CN116060020B - 乙酸自热重整制氢用钙铬基褐铁矿型镍基催化剂 - Google Patents
乙酸自热重整制氢用钙铬基褐铁矿型镍基催化剂 Download PDFInfo
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- QTBSBXVTEAMEQO-UHFFFAOYSA-N Acetic acid Chemical compound CC(O)=O QTBSBXVTEAMEQO-UHFFFAOYSA-N 0.000 title claims abstract description 144
- 239000003054 catalyst Substances 0.000 title claims abstract description 77
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 title claims abstract description 45
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- RQLWXPCFUSHLNV-UHFFFAOYSA-N [Cr].[Ca] Chemical compound [Cr].[Ca] RQLWXPCFUSHLNV-UHFFFAOYSA-N 0.000 title claims abstract description 11
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- KRKNYBCHXYNGOX-UHFFFAOYSA-N citric acid Chemical compound OC(=O)CC(O)(C(O)=O)CC(O)=O KRKNYBCHXYNGOX-UHFFFAOYSA-N 0.000 claims description 18
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- ODINCKMPIJJUCX-UHFFFAOYSA-N calcium oxide Inorganic materials [Ca]=O ODINCKMPIJJUCX-UHFFFAOYSA-N 0.000 claims description 7
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- BRPQOXSCLDDYGP-UHFFFAOYSA-N calcium oxide Chemical compound [O-2].[Ca+2] BRPQOXSCLDDYGP-UHFFFAOYSA-N 0.000 claims description 6
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Abstract
本发明涉及一种用于乙酸自热重整制氢的钙铬基褐铁矿型镍基催化剂。本发明针对现有催化剂在乙酸自热重整过程中出现的失活问题,提供了一种活性高、抗积炭、耐氧化的新型催化剂。本发明的催化剂的化学成分是(NiO)a(CaO)b(CrO1.5)c,其中a为0.38‑0.43,b为0.88‑1.33,c为1.27‑1.61。本发明采用Pechini溶胶凝胶法制备了Ni物种负载于钙铬褐铁矿型Ca2Cr2O5载体上且形成Ni‑Ca‑Cr‑O活性中心的新型催化剂。本发明催化剂有效提高了乙酸自热重整制氢反应活性。
Description
技术领域
本发明涉及一种特定的以钙铬基褐铁矿型氧化物Ca2Cr2O5负载Ni基催化剂的制备方法及其在乙酸自热重整制氢过程中的应用,属于乙酸自热重整制取氢气的技术领域。
