catalysts Article
Effect of the Cr2O3 Promoter on Pt/WO3-ZrO2 Catalysts for n-Heptane Isomerization Guangxiang He 1,2 , Rongrong Zhang 1,2 , Qian Zhao 1,2 , Suohe Yang 1,2 , Haibo Jin 1,2, * Xiaoyan Guo 1,2, * 1
2
*
and
Department of Chemical Engineering, Beijing Institute of Petrochemical Technology, No.19, Qingyuan Road, Huangcun Town, DaXing District, Beijing 102617, China;
[email protected] (G.H.);
[email protected] (R.Z.);
[email protected] (Q.Z.);
[email protected] (S.Y.) Beijing Key Laboratory of Fuel Cleanliness and Efficient Catalytic Emission Reduction Technology, Beijing 102617, China Correspondence:
[email protected] (H.J.);
[email protected] (X.G.); Tel.: +86-186-1822-9069 (H.J.); +86-136-8102-9163 (X.G.)
Received: 28 September 2018; Accepted: 3 November 2018; Published: 6 November 2018
Abstract: Isomerate, the product of a light naphtha Isomerization unit, is a clean, high-octane gasoline blending component, which is free of sulfur content, aromatics, and olefins. However, the isomerization of the long-chain alkanes, such as n-heptane, is pretty difficult. As a result, this process has not been commercialized yet. In recent years, much attention has been paid to Pt/WO3 /ZrO2 as an n-heptane isomerization catalyst due to its good thermal stability, strong acidity, simplicity of preparation, reusability and good isomerization activity. In this work, the Pt/WO3 /ZrO2 catalyst was modified by various loading of metal Cr to improve the catalytic performance. The effects of WO3 content, Cr metal loading and calcination temperature on the catalyst characters and catalytic activity were studied. It is shown that Cr-Pt/WO3 /ZrO2 with the loading of 18 wt% WO3 and 1.0–1.4 wt% Cr, prepared at the calcination temperature of 800 ◦ C, has the highest activity. It was found that the octane number increases by 28 units through the isomerization of light naphtha feedstocks. In addition, the study on the stability of Cr-Pt/WO3 /ZrO2 indicates that the catalyst is not deactivated after 500 h of the n-heptane isomerization reaction. Keywords: Solid superacid; Pt/WO3 /ZrO2 ; n-heptane; hydrogen isomerization; Cr
1. Introduction Current stringent environmental protection regulations have forced the implementation of severe controls for reducing aromatic, olefin, and sulfuric compounds and led to a sharp rise in the demand for clean fuels with none-aromatic and high octane number [1,2]. Heptane isomerization to branched isomers with high octane numbers is a desirable reaction for compensating for the octane number [3,4]. However, no process that is suitable for heptane fraction isomerization in industrial oil refining is currently known; since the implementation of such a process will allow for the meeting of long-term environmental requirements for commercial gasoline, it has aroused increased interest [4]. The main concern for the commercialization of n-heptane isomerization is that β-cleavage occurs during the isomerization reaction. A suitable catalyst needs to be developed to prevent n-heptane from cracking while promoting the conversion of n-heptane to multi-branched isomers [5]. SO4 2− /ZrO2 catalysts have attracted significant attention because of their ability to isomerize light alkanes at low temperature, but suffer from the disadvantages of deactivation and possibly from sulfur loss during reaction and regeneration, limiting their applicability in isomerization. As an alternative to SO4 2− /ZrO2 , WO3 /ZrO2 has become increasingly important since its discovery by Catalysts 2018, 8, 522; doi:10.3390/catal8110522
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Arata and Hino [6] because of its good stability both in reducing and oxidizing conditions [7]. While the WO WO33/ZrO /ZrO catalyst shows high selectivity in the isomerization reaction of linear alkane, 2 type catalyst shows high selectivity in the isomerization reaction of linear alkane, the 2 type catalyst activity is is is too high [8].[8]. Thus, thethe catalyst needs to the catalyst is relatively relativelylow lowand andthe thedegree degreeofofcracking cracking too high Thus, catalyst needs modifiedtotomeet meetthe theindustrial industrialproduction production requirements. requirements. The catalyst to bebemodified catalyst consisting consistingof ofPt Ptdeposited deposited on 3WO 3/ZrO 2 n-heptane for n-heptane isomerization has attracted the attention ofnumber a largeofnumber of on WO /ZrO isomerization has attracted the attention of a large researchers 2 for researchers as the most promising catalyst [9,10]. The structure of the WO 3 /ZrO 2 catalyst varies as the most promising catalyst [9,10]. The structure of the WO3 /ZrO2 catalyst varies depending on the on theleading WO3 content [11,12], leading to different catalyst activities. The structure andof WO3depending content [11,12], to different catalyst activities. The structure and properties of this kind properties this kindby ofchanging catalyst can modified by changing the WO3 content [13]. While most catalyst can beofmodified thebe WO 3 content [13]. While most researchers believe that the researchers believe that the best catalytic activity is obtained on when 3 is highly dispersed on the best catalytic activity is obtained when WO3 is highly dispersed the WO surface of the catalyst, there has surface of the catalyst, there has been some debate about this subject. It was shown that the catalyst been some debate about this subject. It was shown that the catalyst has better catalyst activity with the has better catalyst activity with the WO3 density of seven–eight atoms∙nm−2 [14,15]. Many WO3 density of seven–eight atoms·nm−2 [14,15]. Many researchers have found that the addition of researchers have found that the addition of small amounts of metals such as Fe, Al, In, and Ga to small amounts of metals such as Fe, Al, In, and Ga to Pt/WO3 /ZrO2 enhances catalysis and stability of Pt/WO3/ZrO2 enhances catalysis and stability of n-heptane isomerization, and it was demonstrated n-heptane isomerization, and it was demonstrated that Cr can be used as a modifier in Pt/WO3 /ZrO2 that Cr can be used as a modifier in Pt/WO3/ZrO2 catalysts. Based on economic considerations, Cr catalysts. Based on economic considerations, Cr metal is a more cost-effective option, and is therefore metal is a more cost-effective option, and is therefore a more suitable for industrial production than a more suitable for industrial production than Ga metal [16–18]. In this work,Pt/WO n-heptane isomerization Ga metal [16–18]. In this work, n-heptane isomerization over Cr-promoted 3/ZrO2 was studied overand Cr-promoted Pt/WO studied and compared to that over Pt/WO3 /ZrO2 . 