exporting asp.net literal control to excel workbook problem

 protected void btnExport_excel_Click1(object sender, EventArgs e)
    {
        HttpContext.Current.Response.Clear();
        HttpContext.Current.Response.Buffer = true;
       // HttpContext.Current.Response.ContentType = "application/vnd.ms-excel";
        HttpContext.Current.Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
        HttpContext.Current.Response.AddHeader("content-disposition", "attachment;filename=Report.xlsx");
        HttpContext.Current.Response.Charset = "";
        this.EnableViewState = false;

        System.IO.StringWriter sw = new System.IO.StringWriter();
        HtmlTextWriter hw = new HtmlTextWriter(sw);
        ltATPCLP = (LiteralControl)(Session["exportData"]);
        ltATPCLP.RenderControl(hw);

        string k = sw.ToString();

        HttpContext.Current.Response.Write(k);

        HttpContext.Current.Response.End();
    }