Steam-assisted crystallization of highly dispersed nanosized hierarchical zeolites from solid raw materials and their catalytic performance in lactide production

A solvent-free route based on solid raw materials affords higher product yield and lower waste production compared to the traditional hydrothermal synthesis. However, the as-made zeolites usually present blocky aggregation states, limiting their mass transfer and exposure of active sites in catalytic applications. Herein, highly dispersed nanosized hierarchical Beta zeolites with varied Si/Al ratios were prepared via steam-assisted crystallization from ball-milled solid raw materials. Thanks to the sufficient mixing of solid raw materials and favorable migration of solid mixture, nanosized Beta zeolites are obtained that are assembled from nanoparticles (∼15 nm) and possess abundant interconnected intraparticle mesopores. The strategy can also be extended to synthesize nanosized hierarchical ZSM-5 zeolites. The as-prepared Beta zeolite (Si/Al = 10) exhibits outstanding catalytic performance in conversion of lactic acid to lactide (as high as 77.5% in yield). This work provides avenues for simple and cost-efficient synthesis of highly dispersed nanosized hierarchical zeolites, promising their important catalytic applications.

solution at 80 o C for 3 h, and then calcined at 500 o C for 6 h. The obtained samples are denoted as Beta-10-BM-SAC, Beta-20-BM-SAC, and Beta-40-BM-SAC with Si/Al ratios of 10, 20, and 40, respectively.

Synthesis of Beta-20-BM-SAC zeolites with various mixing methods and crystallization approaches
To investigate the effect of mixing methods and crystallization approaches on the morphologies and structures of as-prepared Beta zeolites, the molar composition of the mixture was set to 1.0 SiO2: 0.025 Al2O3: 0.133 Na2O: 0.28 TEABr: 3 H2O. The synthesis procedure was exactly the same as above, except that the crystallization approach was adjusted as heating treatment at 140 o C for 72 h. The as-prepared sample is denoted as Beta-20-BM-HT, where BM indicates the ball-milling and HT indicates the heating treatment. When the mechanical ball-milling was replaced by the manual grinding for 20 min, then followed the steam-assisted crystallization at 140 o C for 72 h, the as-prepared sample is denoted as Beta-20-MG-SAC. Similarly, Beta-20-MG-HT sample was prepared via heating treatment from manual grinded solid mixture, and the condition of HT and MG was exactly the same as above.

Synthesis of Beta-20-BM-SAC-y zeolites with various H2O/SiO2 ratios
To investigate the effect of the H2O/SiO2 ratios on the morphologies and structures of as-prepared Beta zeolites, the molar composition of the mixture was set to 1.0 SiO2: 0.025 Al2O3: 0.133 Na2O: 0.28 TEABr: (0-6) H2O for the synthesis of Beta-20-BM-SAC with different H2O/SiO2 ratios. The synthesis procedure was exactly the same as above, except that the H2O/SiO2 ratios were adjusted from 6 to 1.5 and 0. The asprepared samples are denoted as Beta-20-BM-SAC -y, where y indicates the H2O/SiO2 ratios. After ion-exchange and calcination, the obtained samples are denoted as Beta-20-BM-SAC -6, Beta-20-BM-SAC-1.5, and Beta-20-BM-SAC-0 with H2O/SiO2 ratios of 6, 1.5 and 0, respectively. The detailed synthesis conditions are listed in Table S1.

Synthesis of Beta-20-BM-SAC-z zeolites with various seed additions
To investigate the effect of amount of seed additions on the morphologies and structures of as-prepared Beta zeolites, the molar composition of the mixture was set to 1.0SiO2: 0.025 Al2O3: 0.133 Na2O: 0.28 TEABr: 3 H2O for the synthesis of Beta-20-BM-SAC with different amount of seed additions. The synthesis procedure was exactly the same as above, except that the seed additions were adjusted from 20 wt% to 5 wt% and 0 wt%. The as-prepared samples are denoted as Beta-20-BM-SAC-z, where z indicates the seed addition. After ion-exchange and calcination, the obtained samples are denoted as Beta-20-BM-SAC-20wt% and Beta-20-BM-SAC-5wt%, and Beta-20-BM-SAC-0wt% with seed addition of 20 wt%, 5 wt% and 0 wt%, respectively. The detailed synthesis conditions are listed in Table S1.

Synthesis of Beta-20-BM-SAC-m zeolites under various ball-milling time
To investigate the effect of the ball-milling time on the morphologies and structures of as-prepared Beta zeolites, the molar composition of the mixture was set to 1.  Table S1.
Details synthetic conditions are listed in Table S1. Typically, solid silica gel, NaAlO2, TPABr, NaOH, H-ZSM-5 and H2O were mixed into ball-milling jar. After mechanical mixing for 5 h, 1.0 g of the powder mixture was transferred into a 50 mL Teflon liner, and 2.0 mL water was added into the bottom of the liner without contacting the powder mixture. The crystallization was conducted at 160 °C for 72 h under static conditions. The as-synthesized solid products were centrifuged, washed with water and ethanol several times, and then dried at 80 o C in the oven overnight, followed by calcination at 550 o C for 6 h. To obtain the H-type ZSM-5, the sample was ion-exchanged three times in 1 M NH4NO3 solution at 80 o C for 3 h, and then calcined at 500 o C for 6 h. The obtained sample is denoted as ZSM-5-20-BM-SAC.

Synthesis of conventional ZSM-5 zeolite under hydrothermal condition
A mixture with a molar composition of 1.0 SiO2: 0.30 TPAOH: 0.025 Al2O3: 0.015 Na2O: 50 H2O was set for the synthesis of conventional ZSM-5 zeolite under hydrothermal condition. Typically, TEOS, TPAOH, and deionized water were mixed at room temperature under stirring until TEOS was hydrolyzed completely, followed by the addition of NaAlO2 and NaOH. The gel mixture was kept on stirring for 1 h and then transferred into a Teflon liner for crystallization at 170 °C for 72 h under static conditions. The next procedure was exactly the same as ZSM-5-20-BM-SAC, and the obtained sample is denoted as ZSM-5-20-Con.