3 /ZrO 2 was compared to that over Pt/WO 3/ZrO2. 2. Results andand Discussion 2. Results Discussion 2.1. Effect of Cr2 O3 Loading on Pt/ WO3 /ZrO2 2.1. Effect of Cr2O3 Loading on Pt/ WO3/ZrO2 2.1.1. Characterization of the Catalysts 2.1.1. Characterization of the Catalysts The XRD patterns of Cr-Pt/WZ catalysts with different Cr loading are presented in Figure 1. The XRD patterns of Cr-Pt/WZ catalysts with different Cr loading are presented in Figure 1. All All of the catalysts show a mixture of the tetragonal and monoclinic zirconia phases, while they are of the catalysts show a mixture of the tetragonal and monoclinic zirconia phases, while they are dominated by the tetragonal phase. The Cr-Pt/WZ diffraction peak peakintensities intensities dominated by the tetragonal phase. The Cr-Pt/WZcatalyst catalystexhibits exhibits lower lower diffraction of monoclinic ZrO than the Pt/WZ catalyst. The intensities of the diffraction peaks of the tetragonal 2 of monoclinic ZrO2 than the Pt/WZ catalyst. The intensities of the diffraction peaks of the tetragonal ZrOZrO increase at first and then decrease with increasing of Cr loading. This result shows that the 2 phase 2 phase increase at first and then decrease with increasing of Cr loading. This result shows that appropriate amount of Cr incorporation helpshelps to stabilize thethe tetragonal ZrO 2 2phase the appropriate amount of Cr incorporation to stabilize tetragonal ZrO phaseof of the the Pt/WZ Pt/WZ catalysts and inhibits the transformation from the tetragonal phase to the monoclinic phase during catalysts and inhibits the transformation from the tetragonal phase to the monoclinic phase during calcination at high temperature, leading to higher isomerization activity activity [19]. The[19]. peaks of crystalline calcination at high temperature, leading to higher isomerization The peaks of WO3crystalline almost disappear when the Pt/WZ catalysts were loaded with Cr.loaded The interaction between Cr and WO3 almost disappear when the Pt/WZ catalysts were with Cr. The interaction and WO3 reduces the energy WO on the surface of the catalyst, promotingof WO3between reducesCr the energy of crystalline WO3 of oncrystalline the surface of3the catalyst, promoting the dispersion dispersion WO3. Based on experiments, the optimal Cr wt%. content is 1.0–1.4 wt%. WOthe on theofexperiments, thethe optimal Cr content is 1.0–1.4 3 . Based
*: WO3
T T
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Figure 1. XRD patterns of the Cr-Pt/WZ catalyst with different Cr contents. Figure 1. XRD patterns of the Cr-Pt/WZ catalyst with different Cr contents.
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Thespecific specificsurface surfacearea, area,pore poresize sizeand andvolume volumeofofthe thePt/WZ Pt/WZsamples sampleswith withdifferent differentCr Crcontents contents The werecharacterized characterizedusing usingBET BETanalysis, analysis,and andthe theresults resultsare arepresented presentedininTable Table1.1.The Theaddition additionofofCr Cr were changes the specific surface areas of WZ. It is shown that the optimal Cr content of approximately changes the specific surface areas of WZ. It is shown that the optimal Cr content of approximately 1.0wt% wt%results results larger pore diameter specific surface which in leads turn leads to more 1.0 in in larger pore diameter andand specific surface area,area, which in turn to more activeactive sites sites and acid [20]. the Cr-Pt/WZ activeisomerization. in n-heptane and acid sites [20].sites Thus, the Thus, Cr-Pt/WZ catalyst willcatalyst becomewill morebecome active inmore n-heptane isomerization. The result also shows thatthe Cr pore can improve pore structure ofthe thedispersed catalysts, oxides but the The result also shows that Cr can improve structurethe of the catalysts, but dispersed oxides be found in the Pt/WZincreases. as the Cr content increases. will be found in thewill pores of Pt/WZ as pores the Crofcontent Table1. 1.The Thespecific specific surface area and pore size and volume the Cr-Pt/WZcatalysts catalystswith withdifferent different Table surface area and pore size and volume ofof the Cr-Pt/WZ Cr contents. Cr contents. 2 −1 3∙g3 −1) −1 Sample SBET/(m Sample SBET∙g/(m) 2 ·g−1 ) VP/(cm VP /(cm ·g ) Pt/WZPt/WZ 77.06 77.06 0.153 0.153 0.2 wt% 80.94 80.94 0.161 0.2Cr-Pt/WZ wt% Cr-Pt/WZ 0.161 0.6Cr-Pt/WZ wt% Cr-Pt/WZ 0.168 0.6 wt% 105.26105.26 0.168 1.0 wt% Cr-Pt/WZ 103.5 0.177 1.0 wt% Cr-Pt/WZ 103.5 0.177 1.4 wt% Cr-Pt/WZ 92.94 0.163 1.4 wt% Cr-Pt/WZ 92.94 0.163 1.8 wt% Cr-Pt/WZ 89.94 0.161 1.8 wt% Cr-Pt/WZ 89.94 0.161
dP/nm dP /nm 6.54
6.54 6.536.53 6.146.14 6.856.85 5.84 5.84 5.83
5.83
The 3 -TPD. Thesurface surfaceacidity acidityofofthe thecatalyst catalystwas wascharacterized characterizedby byNH NH 3-TPD.The Theeffect effectofofCr Crloading loadingon on ◦C the catalyst acidity was studied. As shown in Figure 2, the area of the desorption peaks at 350 the catalyst acidity was studied. As shown in Figure 2, the area of the desorption peaks at 350 °C and ◦ C increase at first and then decrease with the increase of the Cr content, indicating that and 650 650 °C increase at first and then decrease with the increase of the Cr content, indicating that the the medium-strong acid sites superacid sites catalyst increase at first decrease. medium-strong acid sites andand superacid sites on on thethe catalyst increase at first andand thenthen decrease. The The Cr-Pt/WZ catalyst with 1.0 wt% Cr has the most acidic sites. As a result, it has the greatest Cr-Pt/WZ catalyst with 1.0 wt% Cr has the most acidic sites. As a result, it has the greatest isomerization isomerizationactivity activity[21]. [21].
