Creating Saving and Loading Materials in Runtime

I'm trying to create a material during runtime like this:

1. In my Assets I have a Render Texture and one Material
2. I created an Script where I attach the Render Texture to Material

3. I save in my project a PNG Image File for later use

   public void CreateAndSaveMaterial(){
   
   RenderTexture.active = _renderTexture;
   Texture2D tex = new Texture2D(_renderTexture.width, _renderTexture.height, TextureFormat.ARGB32, false);
   tex.ReadPixels(new Rect(0, 0, _renderTexture.width, _renderTexture.height), 0, 0);
   tex.Apply();
   
   File.WriteAllBytes(Application.dataPath + url, tex.EncodeToPNG());
   
   tempObject.mainTexture = tex;
   RenderTexture.active = null;
   

   }

*The PROBLEM:

1.The Material works in rutime, after quit playing, the Material resets :( 2.The created PNG File not working only if the file is reimported in Unity, I tried with:

AssetDatabase.CreateAsset(obj, fileName);
AssetDatabase.SaveAssets();
AssetDatabase.Refresh();

But unsuccessfully, works only in Editor not in build app :(

Any suggestions? Thanks in advance!