液化石油气中低碳烷烃脱氢新型催化剂设计及应用基础研究

Liquefied petroleum gas (LPG), manufactured by refining petroleum or extracted from underground oil or gas stream, is used as a civil fuel for a long time, representing a wasteful use of fossil fuel resources. In China, with the successful implementation of the "west-to-east gas pipeline project", LPG will be unavoidably substituted by natural gas as a civil fuel and the supply of LPG will be in an excess. Therefore, in order to improve the competence of the refinery entreprises and optimize the resource exploitation, it is urgent to find alternative ways to the rational use of LPG. LPG is a hydrocarbon mixture, comprising mainly lower alkanes, like propane and butane, with low concentrations of propylene and butylenes. On the other hand, with the soaring increase of market demand, propylene supply is now in a shortage. Considering the big price gap between propane and propylene (propylene is more expensive), it is desirable to convert propane to propylene via dehydrogenation from the economic point of view. The conversion of butane to propylene can also be realized by dehydrogenation plus olefin metathesis reaction. To meet this end, the key is to develop highly effcient catalysts with high activity, high selectivity and high stability in the dehydrogenation of lower alkanes. There are two types of catalysts now in commercial practice, namely Pt- and Cr-based catalysts. Since the Pt catalysts are sensitive to sulfur compounds and get poisoned fast in processing feedstock containing sulfur, only Cr-based catalysts are suitable for the application in the dehydrogenation of lower alkanes in LPG. The most successful Cr catalyst is chromia/alumina catalyst, however possessing some limitations, such as fast deactivation due to the excessive coking on the catalyst surface and consequently the requirement of frequent regeneration, compromising the process economics. Novel catalysts with longer lifetime are strongly desirable. Furthermore, the deactivation mechanism for the Cr catalysts still remains elusive and more fundamental research work is needed to clarify this mystery. In this project, a chromia/alumina nanofiber material developed using a cation-anion double hydrolysis method in our group will be applied as a catalyst in the dehydrogenation of lower alkanes. Significant amount of efforts will be put into the investigation of the relationship between coking and catalyst structural properties as well as the influence of coking on the catalytic performance, which will gain insights into the mechanistic aspects of catalyt deactivation and provide theoretical guidance for the molecular design of novel efficient dehydrogenation catalysts. The successful implementation of this project will also refill the pool of catalytic materials for the dehydrogenation of lower alkanes.

液化石油气（LPG）长期以来一直被用作民用燃料，造成资源的浪费。随着我国"西气东输"工程的顺利实施，天然气取代LPG的趋势已经形成。如何合理、经济地利用LPG资源已经成为炼油企业优化资源利用，提高自身竞争力的一个重要课题。LPG中主要为丙烷和丁烷，可以通过脱氢和烯烃易位反应将丙烷和丁烷都转化为丙烯，缓解目前丙烯供需缺口大的问题。而解决问题的关键在于研制具有高活性，高烯烃选择性和高稳定性的新一代低碳烷烃脱氢催化剂。本课题在前期工作的基础上，综合考虑催化剂设计以及工艺条件控制，拟采用阴阳离子双水解法合成的多孔氧化铬/氧化铝纳米纤维为低碳烷烃脱氢催化剂，揭示催化剂结构参数与积炭，积炭与催化剂性能之间的关联机制。在此基础上，建立低碳烷烃脱氢催化剂优化设计理论。本课题的顺利实施，将为新一代高性能低碳烷烃催化剂设计奠定理论基础，同时也将丰富低碳烷烃脱氢催化材料的理论体系。

液化气长期以来被用作民用燃料，造成了资源的浪费。随着我国“西气东输”工程的推进，天然气取代液化气的趋势已经形成，造成液化气的过剩。液化气中大部分为C3-C4低碳烷烃，如果能将这一部分烷烃高效地转化为相应的低碳烯烃，将大大提高产品的附加值。而解决这一问题的关键就是具有优越催化性能的低碳烷烃脱氢催化剂的研制。本课题采用阴阳离子双水解法制备了氧化铬/氧化铝催化剂，采用TEM， XRD，低温氮气吸脱附，NH3-TPD，H2-TPR，UV-vis， Raman，XPS等表征手段对催化剂进行了系统的表征。考察了制备条件对于催化剂结构特性的影响。并将催化剂应用于丙烷脱氢反应中，考察了反应工艺条件对催化剂反应性能的影响，得到了优化的丙烷脱氢工艺条件。同时，我们将催化剂的孔结构，晶体结构，表面酸性，铬物种的聚集状态及还原性等因素与其丙烷转化率，丙烯选择性以及稳定性相关联，得出了催化剂结构参数对于其催化性能的影响规律。多次实验结果表明，与单核和低聚表面Cr物种相比，多聚Cr物种的丙烯选择性较高。in-situ DRIFTS结果表明，单核和低聚的Cr物种能诱导表面甲酸盐、脂肪族烃类，不饱和烃类以及芳香族烃类物种的形成，这些表面物种都是形成积炭的前驱体，随着反应的进行，这些前驱体会进一步转化为积炭，覆盖在催化剂的活性位上，从而造成催化剂失活及副产物增加。进一步的研究表明，表面Cr物种的聚集状态能通过调整助剂K的加入顺序来进行调控，在浸渍Cr之前加入助剂K有利于多聚Cr物种的形成。通过简单地改变K的加入顺序，我们将丙烯的选择性提高了5%，这对于工业上大规模丙烷脱氢过程（比如100万吨/年丙烷脱氢）具有重要的经济价值。合成温度对于Cr物种的聚集状态有很大的影响，低温有利于聚合态Cr物种形成，而高温则有利于α-Cr2O3晶体的形成。相比之下，聚合态Cr物种具有更高的丙烯选择性。除此之外，我们还考察了V系催化剂在丙烷和丁烷脱氢反应中的性能，与Cr催化剂相比，V系催化剂的反应活性较高，但丙烯选择性较低，需要进一步研究以改善其丙烯选择性。因此，本课题的研究成果，对工业低碳烷烃脱氢催化剂的设计和优化具有重要的指导意义，同时也丰富了低碳烷烃脱氢催化材料的理论体系。