Structure and Surface Chemistry of Two-Dimensional Phosphate Catalysts

磷酸溶液在减压下除去磷酸锆凝胶中的水分时，磷酸锆会结晶出来。对于给定浓度的磷酸，根据温度和处理时间的不同，通过这种方法可以得到几种形式的微晶。在约800K真空条件下，合成的ZrP_2O_2和epsilon-Zr(HPO_4)_2催化剂对烯烃异构化的催化活性高于其他形式的磷酸锆或其他传统的固体酸催化剂。为了解释这些更高的催化活性，我们用魔角旋转核磁共振(^&lt；31&gt；P-MASNMR)研究了不同温度下抽真空后的磷酸锆盐。随着层间距的增大，化学位移向更高的磁场移动。表现出较高活性的催化剂在-38ppm附近表现出较高的H_3PO_4化学位移。这些结果表明，在800K左右抽空后剩余的磷酸盐起泡可能会增强质子特性，这是因为电子从表面的磷酸基团转移到位于Zr原子平面之间的磷原子。

Zirconium phoshate are crystallized during the removal of hybration water in the zirconium phosphate gel with phosphoric acid slution under reduced pressure. Several forms of crystallites are obtained by this procedure, depending on the temperature and process time, for a given concenttration of phosphoric acid. Synthetic ZrP_2O_2 and epsilon-Zr(HPO_4)_2 catalysts, which were evacuated at ca. 800 K, showed higher catalytic activities for catalytic isomerization of alkenes than other forms of zirconium phosphates or other conventional solid acid catalysts. In order to elucidate those higher catalytic activities, ^<31>P magic angle spinning nmr (^<31>P-MASNMR) has been employed to study zirconium phosphates after evacuation at different temperatures.The ^<31>P chemical shifts move towards higher magnetic fields as the layer separations became amaller. The catalysts which showed higher activities showed higher chemical shifts at around -38 ppm from H_3PO_4. These results suggest that phosphate froups remaining after evacuation at around 800 K may enhance the protonic characteristics, since the accumulation of electrons moves from the phosphate groups on the surface to phosphorus atoms which are located between Zr atom planes.