背景技术
化石能源的大量使用带来了环境问题,如温室效应、酸雨、臭氧层破坏和颗粒物污染等。氢气,作为一种清洁燃料,具有高达122MJ/Kg的能量密度和环境友好的特点,被视为最有潜力的可替代能源之一。目前,常见的制氢方式有化石燃料制氢、工业副产物制氢、水电解法制氢等。化石燃料制氢存在着副产物和气体杂质多,同时对环境有不利影响的问题。工业副产物制氢、电解法制氢等制氢方式对技术和设备有比较严格的要求,且成本较高,氢气产量有限。由可再生的生物质油衍生的乙酸重整制氢是一种比较有前途的绿色制氢方式。
在乙酸进行重整反应制氢时,根据参与反应的原料乙酸、水和氧气配比不同,可以分为蒸汽重整制氢(Steam reforming,SR)、部分氧化重整制氢(Partial oxidation,POX)以及自热重整制氢(Auto-thermal reforming,ATR)三种途径。蒸汽重整制氢为吸热反应,需要外界供给热量,增加了制氢成本;而部分氧化重整制氢引入的氧气容易造成催化剂的氧化失活,且氢气产率不高;自热重整制氢,通过引入适量氧气,平衡了反应热,降低了制氢成本。
在乙酸自热重整制氢过程中,催化剂对反应起到非常关键的作用。重整催化剂包括贵金属催化剂和过渡金属催化剂,而镍基催化剂由于具有良好的活化乙酸分子中的C-C、C-H和O-H键的能力,且其对氢气有着较高的选择性,因此镍基催化剂常作为研究对象。然而,乙酸在催化剂上的Ni0活性位点上转化生成的CH3CO*等中间物种会进一步分解生成CH3*物种,其脱氢产物C*的积聚会形成积炭并沉积在Ni0的表面,导致催化剂的活性位点被覆盖,造成催化剂的失活。另一方面,由于引入的氧气主要在反应前端被消耗,导致反应床层前端温度急剧升高,可达1000℃以上,活性组分Ni在此温度下易发生聚集、烧结,同时在自热重整过程中的氧化性氛围易导致活性金属Ni0的氧化,从而导致反应前端不断向后移动,最终导致整个催化床层发生烧结、氧化而失活。因此,对于在乙酸自热重整制氢反应过程中,提高Ni基催化剂的抗积炭性、抗烧结性、抗氧化性,并降低对副产物如甲烷、丙酮等的选择性及提高氢气产率是乙酸自热重整制氢过程的关键因素。
为解决以上问题,本发明创造性地引入具有A2B2O5褐铁矿结构的Ca2Cr2O5氧化物负载的Ni基催化剂。
首先,褐铁矿结构Ca2Cr2O5中,通过引入有序的氧空位阵列和形成八面体和四面体配位过渡金属阳离子Ca2+/Cr3+的交替层结构,具有丰富的氧空位,并具备较强的晶格氧(O2-)转移能力,而活性组分Ni也对活性氧的释放起到积极的作用,从而可以促进乙酸转化过程中形成的*CHx(x=0-3)等中间物种的转化,提高对积炭前驱体*C的气化,获得CO/CO2产物(C*+O*→CO,CO+O*→CO2);同时,该结构组分促进了WGS反应(CO+H2O→CO2+H2)的正向进行,促进更多的氢气生成,也极大的提高了催化剂的抗积炭能力。
其次,褐铁矿型Ca2Cr2O5具有良好的热稳定性,在乙酸的转化过程中可以提供稳定的反应界面。其中,Ca物种作为碱土金属,提高催化剂的总体碱性,有助于吸附活化水分子,形成*OH和*O等中间物种,其中*OH物种参与重整反应,而*O物种对积炭前驱体的气化有利;而Cr物种具有热稳定性和可变价态,其中三价Cr3+物种能有效改善反应物H2O的吸附和活化,同时还能促进乙酸在自热重整过程中可能产生的甲醛、丙酮、甲苯和乙酸乙酯等副产物的氧化,从而提高氢气的选择性和产率;同时,活性组分Ni和Ca、Cr之间存在协同作用,形成了Ni-Ca-Cr-O活性中心,提高了对反应物乙酸、水、氧气的吸附和活化,促进乙酸衍生的CH3COO*、CH3CO*等中间物种的形成和转化,获得H2等目标产物,提高了催化剂的活性和抗积炭能力。
