The surface decarburized WC-Co gradient cemented carbide based on impregnation liquid of ammonium metatungstate solution (AMT) can greatly enhance the comprehensive performance of cemented carbide; and with the simple process, easy to operate, it is suitable for industrial production. The preparation steps are as follows: 

1. Put the conventional WC-Co carbide under vacuum conditions at 100~500°C for dewaxing, then to carry out presintering in 800~900°C to obtain presintering billet with a certain strength and porosity; 

2. After them, the sample is dipped into ammonium metatungstate solution to form concentration gradient of AMT with a certain depth and distribution from the surface layer to the inner; 

3. Then, after the sample is dried, put it in vacuum furnace for sintering along with slow heating in low temperature stage, to make the ammonium metatungstate decomposed, which the broken down products (tungsten oxide) will react with surface substance of WC, Co, then we can get surface decarburized WC-Co gradient cemented carbide. 

 

Cemented carbide is a kind of metal ceramic tool material which prepared by powder metallurgy method by taking refractory metal compounds (such as WC, TiC, TaC, NbC, etc.) as the matrix, and transition of metals (like Co, Fe and Ni) as bonding phase. Due to its high strength, hardness and elastic modulus, wear resistance, corrosion resistance, low thermal expansion coefficient and stable chemical properties, etc, it in has important application in metal cutting, wood processing, oil drilling in mines, composite materials processing and other respects. Conventional class of WC-Co carbide is uniformly consisted with two phases of WC and binding phase, however it exists contrary relationship between the hardness, wear resistance and strong toughness. The WC - Co gradient cemented carbide structure with high wear-resistance and hardness surface, toughness core part was developed in the 1980s, while the appearance of surface decarburized WC-Co gradient cemented carbide makes it easier to control surface eta phase formation depth, and thereby to prepare the superior performance of WC-Co carbide. 

It is found that the supported ammonium metatungstate (AMT) catalysts with the carriers of silicon dioxide, titanium dioxide and aluminium oxide have exhibited good catalytic activity for the synthesis of ortho - light anisole by gas - phase reaction of catechol and methanol. Among them, the AMT / silicon dioxide catalyst with tungsten loading of 7.9% has the best catalytic performance; in addition, the species with weak acidity and alkalinity which formed by the partial decomposition of ammonium metatungstate in the calcination process has been indicated to be the active center of the catalytic reaction.


 

Preparation of silicon dioxide supported ammonium metatungstate catalyst is using the traditional impregnation method, which is dipping carrier of silicon dioxide with ammonium metatungstate (AMT) aqueous solution in equal volume, followed by drying at 363 K for 6 hours, and then calcining at a certain temperature for 3 hours under air atmosphere to obtain SiO₂/AMT catalyst. The study found that the calcination temperature affects the catalytic activity of silicon dioxide supported ammonium metatungstate catalyst; while the catalysts obtained at calcination temperatures of 533K and 573K showed the highest catechol conversion rate and selectivity of adjacent hydroxyl benzene methyl ether within 10 hours, which is respectively maintaining at about 97% and 90%.

 

Moreover, with the extension of reaction time, the activity of silicon dioxide supported ammonium metatungstate catalyst is decreased slowly, which is mainly attributed to the carbon deposition on the catalyst. The study of regeneration of deactivated SiO₂ / AMT catalyst has proved that both the alcohol washed purging and low temperature (533k) calcination could not obviously restore the activity of the catalyst, and also the regenerated catalyst has stability no better than the fresh catalyst. This may be due to the change of the properties (including the acidity and alkalinity) of active centers on catalyst surface after high temperature regeneration treatment, and to make the easier deactivation of regenerated catalyst.

In recent years, solid acid alkali catalysis of gas-solid-phase reaction of alkylation of catechols has attracted attention due to the environmentally friendly economy and overcoming the environmental pollution, separation and recovery difficulties and equipment corrosion and other defects, compared to conventional synthesis methods, and has shown significant advantages in saving resources and improving economic efficiency. It is found that Al-Ti-P-O, Al-Ti-Si-P-O and silica-supported ammonium metatungstate (AMT/SiO₂) catalyst exhibited high catalytic activity.

 

Hydrotalcite is a kind of anionic clay with layered structure, because of its special layered structure and high catalytic performance in acid-base catalysis, condensation, addition polymerization, hydrogenation and alcohol conversion reaction, hydrotalcite has attracted many concerns. The exchangeable property of hydrotalcite anions make all kinds of anions (such as metatungstate ion) can be introduced into the interlayer of hydrotalcite in succession, thus to form a large class of materials - pillared hydrotalcite layered material.

 

A study has pointed out a preparation method of ammonium metatungstate hydrotalcite catalyst, and studied its catalytic performance, while the preparation steps are as follows:

1. synthesis of hydrotalcite material, take the raw material of magnesium nitrate hexahydrate, aluminum nitrate and sodium hydroxide, and to control the final pH value among 8 ~ 10 of the system, then go on aging, filtration and washing to get hydrotalcite NO₃-MgAl;

2. Weigh a certain amount of fresh prepared hydrotalcite NO₃-MgAl to disperse in distilled water, stirring at room temperature for overnight to fully expand;

3. Add dilute nitric acid dropwise to adjust the pH value of hydrotalcite slurry among 5.5~6, then rise the temperature to 348K;

4. Rapidly add ammonium metatungstate solution under the condition of stirring, and to react at 348K for 2 to 3 hours, then pumping wash the sample and dry at 348K to get the ammonium metatungstate hydrotalcite catalyst, i.e. AMT-Mg₂Al.

The results showed that after changed with AMT, hydrotalcite will exhibit activity to the reaction of gas-solid phase reaction of catechol and methanol, while the conversion of catechol is 75%, the selectivity of guaiacol is 90%, but the catalytic activity decays.