1.8 wt%Cr TCD signal(a.u.)
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1.0 wt%Cr 0.2 wt%Cr 0.6 wt%Cr 0 wt%Cr
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Temperature/ °C Figure Figure2.2.NH NH 3-TPDcurves curvesofofthe theCr-Pt/WZ Cr-Pt/WZcatalyst catalystwith withdifferent differentCr Crcontents. contents. 3 -TPD
2.1.2. Catalytic Activity of the Catalysts 2.1.2. Catalytic Activity of the Catalysts As shown in Figure 3, the reaction temperature has a great influence on the activity of the catalyst. As shown in Figure 3, the reaction temperature has a great influence on the activity of the The conversion of n-heptane gradually increases with the increase of reaction temperature, indicating catalyst. The conversion of n-heptane gradually increases with the increase of reaction temperature, the increases of the isomerization rate. However, the isomerization equilibrium constant Kp decreases, indicating the increases of the isomerization rate. However, the isomerization equilibrium constant which is unfavorable to the isomerization rearrangement reaction. At the same time, the rate of the Kp decreases, which is unfavorable to the isomerization rearrangement reaction. At the same time, cracking is promoted when the temperature increases because cracking is an endothermic reaction. the rate of the cracking is promoted when the temperature increases because cracking is an endothermic reaction. A comprehensive analysis shows that the yield of isomeric heptane is
temperature of approximately 220 °C, which leads to a higher octane number. At the reaction temperature of 220 °C, the yield of isomeric heptane increases at the beginning and then decreases as the Cr content increases. When the Cr content increases from zero to 1.0 wt%, the isomeric heptane yield increases from 40.5% to 73.7%. Further increase of the Cr content to 1.4 Catalysts 2018,not 8, 522 4 of 15 wt% does bring an apparent yield change. Upon an increase to 1.8 wt%, the yield decreases to 68.9%. Based on Figure 1, the peak intensity of the tetragonal ZrO2 phase that endows the catalyst with its catalytic activity is the strongest, and the catalyst has the strongest acidity and reduction A comprehensive analysis shows that the yield of isomeric heptane is relatively high, and products ability, resulting in the best catalytic activity. have a higher proportion of multi-branched isomers at the reaction temperature of approximately 220 ◦ C, which leads to a higher octane number. 80
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Figure 3. The activity of Cr-Pt/WZ catalysts with different Cr content. Reaction condition: n(H2 ):n(C7 ) Figure 3. The activity of Cr-Pt/WZ catalysts with different Cr content. Reaction condition: n(H2):n(C7) = 9 mol/mol, LHSV(liquid hourly space velocity) = 0.7 h−1 , and P = 1.0 MPa, T = 200~280 ◦ C. S-Yield: and P M/S: = 1.0 MPa, T = 200~280 = 9 mol/mol,isomer LHSV(liquid hourly space velocity) = isomer 0.7 h−1, yield, Single-chain yield, M-Yield: Multi-branched M-Yield/ S-Yield. °C. S-Yield: Single-chain isomer yield ,M-Yield: Multi-branched isomer yield, M/S: M-Yield/ S-Yield.
At the reaction temperature of 220 ◦ C, the yield of isomeric heptane increases at the beginning 2.2. Effect of WO3 on as Pt/WZ and then decreases the Cr content increases. When the Cr content increases from zero to 1.0 wt%, the isomeric heptane yield increases from 40.5% to 73.7%. Further increase of the Cr content to 1.4 wt% 2.2.1.not Characterization of theyield Catalysts does bring an apparent change. Upon an increase to 1.8 wt%, the yield decreases to 68.9%. BasedThe onXRD Figure 1, theofpeak intensity of the tetragonal endows thea catalyst with pattern the Cr-Pt/WZ catalyst is shown ZrO in Figure 4. Itthat is known that characteristic 2 phase its catalytic activity be is the strongest, andits thecontent catalystishas the strongest acidity and reduction WO 3 peak cannot observed when lower than 18 wt% because WO3 is ability, highly resulting the best catalytic activity. dispersedinon the surface of the catalyst and no WO3 crystals are formed. WO3 exists on the surface of the catalyst due to the stable W-O-Zr bonding [21]. W-O-Zr bonding prevents the tetragonal ZrO2 2.2. Effect of WO3 on Pt/WZ from transforming to a monoclinic one. It can be observed in Figure 3 that the catalyst with 18 wt% WO3 Characterization has a stronger diffraction intensity of tetragonal ZrO2 and a weaker diffraction intensity of 2.2.1. of the Catalysts WO3. Thus, it will form a relatively stable solid superacid structure resulting in a higher catalyst TheatXRD pattern of the Cr-Pt/WZ catalyst is shown in Figure 4. It is known that a characteristic activity a lower temperature. WO3 peak cannot be observed when its content is lower than 18 wt% because WO3 is highly dispersed on the surface of the catalyst and no WO3 crystals are formed. WO3 exists on the surface of the catalyst due to the stable W-O-Zr bonding [21]. W-O-Zr bonding prevents the tetragonal ZrO2 from transforming to a monoclinic one. It can be observed in Figure 3 that the catalyst with 18 wt% WO3 has a stronger diffraction intensity of tetragonal ZrO2 and a weaker diffraction intensity of WO3 . Thus, it will form a relatively stable solid superacid structure resulting in a higher catalyst activity at a lower temperature.
