective was to discover the mechanisms of HCl-Carbon interactions by investigating the effects of O₂ activation and metal addition to the interaction thereby ascertaining the role of carbon active sites and some metal catalyst in the formation of organic chloride species.

In order to analyze the main objective of possible HCl-Carbon interactions, Tsubouchi et al. (2016) applied means of X-ray Photoelectron Spectrum (XPS) and Temperature-Programmed Desorption (TPD). XPS measurement was utilized in order to determine functional forms of chlorine present in pure and metal-doped activated carbons (AC) samples.

For the experiment set up, phenol resin was first carbonized in stream of Helium as carbon sample were denoted by AC-0, AC-50, AC-100, AC-150 with respect to their increasing surface area. Several AC-100 samples were loaded with either Potassium (K), Calcium (Ca) or transitional metals such as Copper (Cu) and Zinc (Zn) cations via an impregnation method using ethanol solution. The subsequent samples was immediately exposed to a stream of 100ppm of HCl in high-purity N₂. The number of active sites of each surface of pure and metal-doped AC samples was also determined by CO/CO₂-TPD method and all experiments were carried out in a cylindrical flow-type quartz reactor.

From the results, Tsibouchi et al. (2016) discovered that when AC-0. AC-50, AC-100 and AC-150 (without any metal cations added) were passed into a stream of 100ppm HCl/N₂ immediately after heat treatment of 500⁰C, concentration of HCl decreased with respect to longer time periods. The amount of HCl which reacted with AC-150 sample was 8 times greater than AC-0 sample.

The rate of HCl reactions in HCl-treatment trials with 0.5 mass % of K/C, Ca/C. Cu/C and Zn/C specimen showed that K/C sample shared almost the same profile with AC-100 (when no metal cations were added) with initial increase with time on stream but decreased steeply after reaching a maximum at approximately 6min while Ca/C, Cu/C and Zn/C specimens decreased with an increasing time after 8-9 min. It can be inferred that metal cations increases the amount of HCl reaction concept for K⁺ with Zn/C having the highest increase of HCl reacted.

On investigation of the CO/CO₂-TPD behaviors, AC-0 generated small amounts of CO and CO₂ over the range of 150-200⁰C. AC-50, AC-100 and AC-150 specimens evolved CO at approximately 200⁰C and rate of CO formation increased rapidly beyond 400⁰C while CO₂ formation started around 150⁰C reaching its shoulder peak at approximately 250⁰C and also CO₂ formed increased remarkably at 400⁰C while CO₂ formation which also increased significantly at the rate of 350-450⁰C depending on the metal species with Cu greatly influencing the rate of CO₂ formation at approximately 350⁰C.

From XPS measurements to treat Cl forms in HCl treated carbon samples was seen that AC-100 and K/C solid lines generated a weak XPS signal binding energy range of 197-203eV with a peak maximum of 200eV while Ca/C, Cu/C and Zn/C showed broad peaks in range of 196-204ev with different peak maxima, Cl 2_p3/2 XPS peaks of KCl, CaCl₂, CuCl₂, ZnCl₂ and C₆H₅Cl appeared at 198.3, 198.5, 199.3, 198.5 and 200.5eV which shows that there is probability of both organic and inorganic  chlorides were present on surface of five carbon samples.

Deconvolution analysis of the five HCl-treated carbon samples showed the proportions of inorganic and organic chloride were 15-58 and 42-85mol% with Cl/C ratios in range of 1.6-4.4×10^-3. The ratios obtained from Ca/C, Cu/C and Zn/C samples were 2.0-2.3 times the value without any addition which showed inorganic chlorides can promote the formation of organic chloride species.

Tsubouchi et al. (2016) conclude on the basis of discussed results that the addition of these metals promoted the formation of organic chloride forms. The results helps in understanding organic chemistry during high temperature combustion process and efficient methods developed in order to inhibit formation of hazardous chlorinated compounds.