Structure and Surface Chemistry of Two-Dimensional Phosphate Catalysts

磷酸溶液在减压条件下除去磷酸锆凝胶中的杂化水，使磷酸锆结晶。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晶体。合成的ZrP_2O_2和epsilon-Zr(HPO_4)_2催化剂在800 K的真空条件下对烯烃异构化的催化活性高于其他形式的磷酸锆或其他常规固体酸催化剂。为了阐明这些较高的催化活性，采用^<31>P- masnmr （^<31>P- masnmr）研究了不同温度下疏散后的磷酸锆。当层间距变小时，^<31>P的化学位移向更高的磁场移动。活性较高的催化剂在H_3PO_4 -38 ppm左右表现出较高的化学位移。这些结果表明，在800 K左右疏散后留下的磷酸基团可能会增强质子特性，因为电子的积累从表面的磷酸基团转移到位于Zr原子平面之间的磷原子上。

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.