▎ 摘　　要

NOVELTY - A catalyst has a core-shell structure with a double-shell layer whose inner shell layer comprises an active component and a water-absorbing metal oxide in a mass ratio of (5-80):1, preferably (20-35):1, and outer shell layer comprises an active component and an inorganic carbon material in a mass ratio of (10-100):1, preferably (40-55):1. The core of the catalyst is inert ceramic balls. The mass ratio of the inner shell layer to the outer shell layer is (0.5-2):1, preferably (1-2):1. The mass of the inner shell layer and the outer shell layer is 30-50%, preferably 35-45% of the total mass of the catalyst. USE - The catalyst is useful for producing methacrylic acid (claimed). ADVANTAGE - The catalyst constructs double-layered structure through secondary rotating roller coating, which can effectively solve the problem of the activity of heteropolyacid catalyst being weakened due to moisture absorption during production, transportation, storage and filling, can effectively avoid the problem of easily reducing the reaction activity of the catalyst or forcing the reaction to stop due to flying temperature in the process of strong exothermic reaction, and can effectively resist the stress caused by the bed pressure and reduce the impact and abrasion caused by the gas flow during the reaction process. The heteropolyacid catalyst has excellent compressive and abrasion resistance, and catalytic activity, and prolonged service life. DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following: (1) preparation of the catalyst, which involves (1) uniformly mixing pore-forming agent, active component powder and water-absorbing metal oxide to obtain a mixed powder, adding the mixed powder, inert ceramic balls and binder to a rotary coater, coating the mixed powder on the surface of the inert ceramic ball by adjusting the spraying speed of the binder in the coater, molding, drying and pre-baking at low temperature to obtain an inert ceramic ball covering the inner shell layer, and (2) uniformly mixing pore-forming agent, active component powder and inorganic carbon material to obtain a mixed powder, then adding the inert ceramic ball covering the inner shell layer, the mixed powder and the binder to the rotary, adjusting the spraying speed of the binder in the coater to realize the mixed powder coating on the surface of the inner shell layer, molding, drying, and calcining; and (2) preparation of methacrylic acid, which involves preheating and mixing raw materials methacrolein, oxygen-containing gas, which is air, and water vapor in a fixed-bed tubular reactor, and subjecting to gas-phase catalytic oxidation reaction under the action of the catalyst. The catalyst is packed in the fixed-bed tubular reactor, and both the upper and lower ends of the catalyst are filled with inert ceramic balls. The oxygen-containing gas is air, a mixture of air and diluent gas, or a mixture of molecular oxygen and diluent gas. The molecular oxygen is pure oxygen or air. The diluent gas is nitrogen and/or carbon dioxide that are inert.