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*: WO3
T
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M M
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Figure 4. 4. XRD patterns of of thethe Cr-Pt/WZ catalysts with different WO 3 contents. Figure XRD patterns Cr-Pt/WZ catalysts with different WO 3 contents.
The specific area, pore size, and volume of the Pt/WZ samples with different WO3 contents The specific area, pore size, and volume of the Pt/WZ samples with different WO3 contents were were analyzed, and the results are presented in Table 2. The results show that the catalyst with analyzed, and the results are presented in Table 2. The results show that the catalyst with approximately 15 wt% WO3 has a relatively large pore diameter and higher specific surface area. approximately 15 wt% WO3 has a relatively large pore diameter and higher specific surface area. The The highly dispersed WO3 on the surface of the catalyst prevents the catalyst from agglomeration highly dispersed WO3 on the surface of the catalyst prevents the catalyst from agglomeration at high at high temperatures [22]. In addition, WO3 can also increase the exposed structure of the catalyst, temperatures [22]. In addition, WO3 can also increase the exposed structure of the catalyst, resulting resulting in a high specific surface area. However, as the WO3 content increases, the WO3 is found as in a high specific surface area. However, as the WO3 content increases, the WO3 is found as coke coke deposition and forms the WO3 crystal. The WO3 crystals are dispersed in the catalyst channel, deposition and forms the WO3 crystal. The WO3 crystals are dispersed in the catalyst channel, leading to the reduction of the catalyst specific surface area. The specific surface area of the Cr-Pt/WZ leading to the reduction of the catalyst specific surface area. The specific surface area of the decreases when the content of WO3 increased to 24 wt%. Cr-Pt/WZ decreases when the content of WO3 increased to 24 wt%. Table 2. The specific surface area and pore size and volume of the Cr-Pt/WZ catalyst with different Table 2. The specific surface area and pore size and volume of the Cr-Pt/WZ catalyst with different WO3 contents. WO3 contents. Sample
S
/(m2 ·g−1 )
BET 2 −1 Sample SBET/(m ∙g ) Cr-Pt/12 wt%WZ 102.8 Cr-Pt/12 wt%WZ 102.8 Cr-Pt/15 wt%WZ 128.3 Cr-Pt/15 wt%WZ 128.3 Cr-Pt/18 wt%WZ 103.5 Cr-Pt/18 wt%WZ 103.5 Cr-Pt/21 wt%WZ 89.54 Cr-Pt/24 wt%WZ 75.21 Cr-Pt/21 wt%WZ 89.54 Cr-Pt/24 wt%WZ 75.21
V /(cm3 ·g−1 )
P 3∙g−1) VP/(cm 0.180 0.180 0.182 0.182 0.177 0.177 0.166 0.124 0.166 0.124
dP /(nm)
dP/(nm)
6.86 6.86 6.90 6.856.90 6.426.85 6.576.42
6.57
The surface acidity of the catalyst was characterized by NH3 -TPD, and the effect of WO3 loading surface acidity of studied. the catalyst was characterized byin NH 3-TPD, and the effect of WO3 loading on theThe catalyst acidity was The results are shown Figure 5. The desorption peak area on the catalyst acidity was studied. The results are shown in Figure The desorption peak area of ◦ of NH3 at 350 C increases at first and then decreases with the increase5.in WO 3 content, indicating NH 3 at 350 °C increases at first and then decreases with the increase in WO3 content, indicating that that the medium-strong acidic sites increased at first and then decreased. The Cr-Pt/WZ catalyst thethe medium-strong acidic sitesthe increased at first is and thenwt%, decreased. The Cr-Pt/WZ catalyst hasthe the has most acidic sites when WO3 content 18–21 and therefore, the catalyst has most acidic sites when the WO 3 content is 18–21 wt%, and therefore, the catalyst has the greatest ◦ greatest isomerization activity. However, the desorption peak area of NH3 at 650 C increases with isomerization activity. However, the desorption peak area of NH3 at 650 °C increases with the the increase of WO 3 content, implying that the superstrong acid sites increase with WO3 content increase of WO 3 content, implying that the superstrong acid sites increase with WO3 content increasing. An appropriate WO3 content prevents the transformation of ZrO2 from the tetragonal increasing. An appropriate of ZrOresulting 2 from the tetragonal phase to the monoclinic phase.WO As 3a content result, itprevents increasesthe thetransformation acidity of the catalyst, in a higher phase to the monoclinic phase. As a result, it increases the acidity of the catalyst, resulting in a higher activity. However, with the increase in WO3 content, the WO3 on the surface of the catalyst aggregates activity. However, with the increase in WO 3 content, the WO3 on the surface of the catalyst and blocks the acidic sites of the catalyst. Thus, the activity of the catalyst decreases due to the aggregates andacid blocks acidic sites of catalyst. the catalyst. the activity of the catalyst decreases due decrease of the sitesthe number on the ThisThus, is approved by the results of the XRD and to the decrease of the acid sites number on the catalyst. This is approved by the results of the XRD BET characterizations. and BET characterizations.
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TCD signal (a.u.)
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Figure 5.Figure NH3 -TPD of the Cr-Pt/WZ withdifferent different 5. NH3curves -TPD curves of the Cr-Pt/WZcatalyst catalyst with WOWO 3 contents. 3 contents.
