Preparation Of Nickel-Supported Hierarchical Beta Zeolite Catalysts And Their Catalytic Performance For Ethanol Steam Reforming

Hydrogen production by ethanol steam reforming(ESR),as a means to support sustainable energy development,is favored by the chemical industry because of its high hydrogen selectivity and ethanol conversion rate,and will certainly be applied widely.At present,the main problems faced by the ESR reaction failing to achieve industrialization are:(1)high production cost and high thermal energy consumption;(2)separation of carbon dioxide in the mixed gas and the practical difficulty of the elimination of by-products;(3)stability of the catalyst.There is still certain gap in the requirements for industrialization.The former mainly involves the selection of a catalyst carrier and a suitable active metal,while the latter requires in situ elimination of the properties of carbon dioxide.Therefore,designing and preparing a catalyst used in ESR with low energy consumption,reasonable production cost,high efficiency,long service life and easy regeneration has important theoretical significance and practical application value.The use of non-precious metal nickel as the active metal for ethanol steam reforming has the advantages of high activity,good performance and low cost.However,nickel-based catalysts generally have problems of easy sintering and deactivation,which limits the application of nickel-based catalysts.How to improve the anti-sintering performance of nickel has become an urgent problem to be solved.In this thesis,hierarchical Beta molecular sieve with micro-mesoporous connectivity was constructed by soft template method,and nickel was used as the only active metal.The volume immersion method,ion exchange method and chemical reaction method were used.The active metal is supported or embedded on a support to construct different types of nickel-based catalysts for hydrogen production from ethanol steam reforming.The mechanism of soft-template in the construction of multi-stage pore molecular sieves was studied by means of related characterization methods.The structure and catalytic mechanism of the catalyst were studied from the microscopic point of view.The main research contents and related conclusions are as follows:(1)Hierarchical Beta molecular sieve(MBeta)having rich intragranular mesopores structure was synthesized via one-step hydrothermal synthesis method by using gemini surfactant[C12H25(CH3)2N+(CH2)6N+(CH3)2C12H25][Br-]2(C12-6-12Br)and tetraethylammonium hydroxide(TEAOH)as templates.By studying the effects of two templating agent ratios,solution alkalinity and crystallization time on MBeta,combined with SAXRD,BET and TEM data analysis,the micro-mesopore structure at the beginning of the reaction was simultaneously observed,indicating that C12-6-12Br played an occupying effect at the early stage of synthesizing MBeta,restricted the molecular sieve precursor in certain area,took synergistic effect with TEAOH in suitable alkali environment,and constructed hierarchical pore structure with micro-mesoporous connectivity inside the crystal.The results show that when the ratio of TEAOH/C12-6-12Br/NaOH is 10:3:4.8,maintaining the crystallization at 120 ? for 96 h,the MBeta in which SBET is 765.7 m2�g-1 and SMeso/SBET is 0.31 can be synthesized.It can be observed from the TEM that the microporous channels have rich worm-like mesoporous structures,and the two pore structures are connected to each other without significant phase separation.(2)MBeta zeolite and Beta zeolite synthesized by two template methods(TEAOH/C12-6-12Br)were used as carriers to load active metal Ni onto the supports using sol-gel isometric impregnation(SIWI)and equal volume impregnation(IWI),preparing a series of nickel-based catalysts which were applied to ESR reaction.The effects of support structure and loading mode on catalytic performance were studied.Firstly,the influence of carrier effect was studied:10Ni-MBeta(G)and 10Ni-Beta(G)catalysts were prepared by SIWI method with 10 wt%Ni.When the reaction temperature is 550 ?,the stability test is carried out for 29 h.The rate of ethanol conversion and hydrogen yield are more than 95%and 75%when lONi-MBeta(G)catalyst is used.However,when lONi-Beta(G)catalyst was used for 19 hours,obvious deactivation occurred and the rate of ethanol conversion and hydrogen yield decreased from 90%and 79%to 50%and 61%respectively.The results show that when MBeta was used as carrier,intracrystalline mesopores like overpass can transfer the reaction media efficiently and lower the carbon deposition rate in the channel.Secondly,the influence of loading method was studied.The nickel-based catalysts lONi-MBeta(G)and lONi-MBeta(N)were prepared by SIWI and IWI with MBeta as support respectively to study ESR catalytic performance.The hydrogen production rate of using 10Ni-MBeta(G)catalyst was about 98%in the stability test at 550 ? for 32 h,while that of using lONi-MBeta(N)catalyst decreased from 77%to 63%.It is concluded that the SIWI method can improve the catalytic activity of active metals.XRD analysis showed that the characteristic peaks of active component Ni in lONi-MBeta(G)were dispersed,and its particle size was 6.7 nm,which was significantly lower than the corresponding value of lONi-MBeta(N)of 16.7 nm.It is concluded that SIWI method is more conducive to the formation of Ni with higher dispersion and smaller particle size by the interaction between active metals and carriers,which can effectively improve the catalytic efficiency of ESR.