Download dialog does not appear

I am generating a xls in memory and it should appear for download , but at the end of the method the download dialog is not appearing.

xhtml

<a4j:commandButton styleClass="Button" value="Download"
                    execute="@form" render="@form"
                    action="#{lotesEnvBean.ImprimirLotesEnviados()}" >
                    </a4j:commandButton>

Bean

public void ImprimirLotesEnviados() {

    try {

        SimpleDateFormat simpledate = new SimpleDateFormat("ddMMyyyyHHMMSS");
        java.util.Date dateFile = new java.util.Date();
        //FileOutputStream  outFile = new  FileOutputStream (new File("/home/joao/app/tmp/envLote"+simpledate.format(dateFile)+".xls"));

        HSSFWorkbook workbook = new HSSFWorkbook();

        HSSFSheet sheet = workbook.createSheet("Lote Enviados");

        HSSFFont font = workbook.createFont();
        font.setBoldweight(HSSFFont.BOLDWEIGHT_BOLD);
        HSSFCellStyle style = workbook.createCellStyle();
        style.setFont(font);            

        HSSFCellStyle my_style_0 = workbook.createCellStyle();
        my_style_0.setAlignment(HSSFCellStyle.ALIGN_CENTER);
        my_style_0.setFont(font);
        my_style_0.setBorderLeft( (short) 100);

        Row header = sheet.createRow(0);

        header.createCell(0).setCellValue("ID Familia");
        header.getCell(0).setCellStyle(my_style_0);         

        header.createCell(1).setCellValue("Famailia");
        header.getCell(1).setCellStyle(my_style_0);

        header.createCell(2).setCellValue("ID Produto");
        header.getCell(2).setCellStyle(my_style_0);         

        header.createCell(3).setCellValue("Produto");
        header.getCell(3).setCellStyle(my_style_0);

        Integer count=1;
        for (LoteEnvDetalheEntity dto :this.lotesEnvDetalhe ) {

            Row dataRow = sheet.createRow(count);

            dataRow.createCell(0).setCellValue(dto.getSeqfamilia());

            dataRow.createCell(1).setCellValue(dto.getFamilia());

            dataRow.createCell(2).setCellValue(dto.getSeqproduto());

            dataRow.createCell(3).setCellValue(dto.getProduto());

            count++;
        }

        //workbook.write(outFile);
        //outFile.close();
        System.out.println("Excel written successfully..");

        FacesContext fc = FacesContext.getCurrentInstance();
        ExternalContext ec = fc.getExternalContext();

        ec.responseReset();
        ec.setResponseContentType("application/download");
        ec.setResponseHeader("Content-Disposition", "attachment; filename=\"lotes_enviados_" + simpledate.format(dateFile) + ".xls");

        System.out.println("attachment; filename=lotes_enviados_" + simpledate.format(dateFile) + ".xls");

        OutputStream output = ec.getResponseOutputStream();

        output.write(workbook.getBytes());
        output.flush();
        output.close();

        fc.responseComplete();


    }catch (Exception e) {
        e.printStackTrace();
    }

}