2.2.2. Catalytic ActivityActivity of the of Catalysts 2.2.2. Catalytic the Catalysts The performance of the Cr-Pt/WZ catalyst with 12–24 wt%WO WO33 was was evaluated and thethe results The performance of the Cr-Pt/WZ catalyst with 12–24 wt% evaluated and results are are shown in Figure 6. It can be observed that the isomer yield of n-heptane increases at the shown in Figure 6. It can be observed that the isomer yield of n-heptane increases at the beginning and beginning and then decreases with the increase of the WO3 content. At the reaction temperature of then decreases with the increase of the WO3 content. At the reaction temperature of 220 ◦ C, the isomer 220 °C, the isomer yield of n-heptane increases to 73.7% when the WO3 content increases to 18 wt%. yield of n-heptane 73.7% when the WO3 content increases to318 wt%.increases However, the isomer However, increases the isomer to yield of n-heptane decreases sharply when the WO content further. According to the XRD characterization and the state diagram of WOx on the WO 3 /ZrO 2 surface [23], yield of n-heptane decreases sharply when the WO3 content increases further. According to the XRD the catalyst in the growth zone ofof tungstate when loading of WO3 is 12–18 wt%. The catalyst characterization andis the state diagram WOx on thethe WO 3 /ZrO2 surface [23], the catalyst is in the activity increases as the WO3 content increases in this range. Then, the catalyst is found in the growth zone of tungstate when the loading of WO3 is 12–18 wt%. The catalyst activity increases as tungstate and WO3 region when the WO3 content is 24 wt%. As a result, the catalyst activity the WO3 content increases this range. Then, the catalyst is found in the tungstate and WO3 region decreases. When theinWO 3 content was 12 wt%, and no WO3 peak was observed in the catalyst when the WO content is 24 wt%. As a result,ZrO the2 catalyst activity decreases. When thecatalytic WO3 content phase was not well suppressed, and the sample, the formation of the monoclinic 3 activity low.3 peak The characteristic peakin of 3 begins to appear the catalyst, andmonoclinic the was 12 wt%, and was no WO was observed theWO catalyst sample, the in formation of the characteristic peak of the monoclinic ZrO 2 phase is weak for the WO3 content of 18 wt%. In this case, ZrO2 phase was not well suppressed, and the catalytic activity was low. The characteristic peak of WO3 on the catalyst surface effectively prevents the conversion of the tetragonal ZrO2 phase to the WO3 begins to appear in the catalyst, and the characteristic peak of the monoclinic ZrO2 phase is monoclinic phase. The tetragonal ZrO2 has a large specific surface area, forming more acidic sites, weak for the of 18 wt%. In thisstructure case, WO onthe thecatalyst catalyst effectively prevents 3 content and WO tetragonal ZrO2 acts a solid superacid so 3that has surface strong activity. the conversion of the tetragonal ZrO2 phase to the monoclinic phase. The tetragonal ZrO2 has a large specific surface area, forming more acidic sites, and tetragonal ZrO2 acts a solid superacid structure so that the catalyst has strong activity. Catalysts 2018, 8, x FOR PEER REVIEW
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6. The activity the Cr-Pt/WZ catalystswith with different different WO 3 contents. Reaction condition: Figure 6. Figure The activity of the ofCr-Pt/WZ catalysts WO 3 contents. Reaction condition: n(H2):n(C7) = 9 mol/mol, LHSV = 0.7−h1−1, and P = 1.0 MPa, T = 200–280 °C. ◦ n(H2 ):n(C7 ) = 9 mol/mol, LHSV = 0.7 h , and P = 1.0 MPa, T = 200–280 C.
2.3. Effect of Calcination Temperature on Catalyst Activity The calcination temperature is an important factor affecting the degree of reduction and dispersion of the WO3 over the WO3/ZrO2 catalyst. A suitable calcination temperature is a prerequisite for preparing a high activity catalyst. A suitable calcination temperature is generally considered to be between 600 and 900 °C.
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2.3. Effect of Calcination Temperature on Catalyst Activity The calcination temperature is an important factor affecting the degree of reduction and dispersion of the WO3 over the WO3 /ZrO2 catalyst. A suitable calcination temperature is a prerequisite for preparing a high activity catalyst. A suitable calcination temperature is generally considered to be between 600 and 900 ◦ C. 2.3.1. Characterization of the Catalysts The TG-DTA curves of Pt/WZ precursor catalyst are shown in Figure 7. The endothermic peak of the DTA curve between 100 and 300 ◦ C is attributed to the removal of adsorbed water and crystalline water from the catalyst surface. The weight loss rate of the catalyst is greater below 200 ◦ C, mainly due to the removal of the physically adsorbed water on the surface of the catalyst. The catalyst weight loss rate is relatively slow between, 450 and 600 ◦ C, which is mainly due to the decomposition of ammonium tungstate and changing to WO3 . The thermal weight loss peak at approximately 600 ◦ C is attributed to the formation of tetragonal ZrO2 phase crystals. The catalyst weight loss rate is low and the weight tends to be constant for temperatures greater than 600 ◦ C. Therefore, the minimum calcination temperature of the catalyst is set as 600 ◦ C. Catalysts 2018, 8, x FOR PEER REVIEW 8 of 15 2
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Temperature/ °C Figure 7. 7. TG-DTA TG-DTA curves curves of of the the Pt/WZ Pt/WZprecursor precursorcatalyst. catalyst. Figure
Intensity(a.u.)
It is observed from Figure 8 that the intensity of the characteristic peak of WO3 gradually increases as the calcination temperature increases. There is a strong characteristic peak of tetragonal ZrO2 , and no : WO3 monoclinic ZrO2 phase and WO3Tcrystals are detected when*the calcination temperature is lower T: Tetragonal ZrO2 than 600–700 ◦ C. This indicates M that WO3 has good dispersibility on the surface of the catalyst at the M: Monoclinic ZrO2 M T calcination temperature range. inhibit the transformation ∗ The highly T dispersed WO3 may effectively T of the metastable tetragonal ZrO2 . It could also be that the tetragonal ZrO is more stable and 2 phase 900 °C does not convert to a monoclinic phase at low temperature. The monoclinic ZrO2 does not show isomerization catalytic activity so that the Cr-Pt/WZ catalyst has the preferred n-heptane isomerization catalytic structure. The monoclinic ZrO2 and WO3 crystals begin to form at800 the°C calcination temperature of 800 ◦ C. Sharp WO3 and monoclinic ZrO2 characteristic peaks appear in the catalyst prepared at the calcination temperature of 900 ◦ C. The catalyst pores collapse, the active components on the surface of 700 °C the catalyst appears to be sintered, and the crystallinity is higher under a high calcination temperature, which leads to the decrease of the catalyst activity in the n-heptane isomerization reaction. 600 °C 20
30
40
2θ/ °
50
60
70
5.8 0
100
200
300
400
500
600
700
-12 900
800
Temperature/ °C Catalysts 2018, 8, 522
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Figure 7. TG-DTA curves of the Pt/WZ precursor catalyst.
