CIT-16P Molecular Sieve Transformation for Stable MTO Catalysis
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Solution Overview
Problem
Existing AlPO4-based molecular sieves are structurally challenging to analyze due to their instability and complex coordination behaviors, and the removal of organic structure-directing agents (OSDAs) often leads to framework transformation, complicating the understanding and utilization of their catalytic properties.
Innovation Solution
The synthesis of a new SAPO material, CIT-16P, which incorporates DiQ-C4 or DiQ-C3 OSDAs, allowing for the development of SAPO-17 catalysts with a high initial methanol conversion rate and extended lifetime in the MTO reaction by maintaining a significant proportion of pentacoordinate Al-sites after OSDA removal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If new silicoaluminophosphate molecular sieves are synthesized with specific structures, then catalytic performance in MTO reaction is improved, but structural instability and complex coordination behaviors make framework integrity difficult to maintain
Solution Approach 1:
An organic structure-directing agent (OSDA) is used as an intermediary during the synthesis of CIT-16P molecular sieve. The OSDA templates the formation of the desired framework structure, stabilizing the complex coordination behaviors during synthesis and enabling the formation of pentacoordinate Al sites that would otherwise be difficult to maintain. The OSDA is removed after synthesis to leave the stable final structure.
Solution Approach 2:
The synthesis process utilizes controlled changes in chemical parameters including pH, temperature, and composition ratios to stabilize the framework during formation. The presence of OSDA and specific synthesis conditions allow the formation and stabilization of pentacoordinate Al sites, which transform to tetrahedral sites upon OSDA removal, maintaining framework integrity throughout the process.
2Productivity
If occluded organic molecules are removed from molecular sieves, then catalytic activity is enhanced, but structural instability occurs affecting framework integrity
Solution Approach 1:
The molecular sieve structure is pre-stabilized during synthesis with the OSDA present, forming a robust framework with pentacoordinate Al sites before the OSDA is removed. This preliminary stabilization ensures that when the OSDA is subsequently removed to enhance catalytic activity, the framework maintains its integrity and does not collapse.
Solution Approach 2:
The removal of occluded organic molecules induces a phase transition in the framework structure where pentacoordinate Al sites transform into tetrahedral sites. This controlled phase transition, occurring after the framework is already stabilized during synthesis, results in an open, catalytically active structure while maintaining framework integrity.
3Reliability
If pentacoordinate Al sites are formed in molecular sieve, then catalytic stability and methanol conversion rates are improved, but structural complexity increases
Solution Approach 1:
The OSDA acts as a mediator that facilitates the formation of pentacoordinate Al sites during synthesis by controlling the local coordination environment. The OSDA's structure and positioning guide the formation of these complex sites, and its subsequent removal leaves behind the simplified yet stable pentacoordinate/tetrahedral Al site structure with enhanced catalytic properties.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
CIT-16P transforms into SAPO-17 with improved catalytic performance in the MTO reaction, demonstrating enhanced methanol conversion and catalyst longevity, attributed to its unique structural characteristics and large crystal size.
Implementation Method 1
CIT-16P, with an occluded OSDA that transforms into SAPO-17 upon OSDA removal, featuring a significant proportion of pentacoordinate Al sites that condense into tetrahedral sites
Implementation Method 2
enhancing the selectivity and stability of the catalytic process, compared to other ERI-type materials like ECNU-38P
Data Source
AI summary
The disclosure provides a novel silicoaluminophosphate molecular sieve, referred to as CIT-16P, that comprises a silicoaluminophosphate framework with an occluded OSDA that is DiQ-C4 or DiQ-C3. The synthesis of CIT-16P, its conversion to SAPO-17, and the use of the so-derived SAPO-17 in the MTO reaction are also disclosed.