《Table 1 NH3-TPD results of the catalysts》

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《ZSM-5/MAPO Composite Catalyst for Converting Methanol to Olefins in a Two-Stage Unit with a Dimethyl Ether Pre-Reactor》

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The number of strong acid sites and weak acid sites in the composites obviously decreased compared with those in ZSM-5.The high-and low-temperature peaks shifted from 430°C to 410°C and from 260°C to 205°C，respectively（Table 1）.The amount of the total acid sites significantly decreased after the introduction of the MAPO molecular sieve.Moreover，the intensity and temperature of the peaks continued to decrease and shifted toward low temperatures.Hence，the concentration and strength of strong acid sites were decreased by the introduction of P，as evidenced by the high-temperature peak.In generally，the P species，such as polyphosphates，shortchain polyphosphates，and pyrophosphates，could form in the channels of molecular sieves after P modification.Most of these species could interact with both acidic and nonacidic hydroxyl groups，especially at the strong acid sites，leading to partial collapse of the molecular sieve framework[31-32].Furthermore，weak acid sites are considered to be capable of promoting the generation of DME from methanol，and meanwhile，the strong acid sites can easily lead to side reactions，which can produce heavy hydrocarbons that can negatively affect the stability of the catalyst[33].