*: WO3
T M
∗
M
T
Intensity(a.u.)
T
T: Tetragonal ZrO2 M: Monoclinic ZrO2 T 900 °C
800 °C
700 °C
600 °C 20
30
40
2θ/ °
50
60
70
Figure 8. XRD patterns of the Cr-Pt/WZ catalyst calcined at 600~800 ◦ C. Figure 8. XRD patterns of the Cr-Pt/WZ catalyst calcined at 600~800 °C.
The specific surface area and pore size and volume of Cr-Pt/WZ catalysts, prepared at different The specific surfaceare area and pore size3.and Cr-Pt/WZ catalysts, at catalyst different calcination temperatures, shown in Table It isvolume shown of that the specific surfaceprepared area of the 2 − 1 calcination temperatures, are shown in Table 3. It is shown that the specific surface area of the is gradually reduced from 129.8 to 73.2 m ·g when the calcination temperature is increased from 2 −1 ◦ ◦ catalyst gradually reducedthe from 129.8 to 73.2 m ∙g when the calcination temperature is increased 600 C to is 800 C. Meanwhile, pore volume is gradually reduced, and the pore size decreases and from 600 °C to 800 °C. Meanwhile, the pore volume is gradually reduced, and the pore size decreases then increases. It is observed that WO3 crystals increasingly form on the catalyst surface at first with and then increases. It is observed that WOso 3 crystals form catalyst surface at first the increase of the calcination temperature that the increasingly catalyst pores andon thethe pore diameter decrease. with characteristic the increase of the calcination that the catalyst pores and the pore diameter Sharp peaks of WO3 andtemperature monoclinic so ZrO appear, and the catalyst pores collapse at 2 ◦ decrease. Sharp characteristic peaks of WO 3 and monoclinic ZrO 2 appear, and the catalyst pores the calcination temperature of 900 C. As a result, the specific surface area, pore volume, and pore collapseofatthe thecatalyst calcination temperature of 900 As asites result, specific surface area, pore volume, diameter decrease sharply, and the°C. active andthe acidic sites in the catalyst decrease so andthe pore diameter of the catalyst decrease sharply, and the active sites and acidic sites in the catalyst that catalytic activity decreases. decrease so that the catalytic activity decreases. Table 3. Surface area and pore size and volume of the Cr-Pt/WZ catalysts calcined at 600–800 ◦ C. Calcination Temperature/(◦ C)
SBET /(m2 ·g−1 )
VP /(cm3 ·g−1 )
dP /(nm)
600 700 800 900
129.8 107.9 103.5 73.2
0.246 0.189 0.177 0.120
5.81 6.20 6.85 5.20
The NH3 -TPD curves of the Cr-Pt/WZ catalyst calcined at 600~800 ◦ C are shown in Figure 9. It is observed from the figure that the area of the NH3 desorption peak increases at first and then decreases with the increase of the calcination temperature. This indicates that the total number of acid sites on the catalyst increases at first and then decreases. It is also observed from the figure and table that WO3 has good dispersibility on the surface of the catalyst and the Cr-Pt/WZ catalyst has a large specific surface area, resulting in a greater number of acidic sites. The catalyst has a strong acidic structure and good catalytic activity at a lower reaction temperature for the calcination temperature of 800 ◦ C. However, the structure of the catalyst changes strongly, and the acidity is weakened when the calcination temperature of the catalyst is too high.
TCD signal (a.u.)
decreases with the increase of the calcination temperature. This indicates that the total number of acid sites on the catalyst increases at first and then decreases. It is also observed from the figure and table that WO3 has good dispersibility on the surface of the catalyst and the Cr-Pt/WZ catalyst has a large specific surface area, resulting in a greater number of acidic sites. The catalyst has a strong acidic structure and good catalytic activity at a lower reaction temperature for the calcination Catalyststemperature 2018, 8, 522 of 800 °C. However, the structure of the catalyst changes strongly, and the acidity 9isof 15 weakened when the calcination temperature of the catalyst is too high.
800 °C 700 °C
600 °C 900 °C 200
300
400
500
Temperature/ °C
600
700
FigureFigure 9. The9.NH -TPD curves of Cr-Pt/WZ catalyst calcined at 600~800 ◦ C. The3NH 3-TPD curves of Cr-Pt/WZ catalyst calcined at 600~800 °C.
