Progress in selective oxidative dehydrogenation of light alkanes to olefins promoted by boron nitride catalysts

Abstract

Conversion of light alkanes into industrial chemical olefins via oxidative dehydrogenation (ODH) is a promising route because of favorable thermodynamic and kinetic characteristics, but encounters difficulties in selectivity control for olefins because of over-oxidation reactions that produce a substantial amount of undesired carbon oxides. Compared to widely-developed metal oxide-based catalysts, functionalized boron nitride has recently been shown as a competitive system in the ODH of light alkanes because of its more superior selectivity toward olefins as well as negligible formation of CO2. It is also characterized by high productivity to light olefins, remarkable catalyst stability, superior anti-oxidation ability, and excellent thermal conductivity. This feature article highlights the recent developments in applying boron nitride towards the ODH reaction of light alkanes. By correlating structural character with catalytic behavior, we expect to provide more insights into the catalytic nature of boron nitride-based materials in ODH reactions. Finally, we envisage perspective directions for boron-based ODH catalysts.

Effects of Modifying Acidity and Reducibility on the Activity of NaY Zeolite in the Oxidative Dehydrogenation of n-Octane

“…More recent developments have led to the introduction of oxygen to the DH reaction, which rendered the process exothermic, with much reduced thermodynamic limitations and reduced coke deposition on the catalyst. This is the reason why oxidative dehydrogenation (ODH) would be the preferred reaction in the petrochemical industry [9,10]. Very high COx production remains the major disadvantage of ODH reactions.…”

“…The sequence of increasing selectivity to octenes followed the order: CaGa-NaY < Ga-NaY< MgGa-NaY < SrGa-NaY < BaGa-NaY. The highest octene selectivity obtained was 37% at iso-conversion of 6 ± 1% when BaGa-NaY was used at a temperature of 450 • C. The activity of the catalysts was directly proportional to the reducibility of the catalysts, which is in agreement with expectations.A number of systems have been used previously for the ODH of alkanes [9,11]. For example, alkaline earth metal oxide-based catalysts promoted by other elements such as lithium and chlorine showed improved oxidative dehydrogenation activity [12,13].Rare earth metal oxide containing catalysts are used in ODH reactions because of their high thermal stability and their basicity.…”

“…A number of systems have been used previously for the ODH of alkanes [9,11]. For example, alkaline earth metal oxide-based catalysts promoted by other elements such as lithium and chlorine showed improved oxidative dehydrogenation activity [12,13].…”

Catalytic Activity and Coking Resistance on Hydroxyapatite for the Oxidative Dehydrogenation of Isobutane

“…Light olefins (C 2 -C 4 ) are the most important feedstocks (> 250 million metric tons per year) in the chemical industry 1 . Conventionally, they are produced from hydrocarbons via steam cracking of naphtha 2 .…”

Carbonyl Groups Modified Graphite Sheets Catalyze Oxidative Dehydrogenation of Propane to Propene

“…Key words: carbonyl group; graphite sheet; oxidative dehydrogenation; propane; propene 丙烯是全球最重要的化工原料之一, 传统的丙 烯生产工艺主要是利用石脑油作为原料通过热裂解 和催化裂解来生产烯烃, 整个过程的能耗较高且反 应可控性差, 目标产物收率低 [1] 。 通过丙烷直接脱氢 技术可以一步制取烯烃, 实现廉价烷烃的高附加值 利用, 因而成为近几年推动丙烯产量提升的新型工 艺 [2] 。 虽然丙烷直接脱氢技术原子利用率高, 但是从 化学反应角度考虑, 该过程受热力学平衡限制, 反 应强吸热, 需要较高的能耗, 增加了生产成本 [3][4] 。 在反应中引入氧化剂可以不受热力学限制, 并 且可以降低裂解产物的生成并消除积碳的影响, 从 而从根本上解决直接脱氢工艺中存在的问题 [5] 。但 是, 氧化脱氢过程中容易出现烯烃的过度氧化, 降 低目标产物的选择性。这一不利因素严重制约了氧 化脱氢工艺的工业化, 因此, 研究和开发一种高烯 烃选择性的催化剂至关重要 [6][7] 。 在现有的丙烷氧化脱氢制丙烯催化剂中, 负载 在二氧化硅、氧化锆和氧化铝等载体上的氧化钒是 研究最多的催化剂 [8] 。 但是作为金属氧化物, 此类催 化剂的活性较高, 反应过程中容易出现深度氧化产 物, 降低了目标产物的选择性 [9] 。近年来, 很多研 究者把研究方向转移到非金属材料上。2016 年, Hermans 课题组 [10] 首次利用六方氮化硼(h-BN)材料 催化丙烷氧化脱氢制丙烯, 当丙烷转化率为 14%时, 丙烯的选择性依然维持在 79%, 而且副产物主要是 乙烯, 深度氧化产物 CO 2 很少。 h-BN 催化过程主要 是利用 B-OH 和 B-O 之间的相互转换来活化并脱去 丙烷中的氢原子, 正是利用发生在催化剂表面可控 的氧化还原反应, 实现烯烃选择性高的目标 [11] 。h-BN 活性位的构建以及催化机理的研究为发展全新的催 化剂提供了一种新的思路。 石墨作为一种自然矿藏, 在我国储量丰富, 开 采便利。 相比较于氮化硼材料, 石墨片价格低廉, 更 易大规模获取 [12] 12.4%, 丙烯的选择性依然高达 73.9%, 深度氧化产 物 CO x 为 10%左右。羰基化石墨片的烯烃选择性, 远高于利用相同气相氧化处理得到的碳纳米管催化 剂 [13] , 十分接近现有报道中性能最优的氮化硼材 料 [10] 。当反应温度继续升高时, 丙烷的转化率迅速 提高。 结合图 2(b)可以看到, 当丙烷的转化率为 26% 时(温度为 535 ℃), 丙烯的选择性依然有 57.5%, 丙 烯的收率为 15%, 而烯烃的总收率突破了 20%。以 上数据充分体现了羰基化石墨材料催化丙烷氧化脱 氢制烯烃的优异性能 [14] Fig. 7 N 2 adsorption/desorption isotherms (a) and DFT pore size distribution (b) of carbonyl groups graphite sheets before (fresh) and after (spent) ODHP test 比表面积有所增加, 结合图 7(b)中的孔径分布, 可 以发现孔有所增加。这是由于在反应过程中, 氧气 的缓慢侵蚀作用, 使得堆积孔增多, 从而使催化剂 变得更加蓬松, 提高了比表面积。 反应前后的红外光谱图汇总于图 8 中, 通过气 相氧化处理, 石墨片在 1639 cm -1 附近出现对应羰 基的特征峰 [16] , 由此推测羰基化石墨片催化丙烷氧 化脱氢的活性位点应该是石墨片边沿的羰基 [17] 。而 测试完后石墨片的羰基含量明显增多, 且 3450 cm -1 左右的羟基特征峰的面积也在增大 [18] 。前者主要由 于氧气与石墨片的反应, 形成更多的羰基, 后者则 主要来源于羰基与丙烷中的氢原子反应。 为了进一步了解催化剂的成分和活性位的状态, 采用 XPS 对催化剂进行表征, 结果汇总于图 9。由 图 9(a)可以看到, 催化反应后, 氧的含量从 1.94%增 加到反应后的 2.47%, 说明氧气与边沿位的碳继续 反应, 进而形成更多的羰基, 从而提高了氧含量。 图 9(b)的 O1s 谱图中两个峰位分别为 533.0 和 531.8 eV, 分别对应着羟基和羰基两种官能团 [19][20]…”