A Green and Cost-Effective Synthesis of Hierarchical SAPO-34 through Dry Gel Conversion and Its Performance in a Methanol-to-Olefin Reaction

Abstract

In this work, a low-cost and high-performance methanol-to-olefin (MTO) catalyst was synthesized by dry gel conversion (DGC). Using tetraethylammonium hydroxide (TEAOH) as the structure-directing agent (SDA) to prepare the dry gel, nano-SAPO-34 in a range of 50−500 nm was synthesized under the effect of the vapor of triethylamine (TEA) and H 2 O. The TEAOH consumption using this method was only 7.5% of that in the hydrothermal synthesis (HTS), and the yield of nano-SAPO-34 products was up to 94%. When scaling up to the pilot test, obvious heterocrystals appeared with the decrease in SiO 2 content in the dry gel. Our results showed that the heterocrystals were completely eliminated after the introduction of fluoride into the dry gel. A low acid density nanosheet-SAPO-34 molecular sieve product with good micropore, mesopore, and macropore connectivity was obtained. The obtained multikilogram-scale molecular sieve sample can be directly shaped to obtain MTO microsphere catalysts by spray drying without separation and washing. No wastewater was produced during the whole process, demonstrating an entirely green production way. The MTO performance of the microsphere catalysts was tested in a fluidized bed reactor, and results showed that the catalysts exhibited the characteristics of a slow carbon deposition rate and a high activity utilization ratio. Their lifetime was extended by 14%, and their selectivity (C 2 = + C 3 = ) was increased by 1.55%, so they are very suitable for industrial application. This work has provided the theoretical and practical basis for the industrial production and application of DGC in the future.

Synthesis, Types, and Applications of Zeolite Capsule Catalysts

“…There are many unavoidable problems during hydrothermal synthesis, such as low water utilization rate, low yield, serious pollution, high autogenous pressure, and crystallization temperature. [69,74] Dry gel conversion method (DGC) and vapor phase transport (VPT) method provide valuable research for reducing the use of water in zeolite synthesis, [91,92] significantly improving water utilization. However, both methods require water during the preparation of the gel, and they need to be treated with a mixture of water and amine vapor under high temperature and high autogenous pressure in a sealed autoclave, which do not fundamentally solve the drawbacks brought about by hydrothermal synthesis.…”

Synthesis of nano-sized SAPO-34 using a facile micron-meter seed processing method and their enhanced performance in methanol-to-olefin reactions

“…[11][12][13] Previous documents have shown that decreasing the catalyst size is considered to be an effective method to decrease intracrystalline diffusion path length and thus suppress the formation rate of coke deposition, which consequently increases catalyst life span. In recent years, several synthesis methods, such as hydrothermal synthesis, 14 dry gel conversion, 15,16 rapid high temperature methods 17 and microwave and the sonochemical techniques, 18,19 have been used for fabricating nano-sized SAPO-34 catalysts by controlling synthesis parameters (crystallization time and temperature, template type and concentration, raw material type, the concentration of synthesis gel, etc.). 20 However, the expensive tetraethylammonium hydroxide (TEAOH) as the template is needed in most synthesis systems.…”

Controlled synthesis of high performance SAPO-34 with sheet-like morphology and small particle size using concentrated gel system

“…The resultant catalysts demonstrated remarkable performance in the MTO process. Similarly, Li et al [22] utilized TEAOH as the structuredirecting agent to synthesize the dry gel and further introduced uoride into the dry gel to obtain the low acid density nanosheet-SAPO-34 molecular sieve product with good micropore, mesopore, and macropore connectivity. However, the synthesis of SAPO-34 with a nanosheet or plate-like structure is highly restricted, and typically necessitates the utilization of expensive TEAOH, or even involves the incorporation of seed crystals or nucleation inhibitors.…”