(3)MBeta was synthesized by one-step hydrothermal synthesis with bifunctional template[C12H25(CH3)2N+(CH2)2N+(CH3)2C2H5][Br-]2(C12-2-2).Calcium and nickel ions were introduced into the interior of MBeta as the carrier by ion exchange and the content of calcium and nickel was 3.5 wt%and 3.5 wt%by AAS analysis.A series of core-shell structure with a gradient distribution of active metal content in the interior and exterior of 7 wt%,10 wt%and 15 wt%of total nickel were successfully constructed by further loading active metal nickel on the carrier surface by IWI method.Firstly,the effects of calcium promoter and core-shell structure with gradient distribution of active metal content on ESR catalytic performance were studied:the catalyst lONi-Beta(DI)prepared by the above method and the Ca-free catalyst lONi-MBeta prepared by IWI were used to study the catalytic performance of ESR.At the reaction temperature of 450? the ethanol conversion and hydrogen selectivity of the former were 100%and 71%respectively,and the ethanol conversion and hydrogen selectivity of the latter were 60%and 25%respectively.This illustrates that the lONi-Beta(DI)catalyst exhibits excellent catalytic activity due to the adsorption of calcium promoter to CO2 and the catalytic activity of calcium promoter to methane reforming.Meanwhile,the gradient distribution of active metals in the core-shell structure improves the utilization of active metals and enables the reaction medium to react adequately.Secondly,the effects of different concentration gradient catalysts on the catalytic performance of ESR were studied:7Ni-MBeta(DI),10Ni-MBeta(DI),15Ni-MBeta(DI)were prepared and applied to ESR catalytic reaction.The three catalysts showed significant differences in the range of reaction temperature from 250 ? to 550 ? in which the ethanol conversion and hydrogen selectivity of lONi-MBeta(DI)catalysts remained 95%and 71%respectively and almost unaffected by temperature.But for 7Ni-MBeta(DI)and 15Ni-MBeta(DI)catalysts,ethanol conversion and hydrogen selectivity were lower than those of 10Ni-MBeta(DI).This indicates that the calcium promoter and active metal Ni inside the zeolite effectively affect the choice of reaction pathway,and the amount of active metal on the outer surface of zeolite also plays an important role in the catalysis and purification of the final product.The bifunctional templating agent[C18H37(CH3)2N+(CH2)2N+(CH3)2C8H17][Br-]2(C18-2-8)was used to synthesize hierarchical pore mordenite and hierarchical pore zeolite by one-step hydrothermal method,which broadened the range of applications of the functional templating agent studied above.(4)Micro-mesoporous interconnected OH-MBeta zeolite was synthesized in NaOH-free environment using gemini quaternary ammonium base([C12H25(CH3)2N+(CH2)6N+(CH3)2C12H25][OH-]2(C12-6-12OH)).After calcination,NiSiO2/OH-MBeta was prepared by in-situ growth of Ni3Si2O5(OH)4 shell on the outer surface of the nuclear phase namely OH-MBeta zeolite by chemical reaction and after further reduction,and NiNPS/OH-MBeta zeolite of core-shell structure was constructed.The TEM and SEM analysis showed that the synthesized NiSiO2/OH-MBeta shell presented petal-like structure.Compared with OH-MBeta,the Vtotal,VMesa,and SMesO/SBET of NiSiO2/OH-MBeta increased from 0.77 cm3�g-1?0.63 cm3�g-1 and 0.54 to 1.19 cm3�g-1?3.17 cm3�g-1 and 0.85 respectively,from the BET data.This indicated that Ni3Si2O5(OH)4 of petal-like structure was formed by chemical bonding of Ni with skeleton by hydrothermal method,increasing the external surface area of the catalyst effectively.Such a catalyst will be conducive to the full contact between the reaction medium and the active metal and improve the catalytic efficiency.By comparing the ESR catalytic performance of Ni/OH-MBeta catalyst with same loading amout(22 wt%)prepared by IWI with NiNPS/OH-MBeta,the results showed that the conversion of ethanol and hydrogen selectivity were 90%and 73%respectively and there was no ethylene when NiNPS/OH-MBeta was used as catalyst at 400 ?.But the conversion of ethanol and hydrogen selectivity of using Ni/OH-MBeta catalyst were only 509%and 60%and the selectivity of ethylene is 49%.It is obvious that NiNPS/OH-MBeta exhibits excellent catalytic activity at low temperature.The SEM analysis of the reacted NiNPS/OH-MBeta catalyst showed that no fibrous carbon deposition was found on the surface of the catalyst.This indicates that under the condition of high nickel content,the catalyst of core-shell structure by inlaying Ni on the surface of hierarchical porous Beta zeolite with in-situ chemical method can reflect the advantage of hierarchical channels of nuclear phase,increase the contact probility between the reaction media and Ni,improve the catalytic efficiency of ESR,and alleviate the deactivation of catalyst caused by the aggregation of active metals.