Poly(ethylene terephthalate) (PET), with the chemical formula (C10H8O4)n, is obtained by ester exchange of dimethyl terephthalate with ethylene glycol or esterification of terephthalic acid with ethylene glycol to synthesize dihydroxyethyl terephthalate, and then polycondensation reaction. It is crystalline saturated polyester, a milky white or light yellow, highly crystalline polymer with a smooth and glossy surface. It is a common resin in life. It has excellent physical and mechanical properties in a wide temperature range, and the operating temperature can reach 120°C. It has excellent electrical insulation. Even at high temperature and high frequency, its electrical properties are still good, and its fatigue resistance, friction resistance, and dimensional stability are all good.
Chemical upgrading of waste PET to obtain high-value-added raw materials is an environmentally friendly treatment method, but the current catalytic recovery of waste PET only stays at obtaining primary degradation products. In order to further degrade and upgrade the primary products, some studies have obtained mesoporous m-HZSM-5 by multi-level pore regulation of microporous HZSM-5 through alkali-acid treatment, and then obtained Ru/m-HZSM-5 through impregnation-reduction. Under the conditions of 120°C, 3 MPa H2 and 2 h, the conversion rate of Ru/m-HZSM-5 for the benzene ring hydrogenation reaction of PET primary degradation product BHET reached 95.5%, and the selectivity of the target product BHCD was 95.6%. The high catalytic activity of Ru/m-HZSM-5 can be attributed to the large specific surface area and pore volume, and the high Ru dispersion. In addition, under appropriate reaction conditions, Ru/m-HZSM-5 also has excellent catalytic activity for the benzene ring hydrogenation reaction of other PET primary degradation products (DMT, DET and TPA).
The chemical recycling routes of waste PET mainly include glycol hydrolysis, methanol hydrolysis and hydrolysis, and the corresponding products are diethyl terephthalate (BHET), dimethyl terephthalate (DMT) and terephthalic acid (TPA). Among these three routes, glycol hydrolysis of PET is more attractive because its reaction conditions are relatively milder (190~195°C, 1 atm). In addition, the generated BHET can also be used as a raw material for synthesizing recycled PET or another high-value-added monomer 1,4-cyclohexanedimethanol (CHDM). However, most waste PET recycling processes are interrupted to obtain BHET, and its further upgrading and transformation are rarely reported in the literature. Therefore, it is necessary to design a reasonable catalyst to further upgrade and transform BHET.
The mesoporous m-HZSM-5 was prepared by treating the microporous HZSM-5 with alkali-acid (NaOH-HCl). X-ray powder diffraction shows that the m-HZSM-5 after multi-level pore regulation has an MFI-type topological structure with the initial HZSM-5, and the molecular sieve framework has good stability. N2 adsorption-desorption test proves that HZSM-5 has only microporous characteristics, with a specific surface area and pore volume of 405 m2·g-1 and 0.32 cm3·g-1, respectively; in contrast, m-HZSM-5 has mesoporous channels, and the specific surface area and pore volume increase to 503 m2·g-1 and 0.76 cm3·g-1. Scanning electron microscope images confirm the above description. The surface of HZSM-5 is flat and smooth, while the surface of m-HZSM-5 has uniformly dispersed pores, indicating that alkali-acid treatment can produce mesopores.
Ru catalysts loaded on HZSM-5 or m-HZSM-5 (Ru/HZSM-5 or Ru/m-HZSM-5) were obtained by impregnation. Through the H2 programmed temperature desorption test, it was calculated that the Ru dispersion in Ru/m-HZSM-5 (40.6%) is indeed higher than that in Ru/HZSM-5 (34.6%). This is because the specific surface area and pore volume of m-HZSM-5 are larger than those of HZSM-5, which can provide more anchoring points for Ru. In addition, high-resolution transmission electron microscopy images show that Ru is evenly dispersed in m-HZSM-5, and the average particle size of Ru is 3.5 nm.
Ru/HZSM-5 and Ru/m-HZSM-5 are used in the hydrogenation of the benzene ring of the primary degradation product BHET of PET to generate BHCD, and the reaction pathway is shown in the figure. According to the temperature curves of the two Ru catalysts, the conversion rate of the benzene ring hydrogenation reaction of BHET catalyzed by Ru/m-HZSM-5 under the same conditions is significantly higher than that of Ru/HZSM-5. Under low conversion conditions (90 °C), the conversion frequency (TOF) of Ru/m-HZSM-5 is as high as 23.4 h-1, while the TOF of Ru/HZSM-5 is only 5.7 h-1. Under 120 °C, the conversion rate of Ru/m-HZSM-5 for BHET benzene ring hydrogenation reaction reaches 95.5%, and the selectivity of the target product BHCD is 95.6%. Compared with Ru/HZSM-5, the improvement of the catalytic performance of Ru/m-HZSM-5 can be attributed to the increase in the specific surface area and pore volume of the carrier and the improvement in the dispersion of the active species Ru. In addition, after five cycles of experiments, the conversion rate of Ru/m-HZSM-5 for BHET benzene ring hydrogenation remained almost unchanged, indicating that the catalyst has good thermal and chemical stability.
The reaction mechanism of Ru/m-HZSM-5 catalyzed hydrogenation of BHET benzene ring to produce BHCD is as follows: First, hydrogen is dissociated and adsorbed on the Ru site, and the delocalized π bond of the benzene ring in BHET is also activated and adsorbed on the Ru site. Then, the active hydrogen species attacks the delocalized π bond, and one benzene ring consumes three hydrogen molecules in turn to form a cyclohexyl group. Then, the generated BHCD desorbs from the Ru site.
Finally, Ru/m-HZSM-5 was applied to the benzene ring hydrogenation reaction of other PET primary degradation products (DMT, DET and TPA), and the reaction results are shown in the table. Fortunately, under mild reaction conditions, the conversion rates of DMT and DET are higher than 94%, and the selectivity of the corresponding target products is higher than 97%. For TPA, satisfactory conversion and selectivity can also be obtained under appropriate conditions.