最后,在制备过程中,采用Pechini溶胶凝胶法构造出具有介孔结构的新型催化剂,该介孔结构促进反应物分子CH3COOH、H2O、O2和产物分子H2、CO、CO2等的传递和扩散;而且,通过介孔结构的限域效应,有效抑制乙酸衍生的CH2CO*、CH3OCH3等中间物种的缩聚效应,有效抑制积炭的形成,并提高对反应产物H2及CO/CO2的选择性。
发明内容
本发明要解决的问题是,在乙酸的自热重整过程中,Ni基催化剂在高温下容易被积炭占据活性中心,影响乙酸分子的转化,进而影响氢气的产量,同时在高温下,催化剂容易被氧化和出现烧结,影响催化剂的活性,提供了一种抗积炭、抗氧化、抗烧结的新型催化剂。
本发明采用Ni作为活性组分,通过Pechini溶胶凝胶法制备出具有介孔结构的钙铬基褐铁矿型Ca2Cr2O5载体负载的镍基催化剂;将本发明催化剂用于乙酸自热重整制氢反应中,在反应温度为650℃的情况下,优选催化剂乙酸的转化率接近100%,氢气产率达到2.20mol-H2/mol-HAc左右。
本发明技术方案:
本发明针对乙酸自热重整的特点,以Pechini溶胶凝胶法制备了钙铬基褐铁矿型Ni/Ca2Cr2O5催化剂。本发明的催化剂以氧化物计的摩尔组成是(NiO)a(CaO)b(CrO1.5)c,其中a为0.38-0.43,b为0.88-1.33,c为1.27-1.61;以氧化物计的重量百分比组成为:氧化镍为14.0%-16.1%,氧化钙为24.6%-37.7%,氧化铬为48.3%-61.3%,且各组分重量百分比组成之和为100%。
具体的制备步骤如下:
(1)根据催化剂的摩尔组成(NiO)a(CaO)b(CrO1.5)c,其中a为0.38-0.43,b为0.88-1.33,c为1.27-1.61,称取适量的Ni(NO3)2·6H2O、Ca(NO3)2·4H2O和Cr(NO3)3·9H2O溶于去离子水中进行搅拌,直至全部溶解,得到1#溶液;
(2)按金属阳离子摩尔总数:柠檬酸:乙二醇比为1:1:1,配制柠檬酸与乙二醇的混合溶液2#,将溶液2#与1#溶液混合,然后保持水浴温度为70℃,继续搅拌,直至溶胶形成;
(3)将得到的溶胶置于105℃烘箱中24h,然后于管式炉中以5℃/min的速率从室温升到750℃,煅烧保持4h,即可得到钙铬基褐铁矿型镍基催化剂Ni/Ca2Cr2O5,即形成了Ni-Ca-Cr-O活性中心的、NiO负载于钙铬基褐铁矿型Ca2Cr2O5氧化物的晶相结构,其典型结构如附图1所示,同时构造出介孔结构,其典型BJH孔径分布如附图2所示;
(4)催化剂活性测试:本发明催化剂使用前于H2中500-800℃温度下还原1h,然后将乙酸、水的混合溶液以恒流泵注入汽化器经汽化后,混合氧气,并以氮气为内标气体,形成摩尔组成为CH3COOH/H2O/O2/N2=1/(1.3-5.0)/(0.21-0.35)/(2.5-4.5)的反应原料气,并将此原料气导入反应床层,反应温度为500-800℃。
本发明的有益效果:
(1)在钙铬基褐铁矿型Ni/Ca2Cr2O5催化剂中,形成了Ni-Ca-Cr-O活性中心,提高了对反应物乙酸、水、氧气的吸附和活化,促进乙酸衍生的CH3COO*、CH3CO*等中间物种的形成和转化,获得H2等目标产物,提高了催化剂的活性和抗积炭能力。
(2)在催化剂的褐铁矿型Ca2Cr2O5结构中,形成了有序的氧空位阵列和形成八面体和四面体配位过渡金属阳离子Ca2+/Cr3+的交替层结构,形成了丰富的氧空位,并具备了较强的晶格氧(O2-)转移能力,促进了乙酸转化过程中形成的*CHx(x=0-3)等中间物种的转化,提高了对积炭前驱体*C的气化,获得了CO/CO2产物(C*+O*→CO,CO+O*→CO2);同时,该结构也促进了WGS反应(CO+H2O→CO2+H2)的正向进行,促进更多的氢气生成,提高了催化剂的抗积炭能力。
