Abstract:

A series of Ca-ZSM-5 catalysts with different loading amounts were prepared by ion exchange method. The catalytic performance of the catalysts for the preparation of cyanamide from urea dehydration was investigated in a fixed bed reactor. X-ray diffraction (XRD), N₂ physical adsorption, scanning electron microscopy (SEM), ammonia temperature desorption (NH₃-TPD) and pyridine infrared (Py-IR) were used to analyze the changes in the physical structure, microstructure and acidity of the catalysts before and after modification. The results showed that the physical structure of the modified catalyst was basically unchanged, but the specific surface area decreased with the increase of calcium load. After modification, the strength of weak acid and medium strong acid sites did not change significantly, but the strength of strong acid sites diminished, and the acid content of weak acid and medium strong acid increased significantly. In addition, the proportion of L-acid in the catalyst increased significantly, while the amount of B-acid decreased slightly, and thus the value of B/L decreased. When the reaction temperature was 550℃, urea feed rate was 0.2g/min, NH₃ velocity was 3000h^-1, condensate temperature was 0℃, and “0.1Ca-ZSM-5” was used for a 30 min reaction, the urea conversion was 91.1%, the selectivity and yield of cyanamide were 40.5% and 36.9%, respectively.

Key words: urea, molecular sieve, modified ZSM-5, catalysis, dehydration, heterogeneous reaction, cyanamide

摘要：

采用离子交换法制备了一系列不同负载量的Ca-ZSM-5催化剂，通过固定床反应器考察了催化剂催化尿素脱水制备单氰胺的反应性能。利用X射线衍射（XRD）、N₂物理吸附、扫描电子显微镜（SEM）、氨气程序升温脱附（NH₃-TPD）和吡啶红外（Py-IR）等表征手段分析了催化剂改性前后物理结构、微观形貌以及酸性质的变化。研究结果表明，改性前后催化剂的物理结构基本保持一致，但比表面积随着钙负载量的增加而降低；改性后催化剂的弱酸与中强酸位点的强度无明显变化，但强酸位点的强度减弱，并且弱酸与中强酸的酸量大幅增加；此外，催化剂中L酸的比例明显提升，B酸量略微减少，B/L的值有所降低。当反应温度为550℃、尿素进料速度为0.2g/min、氨气空速为3000h^-1、冷凝水温度为0℃、使用“0.1Ca-ZSM-5”为催化剂反应30min时，尿素转化率为91.1%，单氰胺的选择性为40.5%，单氰胺的收率为36.9%。

关键词: 尿素, 分子筛, 改性ZSM-5, 催化, 脱水, 多相反应, 单氰胺