Preparation, Characterization And Catalytic Properties Of Solid-Acid Catalysts For Methanol Dehydration To Dimethyl Ether

In China, there are more coal, less natural gas, and much less petroleum. Presently, the need for coal can be met by the domestic coal production. But,56%of the need for petroleum and25%of the need for natural gas must be met by importing. Consequently, the prices of natural gas (NG), diesel, and liquefied petroleum gas (LPG) rise every year, which seriously affects people’s daily life and the development of agriculture and industry. To solve these problems, an effective way is to produce dimethyl ether (DME) from relatively abundant coal in China because DME has been proved to be an ideal substitute for NG, diesel, and LPG Currently, production of DME in China is mainly by methanol dehydration with the catalysts of modified eγ-Al2O3or Molecular Sieve, such as HZSM-5. Modified γ-Al2O3catalysts are stable and selective at relative high temperature, but not very active at relative low temperature. In contrast, Molecular Sieve catalysts are quite active at relative low temperature, but not very selective and not very stable at relative high temperature. What is more, molecular sieve catalysts are relatively expensive. Obviously, it is necessary to study and develop new catalysts with high catalytic properties and low cost.In this study, three types of solid-acid catalysts (aluminum silicate, modified alumina, and modified bentonite) were prepared with different methods, tested for catalytic properties, and characterized for structure, composition, specific surface area, and acidity by X-ray diffractometer, X-ray flourescence analyzer, BET, pyridine FT-IR, and n-butylamine TPD. The main results are as follows:1. At atmospheric pressure and160-300℃, the prepared γ-Al2O3catalysts and acid-washed bentonite catalysts showed higher catalytic activity than aluminum silicate catalysts.2. A new precipitation method for γ-Al2O3preparation was developed by washing the precipitate with two types of organic solvents. The properties (such as structure, specific area, and catalytic activity) of the γ-Al2O3catalyst prepared with this new method are affected considerably by the concentration of the aluminum sulfate. The lower the concentration, the better the properties are. At the concentration of0.04mol/L, the γ-Al2O3showed a specific area of255m2/g and a methanol conversion of85%(260 ℃, catalyst:0.4g), much higher than the values (222m2/g,83%) of Beijing industrial active alumina.3. After washing with sulfuric acid, the activity of bentonite improved dramatically. The acid-washed bentonite catalyst prepared under optimum conditions showed a methanol conversion of84.3%at240℃and catalyst weight of0.4g.4. The activity of industrial active alumina can be improved by modifying with zirconium sulfate, titanium sulfate, or ammonium sulfate. The ammonium sulfate is a little better than the other twos. The activity of our best γ-Al2O3catalyst can also be further improved by modifying with ammonium sulfate or tungstophosphoric acid. The methanol conversion at260℃and catalyst weight of0.2g is84.3%for ammonium sulfate modifying and85.3%for tungstophosphoric acid modifying.5. The activity of acid-washed bentonite is affected by modifying with tungstophosphoric acid. When the content of the tungstophosphoric acid modifier in the catalyst rise to20wt%, the methanol conversion of the catalyst at220℃reached87%(catalyst weight of0.4g), equal to the activity of industrial HZSM-5catalyst, but much cheaper.6. The main active sites of the catalysts studied can be ascribed to the acid sites corresponding to the mid-temperature desorption peak at about450℃in the n-butylamine TPD spectrum. Both L acid sites and B acid sites were observed on the surface of the catalysts. Both of them might be active, but L acid sites might act in the form of B sites, because they can combine with water and become B sites.