(3)催化剂的褐铁矿型Ca2Cr2O5氧化物具有良好的热稳定性,在乙酸的转化过程中提供了稳定的反应界面,抑制了活性组分Ni的烧结,提高了催化剂的稳定性。在该结构中,Ca物种作为碱土金属,提高催化剂的总体碱性,有助于吸附活化水分子,形成*OH和*O等中间物种,其中*OH物种参与重整反应,而*O物种对积炭前驱体的气化有利;而Cr物种具有热稳定性和可变价态,其中三价Cr3+物种能有效改善反应物H2O的吸附和活化,同时还促进了乙酸在自热重整过程中可能产生的甲醛、丙酮、甲苯和乙酸乙酯等副产物的氧化,从而提高氢气的选择性和产率;活性组分Ni与Ca2Cr2O5载体之间存在强相互作用,可以有效吸附活化乙酸,且Ni也对活性氧的释放起到促进作用,这都有效提高了催化剂的活性及其抗积炭的能力。
(4)本发明构造出具有介孔结构的新型催化剂,介孔结构促进反应物CH3COOH、H2O、O2和产物分子H2、CO、CO2等的传递和扩散;而且,通过介孔结构的限域效应,有效抑制乙酸衍生的CH2CO*、CH3OCH3等中间物种的缩聚效应,有效抑制积炭的发生,并提高对反应产物H2及CO/CO2的选择性。
(5)将本发明催化剂用于乙酸自热重整制氢反应过程,结果显示,本发明的催化剂体现出抗烧结、抗积炭、抗氧化性、催化活性高等优势。
附图说明
图1:本发明催化剂氧化物的X射线衍射谱图
图2:本发明催化剂的BJH孔径分布图
具体实施方式
参照例一
称取12.497g Al(NO3)3·9H2O和1.175g的Ni(NO3)2·6H2O倒入烧杯中,加入一定量的去离子水溶解,得到溶液1#;然后称取7.849g的柠檬酸和和2.318g的乙二醇,溶解之后进行混合,得到溶液2#;然后将2#溶液与1#溶液混合,置于70℃条件下进行搅拌,直至凝胶形成;然后将凝胶放入105℃烘箱中,24h后取出,最后在管式炉中以5℃/min的速率升温到750℃进行焙烧,保持4h,获得CDUT-NA催化剂,形成了负载于Al2O3上的Ni基催化剂;该催化剂以氧化物计的重量百分比组成为:氧化镍(NiO)为15.1%,氧化铝(AlO1.5)为84.9%。
乙酸自热重整反应活性评价在连续流动固定床反应器中进行。将催化剂研磨并压片,然后筛分成20-40目颗粒,称取0.2g装入反应器中,在500-800℃的温度下在H2中还原1h;然后将乙酸、水的混合溶液以恒流泵注入汽化器经汽化后,混合氧气,并以氮气为内标气体,形成摩尔组成为CH3COOH/H2O/O2/N2=1/(1.3-5.0)/(0.21-0.35)/(2.5-4.5)的反应原料气,并将此原料气导入反应床层,反应条件为500-800℃、常压、空速10000-35000ml/(g-catalyst·h),反应尾气采用气相色谱仪在线分析。
催化剂CDUT-NA经乙酸自热重整活性考察,在反应条件为常压、空速25000ml/(g-catalyst·h)、反应温度为650℃、原料气乙酸/水/氧气=1/4.0/0.28时,乙酸的初始转化率为99.6%,并随着反应进行到10小时,乙酸的转化率降低为64.6%,氢气产率逐渐下降至0.65mol-H2/mol-HAc,CO2的选择性在49.0%左右,CO选择性在38.5%左右,CH4选择性为5.6%,副产物丙酮的选择性增长到35.7%左右。XRD、BET等表征结果表明,该催化剂在乙酸自热重整过程中稳定性差,且副产物较多,丙酮化反应未得到有效抑制,发生烧结、积炭及部分氧化,活性较低。
实施例一
