Synthesis of Dimethyl Ether from Methanol over Heteropoly Acid/Nanocrystalline HZSM-5 Complex Solid Acidic Catalyst

Abstract

Exploiting the catalysts with higher activity and stability for methanol dehydration at low temperature has become the central part of research dimethyl ether from synthesis gas or methanol. The crystal size of HZSM-5 zeolite is 50~100 nm. The nanocrystalline HZSM-5 zeolite is thought to be an ideal material for the encapsulation of heteropoly acid (HPA) molecules having a Keggin or Dawson structure since they have the intergranular secondary pores. A HPA/nanocrystalline HZSM-5 complex solid acidic catalyst was prepared by an impregnation method and characterized by FT-IR, UV-Raman, XRD and 31 P MAS-NMR. The 12-phosphotungstic acid (HPW) loaded on nanocrystalline HZSM-5 zeolites was prepared by the following steps. The strip (ф 1 mm×2 mm) nanocrystalline HZSM-5 (Si/Al=4.2) catalyst was prepared by the procedure that reported in the literature. 1.00 g HPW was dissolved in 15 mL deionized water. 10.0 g of the strip nanocrystalline HZSM-5 zeolite were added into the solution and steeped the mixture for 12 h. The slurry mixture was calcined at 350 ℃ for 4 h. This catalyst is denoted HPW-HZSM-5. The FT-IR spectroscopy, UV-Raman spectroscopy and 31 P MAS-NMR of HPW-HZSM-5 characterization results show that the Dawson type structure was well preserved after calcination of the cata- lysts. The reaction was carried out in a continuous flow fixed-bed reactor (diameter 10 mm). Reaction conditions are listed as following: temperature 200 ℃, WHSV=1.6, FRH (methanol flow rate)=0.08 mL/min; FRH (N2 flow rate)=8.0 mL/min; catalyst 2.05 g; system pressure 0.8 MPa. The composition of the products was analyzed by a GC-8820 gas chromatograph equipped with FID and a GDX-403 (2 m×4 mm). The reaction performance results, including methanol conversion and di- methyl ether selectivity were subsequently calculated. Under steady state conditions, the catalyst shows good stability, and the conversion of methanol maintains above 87.0 mol% (90.9 mol%) during 300 h, it also shows the selectivity (99.0 mol%) of dimethyl ether under the experimental conditions. Keywords 12-phosphotungstic acid; nanocrystalline HZSM-5 zeolite; methanol dehydration; dimethyl ether