2.3.2. Catalytic Activity of the Catalysts
2.3.2. Catalytic Activity of the Catalysts
◦ C is shown in Figure 10. It is observed The activity of Cr-Pt/WZ catalysts calcined The activity of Cr-Pt/WZ catalysts calcinedatat600~800 600~800 °C is shown in Figure 10. It is observed ◦ ◦ C to that when reaction temperature is 220 C and thethe calcination temperature 600°C that the when the reaction temperature is 220 °C and calcination temperatureincreases increases from from 600 ◦ 800 C,tothe conversion of n-heptane increases increases from 13.3% to 13.3% 81.5%.toHowever, the heptane 800 °C, the conversion of n-heptane from 81.5%. However, the conversion heptane ◦ conversion ratetodrops when the temperature calcination temperature is increased °C.yield The of rate drops sharply 3.2%sharply when to the3.2% calcination is increased to 900 to C.900 The yield of isomeric heptane theas same trend as the of conversion of n-heptane. yield increases isomeric heptane has the same has trend the conversion n-heptane. The yield The increases from 13.3% from to 73.7% in the 800 °C and then sharply decreasesatsharply at 900 °C. According to to 73.7% in 13.3% the range of 600 torange 800 ◦of C 600 andtothen decreases 900 ◦ C. According to catalyst catalyst characterization results, the catalyst pore size gradually increased with the increase in the characterization results, the catalyst pore size gradually increased with the increase in the calcination calcination temperature. The isomeric heptane intermediate product is more easily diffused in the temperature. The isomeric heptane intermediate product is more easily diffused in the larger channel larger channel so that the catalyst activity gradually increases. The characteristic peak of the so thattetragonal the catalyst activity gradually increases. The characteristic peak of the tetragonal ZrO2 has a ZrO2 has a high intensity at the calcination temperature of 600–700 °C. However, Pt loses ◦ C. However, Pt loses some metallicity on high intensity at the temperature of 600–700 some metallicitycalcination on the tetragonal ZrO2 phase and maintains high metallicity on the monoclinic the tetragonal ZrO andactivity maintains metallicity the monoclinic [24]. than Therefore, 2 phase the phase [24]. Therefore, of thehigh Cr-Pt/WZ catalyston calcined at 600–700 phase °C is lower the ◦ the activity ofof the catalyst calcined activity theCr-Pt/WZ catalyst calcined at 800 °C. at 600–700 C is lower than the activity of the catalyst ◦ C. calcined at 2018, 800 8, Catalysts x FOR PEER REVIEW 10 of 15 100
Conversion/%
80 60
600 °C 700 °C 800 °C 900 °C
40 20
iC7-Isomer Yield/%
0 80
60
40
20
0
200
220
240
260
Temperature/ °C
280
300
◦ C. Reaction condition: n(H ):n(C ) Figure 10. Activities of the Cr-Pt/WZ 2 7) 7 Figure 10. Activities of the Cr-Pt/WZcatalysts catalystscalcined calcinedat at 600~800 600~800 °C. Reaction condition: n(H2):n(C −1 and P = 1.0 MPa, T = 200–300 ◦ C. = 5 mol L/mol, LHSV = =0.7 P = 1.0 MPa, T = 200–300 °C. = 5 mol L/mol, LHSV 0.7hh−1, and
2.4. Catalyst Stability Study The stability of the catalyst is crucial for the life of the catalyst. As is shown in Figure 11, the stability of Cr-Pt/WZ catalysts with the Cr content of 1.4 wt% and the Pt/WZ catalysts were studied in n-heptane isomerization at the reaction temperature of 220 °C, pressure of 1.0 MPa, ratio of n(H2):n(C7) = 9 mol/mol, and LHSV = 1.0 h−1. The conversion rate of n-heptane and the yield of
Temperature/ °C Figure 10. Activities of the Cr-Pt/WZ catalysts calcined at 600~800 °C. Reaction condition: n(H2):n(C7) = 5 mol L/mol, LHSV = 0.7 h−1, and P = 1.0 MPa, T = 200–300 °C.
2.4. Catalyst Catalysts 2018, 8,Stability 522
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The stability of the catalyst is crucial for the life of the catalyst. As is shown in Figure 11, the stability of Cr-Pt/WZ catalysts with the Cr content of 1.4 wt% and the Pt/WZ catalysts were studied 2.4. Catalyst Stability Study in n-heptane isomerization at the reaction temperature of 220 °C, pressure of 1.0 MPa, ratio of stability of the catalyst is crucial for the life of the catalyst. As is shown in Figure 11, n(HThe 2):n(C7) = 9 mol/mol, and LHSV = 1.0 h−1. The conversion rate of n-heptane and the yield of the stability of Cr-Pt/WZ catalysts withstable. the CrCompared content ofto 1.4the wt% and the Pt/WZ wereof heterogeneous heptane are relatively Pt/WZ catalyst, thecatalysts conversion ◦ studied in n-heptane isomerization at the reaction temperature 220 C, pressure 1.0significantly MPa, ratio n-heptane and the yield of heterogeneous heptane over the of Cr-Pt/WZ catalyst ofare −1 . The conversion rate of n-heptane and the yield ofhigher. n(H2 ):n(C ) = 9 mol/mol, and LHSV = 1.0 h 7 Moreover, the Pt/WZ catalyst was deactivated after the reaction was carried out for 300 h. ofHowever, heterogeneous heptane catalyst are relatively to is thea Pt/WZ catalyst, the conversion of the Cr-Pt/WZ showsstable. good Compared stability and promising approach for industrial n-heptane and the yield of heterogeneous heptane over the Cr-Pt/WZ catalyst are significantly higher. application. Moreover, the Pt/WZ catalyst was deactivated after the reaction was carried out for 300 h. However, the Cr-Pt/WZ catalyst shows good stability and is a promising approach for industrial application. 80
Conversion and Yield/%
70 60 50 40 30
Conversion of Cr-Pt/WZ catalysts Conversion of Pt/WZ catalysts
20
iC7 Yield of Pt/WZ catalysts iC7 Yield of Cr-Pt/WZ catalysts
10 0
100
200
300
400
500
Reaction Time/ h Catalysts 2018, 8, x FOR PEER REVIEW Figure 11. Study on stability of Cr-Pt/WZ catalyst.
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Figure 11. Study on stability of Cr-Pt/WZ catalyst.
It can can be beseen seenfrom fromSEM SEMimages imagesofof catalyst before after reaction (shown in Figure 12) It thethe catalyst before andand after reaction (shown in Figure 12) that that the particles on the surface of the catalyst are more uniformly dispersed before the reaction, and the particles on the surface of the catalyst are more uniformly dispersed before the reaction, and some some particles the catalyst are aggregated the reaction. agglomeration catalyst causes particles of theofcatalyst are aggregated afterafter the reaction. TheThe agglomeration of of catalyst causes a a decrease the catalyst activity and the catalyst gradually deactivated with time. decrease ofof the catalyst activity and the catalyst is is gradually deactivated with time.