称取3.043g的Ca(NO3)2·4H2O、5.156g的Cr(NO3)3·9H2O和1.162g的Ni(NO3)2·6H2O倒入烧杯中,加入一定量的去离子水溶解,得到溶液1#;然后称取6.254g的柠檬酸和和1.847g的乙二醇,溶解之后进行混合,得到溶液2#;然后将2#溶液与1#溶液混合,置于70℃条件下进行搅拌,直至凝胶形成;然后将凝胶放入105℃烘箱中,24h后取出,最后在管式炉中以5℃/min的速率升温到750℃进行焙烧,保持4h,获得了钙铬基褐铁矿型镍基催化剂Ni/Ca2Cr2O5,即CDUT-NCC催化剂,其典型晶体结构如附图1所示,在24.7°、33.7°和49.6°出现明显的Ca2Cr2O5衍射峰,而37.5°、43.5°和63.1°出现NiO的衍射峰,形成了NiO负载于钙铬基褐铁矿型Ca2Cr2O5氧化物的晶相结构,并形成了Ni-Ca-Cr-O活性中心;该催化剂经氮气低温物理吸附测试,孔径集中分布在4nm处,典型介孔结构如附图2所示;该催化剂摩尔组成为(NiO)0.40(CaO)1.29(CrO1.5)1.29,该催化剂以氧化物计的重量百分比组成为:氧化镍(NiO)为14.9%,氧化钙(CaO)为36.1%,氧化铬(CrO1.5)为49.0%。
催化剂CDUT-NCC经乙酸自热重整活性考察,在反应条件为常压、空速25000ml/(g-catalyst·h)、反应温度为650℃、原料气乙酸/水/氧气=1/4.0/0.28时,该催化剂对乙酸转化率为100%左右,氢气产率为2.2mol-H2/mol-HAc左右,CO2选择性在45.7%左右,CO选择性在48.8%左右,CH4选择性在5.6%左右,几乎没有副产物丙酮。对CDUT-NCC催化剂进行氮低温物理吸附表征,结果表明,比表面积为2.1m2/g、孔体积为0.02cm3/g、孔径分布集中,平均孔径为8.3nm,最可几孔径为4.0nm,属于介孔材料。表征结果显示,该催化剂未发生烧结现象,无显著积炭,且该催化剂有效抑制了副产物丙酮的产生,催化活性高。
Claims (2)
1.钙铬基褐铁矿型镍基催化剂在乙酸自热重整制氢过程中的应用,其特征在于:将钙铬基褐铁矿型镍基催化剂在500-800℃的温度下在H2中还原1h,然后将乙酸、水的混合溶液以恒流泵注入汽化器经汽化后,混合氧气,并以氮气为内标气体,形成摩尔组成为CH3COOH/H2O/O2/N2=1/(1.3-5.0)/(0.21-0.35)/(2.5-4.5)的反应原料气,并将此原料气导入反应床层,反应温度为500-800℃;所述催化剂由以下方法制备而成:称取适量的Ni(NO3)2·6H2O、Ca(NO3)2·4H2O和Cr(NO3)3·9H2O溶于去离子水中进行搅拌,直至全部溶解,得到1#溶液;按金属阳离子摩尔总数:柠檬酸:乙二醇比为1:1:1,配制柠檬酸与乙二醇的混合溶液2#,将溶液2#与1#溶液混合,然后保持水浴温度为70℃,继续搅拌,直至溶胶形成;将得到的溶胶置于105℃烘箱中24h,然后于管式炉中以5℃/min的速率从室温升到750℃,煅烧保持4h,得到钙铬基褐铁矿型镍基催化剂Ni/Ca2Cr2O5,即形成了Ni-Ca-Cr-O活性中心的、NiO负载于钙铬基褐铁矿型Ca2Cr2O5氧化物的晶相结构;该催化剂以氧化物计的摩尔组成是(NiO)a(CaO)b(CrO1.5)c,其中a为0.38-0.43,b为0.88-1.33,c为1.27-1.61;以氧化物计的重量百分比组成为:氧化镍为14.0%-16.1%,氧化钙为24.6%-37.7%,氧化铬为48.3%-61.3%,且各组分重量百分比组成之和为100%。
2.根据权利要求1所述的钙铬基褐铁矿型镍基催化剂在乙酸自热重整制氢过程中的应用,其特征在于:所述催化剂以氧化物计的重量百分比组成为:氧化镍为14.9%,氧化钙为36.1%,氧化铬为49.0%。
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