1.  Field of the Invention
The present invention relates to a method for the production of N-vinyl-2-pyrrolidone by gas-phase dehydration reaction at atmospheric pressure, in particular to a method for producing N-vinyl-2-pyrrolidone by dehydrating N-(.beta.-hydroxyethyl)-2-pyrrolidone employing a mixed oxide of group IV elements as catalyst at a temperature of 300.degree.-450.degree.  C. and at a space velocity of 500-4500 hr.sup.-1. 
2.  Description of the Prior Art
N-vinyl-2-pyrrolidone is a valuable and useful fine chemical.  Due to its unique physical properties such as water solubility, high polarity, nontoxicity, chemical stability and cation activity, it has been widely applied in the manufacture of adhesives, paints, textiles, foods and personal medicines.  The homopolymers or copolymers thereof have improved film strength, dye compatibility, rigidity and adhesion. 
Conventionally, N-vinyl-2-pyrrolidone is produced by utilizing the "Reppe Reaction" to subject 2-pyrrolidone and acetylene to vinylation.  However, there are various difficulties with the acetylene employing Reppe Reaction.  Moreover, acetylene is easy to explode, and is thus difficult to transport and handle.  Consequently, alternative methods have been proposed.  Among them, process employing N-(.beta.-hydroxyethyl)-2-pyrrolidone as raw materials is deemed as most desirable process.  For example, in U.S. Pat.  No.  2,775,599 issued to Puetzer et al, it is disclosed that N-vinyl-2-pyrrolidone is obtained by reacting N-(.beta.-hydroxyethyl)-2-pyrrolidone with thionyl chloride to form N-.beta.-chloroethyl-2-pyrrolidone, followed by removing hydrogen chloride.  Also, U.S.S.R. Patent No.  125,567 discloses a method which involves reacting N-(.beta.-hydroxyethyl)-2-pyrrolidone with acetic anhydride to form ester, followed by removing acetic anhydride to obtain N-vinyl-2-pyrrolidone.  Methods for producing N-vinylpyrrolidone by directly dehydrating N-(.beta.-hydroxyethyl)-2-pyrrolidone without the formation of intermediates are disclosed in, for example U.S. Pat.  No.  2,669,570.  According to the production method of said patent, the dehydration reaction is carried out by directly contacting N-(.beta.-hydroxyethyl)-2-pyrrolidone with dehydration catalysts at a temperature of 300.degree.-340.degree.  C., and under sub-atmosphereic pressures below 100 mm of mercury at a hourly vapor space velocity of 500-4000 hr.sup.-1.  The employed dehydration catalysts are active aluminum, calcium oxide-aluminum or iron oxide-potassium hydroxide.  By using active aluminum, N-vinyl-2-pyrrolidone is produced by yields above 64 mole %.  The catalytic dehydration process, however, should be carried out at a reduced pressure below 100 mm of mercury, and thus is not practical for industrial production. 
U.S. Pat.  No.  3,821,245 discloses a method for producing N-vinyl-2-pyrrolidone by dehydrating N-(.beta.-hydroxyethyl)-2-pyrrolidone employing an oxide selected from zirconium oxide, thorium oxide, cerium oxide, zinc oxide and chromium oxide as a catalyst.  The yield of the method is improved and the reaction can be conducted at atmospheric pressure.  Among these oxides, zirconium oxide has the best catalytic activity, and when the reaction is carried out at 350.degree.  C. at a space velocity of 1800 hr.sup.-1 for 2.5 hours, the conversion of N-(.beta.-hydroxyethyl)-2-pyrrolidone can reach 95.7 mole %.  However, disadvantages of the method are that the selectivity of N-vinyl-2-pyrrolidone is not high, merely 73.8 mole %, and its catalyst life is short.  When the reaction proceeds for 50 hours, the conversion rapidly drops to 80 mole %, and thus the method is not suitable for industrial production.