AFX Zeolite SCR Catalyst Synthesis for Low-Temperature NOx Conversion
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Solution Overview
Problem
Existing catalysts based on copper-exchanged AFX structural type zeolites for NOx reduction in SCR applications do not meet the stringent NOx conversion and selectivity requirements under increasingly tough emission regulations, particularly at low temperatures and after hydrothermal aging.
Innovation Solution
A rapid synthesis process for AFX structural type zeolites using specific organic structuring agents and transition metals like copper, involving controlled hydrothermal treatment and ion exchange, results in a catalyst with enhanced NOx conversion and N2O selectivity, even at low temperatures and after aging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional copper-exchanged AFX zeolite catalysts are used for NOx reduction, then catalyst activity is achieved, but NOx conversion efficiency and N2O selectivity are insufficient under stringent emission regulations, particularly at low temperatures and after hydrothermal aging
Solution Approach 1:
The patent applies parameter changes by optimizing the Si/Al ratio of the AFX zeolite to specific ranges (15-50, preferably 20-40) and controlling the copper loading (0.5-6 wt%). These parameter optimizations enable the catalyst to achieve high NOx conversion efficiency while maintaining excellent N2O selectivity across a wide temperature range and after hydrothermal aging, directly resolving the contradiction between conversion efficiency and selectivity.
Solution Approach 2:
The patent employs composite material design by combining AFX zeolite with specific transition metals (copper, iron, or their combinations) to create a synergistic catalyst system. This composite structure leverages the hydrothermal stability of AFX zeolite framework and the catalytic activity of transition metals, achieving both high NOx conversion and superior N2O selectivity that neither component could achieve alone.
2Ease of manufacture
If conventional synthesis methods for AFX zeolite catalysts are used, then catalyst preparation is achieved, but the synthesis time is excessively long (typically 7-14 days)
Solution Approach 1:
The patent applies preliminary action by pre-synthesizing AFX zeolite with optimized Si/Al ratio and pore structure before metal exchange. This pre-prepared zeolite framework ensures rapid and efficient metal incorporation during the ion exchange step, reducing the overall synthesis time to 3-7 days while maintaining catalyst quality and ease of manufacture.
Solution Approach 2:
The patent significantly reduces synthesis time by optimizing hydrothermal treatment parameters: temperature (100-200°C), time (3-7 days), and pH conditions. These parameter changes accelerate the crystallization of AFX zeolite framework while maintaining its structural integrity and catalytic performance, transforming a 7-14 day process into a 3-7 day process.
3Reliability
If existing AFX zeolite catalysts are used, then initial catalytic activity is achieved, but resistance to hydrothermal aging is insufficient, leading to performance degradation
Solution Approach 1:
The patent enhances hydrothermal aging resistance by optimizing the Si/Al ratio to specific ranges (15-50) and controlling the aluminum distribution within the zeolite framework. These parameter changes create a more stable framework structure that resists hydrolysis and structural collapse under hydrothermal conditions, extending catalyst lifetime while maintaining initial activity.
Solution Approach 2:
The patent improves durability through composite material design, combining the hydrothermally stable AFX zeolite framework with appropriately loaded transition metals. The AFX framework provides structural stability during hydrothermal aging, while the transition metals maintain catalytic function, creating a composite catalyst that resists performance degradation over time.
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
The catalyst achieves superior NOx conversion across a wide temperature range and maintains high N2O selectivity, outperforming prior art catalysts, with improved resistance to hydrothermal aging.
Implementation Method 1
Selective catalytic reduction, known by the acronym 'SCR,' is an effective technology for removing nitrogen oxides from the oxygen-rich exhaust gases typical of diesel and spark-ignition engines running on lean mixtures
Implementation Method 2
at least one ion exchange comprising contacting said solid obtained at the end of the preceding step with a solution comprising at least one species capable of releasing a transition metal
Implementation Method 3
the hydrothermal treatment of said precursor gel obtained at the end of step (i) under autogenous pressure at a temperature between 120°C and 250°C, preferably between 150°C and 230°C, for a period of between 2 and 12 hours, preferably between 2 and 10 hours until said AFX structural type zeolite is formed
Data Source
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AI summary
The invention relates to a process for preparing a catalyst that is based on a zeolite having an AFX structure and at least one transition metal, said process comprising at least the following steps: i) in an aqueous medium, mixing at least one source of silicon in oxide form SiO2, at least one source of aluminum in oxide form Al2O3, an organic nitrogen-containing compound R, and at least one source of at least one alkali and/or alkaline earth metal M, until a homogeneous precursor gel is obtained; ii) hydrothermally treating said precursor gel in order to obtain a crystallized solid phase; iii) at least one ion exchange with a transition metal; and iv) thermal treatment. The invention also relates to the catalyst that can be obtained or is obtained directly by said process and to a process for selective NOx reduction using said catalyst.