Fixed bed reactor for gas-phase catalytic oxidation and processes for producing acrolein or acrylic acid

A fixed bed reactor for gas-phase catalytic oxidation, the fixed bed reactor comprising

• a reaction tube filled with a first catalyst for producing acrolein by gas-phase catalytic oxidation of propylene with molecular oxygen and
• a reaction tube filled with a second catalyst for producing acrylic acid by gas-phase catalytic oxidation of acrolein with molecular oxygen, wherein
• a solid acid is placed in an end portion(s) of the reaction tube or between catalyst layer of the catalyst, or mixed in the catalyst,
• the solid acid is an oxide or a complex oxide containing at least one kind of element selected from the group consisting of aluminum (Al), silicon (Si), titanium (Ti) and zirconium (Zr), and has a shape of sphere, cylindrical column, cylindrical tube, star, ring, tablet or pellet,
• the first catalyst is the complex oxide containing molybdenum (Mo) and bismuth (Bi),
• the second catalyst is the complex oxide containing molybdenum (Mo) and vanadium (V), and
• acid strength (H0) of the solid acid meets an inequality:
• −5.6≦H0≦1.5.

A fixed bed reactor for gas-phase catalytic oxidation, the fixed bed reactor comprising a reaction tube filled with a gas-phase oxidation catalyst for producing acrylic acid by gas-phase catalytic oxidation of acrolein with molecular oxygen, wherein

• a solid acid is placed in an end portion(s) of the reaction tube or between catalyst layers of the catalyst, or mixed in the catalyst,
• the solid acid is an oxide or a complex oxide containing at least one kind of element selected from the group consisting of aluminum (Al), silicon (Si), titanium (Ti) and zirconium (Zr), and has a shape of sphere, cylindrical column, cylindrical tube, star, ring, tablet or pellet,
• the catalyst is a complex oxide containing molybdenum (Mo) and vanadium (V), and
• acid strength (H0) of the solid acid meets an inequality:
• −5.6≦H0≦1.5.