Before
After
Figure 12. SEM of the Cr-Pt-WZ catalyst before and after the reaction. Magnification of the electron Figure 12. SEM of the Cr-Pt-WZ catalyst before and after the reaction. Magnification of the electron microscope: 10,000. microscope: 10,000.
2.5. Application of Catalyst in the Industrial Raw Material Isomerization To study the activity of the Cr-Pt/WZ catalyst over industrial raw materials, the isomerization reaction was carried out using lighter naphtha fractions above 75 °C under the condition of 220 °C, 1.0 MPa, n(H2):n(C7) = 9 mol/mol, and LHSV = 1.0 h−1. Tables 4–6 show the PIONA(paraffins, isoparaffins, olefins, naphthenes, aromatics, and oxygenates) composition of the lighter naphtha raw material, and the reaction products after 48 h and 100 h operation, respectively. It is observed
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2.5. Application of Catalyst in the Industrial Raw Material Isomerization To study the activity of the Cr-Pt/WZ catalyst over industrial raw materials, the isomerization reaction was carried out using lighter naphtha fractions above 75 ◦ C under the condition of 220 ◦ C, 1.0 MPa, n(H2 ):n(C7 ) = 9 mol/mol, and LHSV = 1.0 h−1 . Tables 4–6 show the PIONA(paraffins, isoparaffins, olefins, naphthenes, aromatics, and oxygenates) composition of the lighter naphtha raw material, and the reaction products after 48 h and 100 h operation, respectively. It is observed that the straight chain alkanes are heterogeneous products of cyclanes, aromatics, or a smaller number of carbon atoms when the Cr-Pt/WZ catalyst is used. The chromatographic octane number of the product increased from the initial 30.6 to 58.9 after 48 h operation and 57.6 after 100 h operation, respectively. It can also be seen from the table that the isomerization works better for C7 and smaller moleculars. It is recommended to separate C5 to C7 from the raw materials and then feed it into the isomerize process. The contents of n-C5, i-C5, n-C6, and i-C6 in the products increase, indicating that bigger molecules such as C8, C9, and C10, are relatively easy to crack. The appearance of C3 and C4 indicates that cracking occurs to a certain degree during the isomerization process. The C3 and C4 content changes in the products indicates that the cracking activity decreases and the isomerization activity also decreases slightly as the reaction progresses. After 100 h of the light naphtha feedstock reaction, the catalyst was not deactivated, and the combined catalyst was not deactivated after 500 h of n-heptane isomerization reaction, indicating that the catalyst has good stability. Table 4. The PIONA composition of the light naphtha raw material above 75 ◦ C. CNum
N-P
I-P
O
N
A
Total
C-5 C-6 C-7 C-8 C-9 C-10 C-11
0.25 4.55 9.92 11.39 8.95 1.84 0.26
0.18 4.04 14.40 16.87 8.51 5.16 1.34
0.00 0.04 0.62 0.00 0.00 0.00 0.00
0.36 2.82 1.33 2.46 3.25 0.36 0.00
0.00 0.02 0.11 0.61 0.10 0.08 0.00
0.79 11.47 26.38 31.33 20.81 7.44 1.60
Table 5. The product of light naphtha Isomerization (48 h). CNum
N-P
I-P
O
N
A
Total
C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10
0.37 1.55 2.11 6.04 7.14 1.32 0.32 0.03
0.00 6.72 10.93 14.36 32.84 6.21 0.79 0.08
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 1.55 3.19 3.21 1.18 0.00
0.00 0.00 0.00 0.00 0.01 0.08 0.00 0.00
0.37 8.27 13.04 21.95 43.18 10.82 2.29 0.11
Table 6. The product of light naphtha Isomerization (100 h). CNum
N-P
I-P
O
N
A
Total
C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10
0.22 1.16 1.91 6.58 8.05 1.35 0.26 0.02
0.00 4.76 9.96 12.96 32.79 6.46 0.89 0.06
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 2.01 1.70 3.91 3.54 1.30 0.00
0.00 0.00 0.00 0.00 0.01 0.09 0.00 0.00
0.22 5.92 13.88 21.24 44.76 11.44 2.45 0.08
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3. Experimental 3.1. Catalyst Preparation The mixed hydroxide Cr(OH)3 -Zr(OH)4 was prepared from ZrOCl2 -containing Cr(NO3 )3 solution by dropwise addition of an ammonia water solution under vigorous stirring up to pH 9~10. The precipitated hydrogel was aged at room temperature for 24 h and refluxed at 100 ◦ C for 24 h. Then, it was filtered and washed with deionized water repeatedly until no chloride ions were detected. The gel was dried at 80 ◦ C overnight. Cr2 O3 /WO3 /ZrO2 was obtained by impregnation of aqueous ammonium tungstate, drying at 80 ◦ C overnight, and calcining at 800 ◦ C in static air for 3 h. Pt/Cr2 O3 /WO3 /ZrO2 was prepared by Cr2 O3 /WO3 /ZrO2 infiltration with an H2 PtCl6 solution with 0.35 wt% Pt content in the catalyst followed by drying at 80 ◦ C overnight and calcining at 450 ◦ C in static air for 3 h. The Pt/WO3 /ZrO2 catalyst was prepared by the same method mentioned above. The catalysts prepared were designated as Cr-Pt/WZ and Pt/WZ, respectively. 3.2. Catalyst Characterization X-ray diffraction (XRD) patterns of the catalysts were obtained using an XRD-7000 instrument (Shimadzu, Tokyo, Japan) with Cu-Kα radiation at 40 kV and 30 mA and 2θ = 20–70(◦ ), with steps of 4(◦ )/min. The Brunner−Emmet−Teller measurements (BET) surface area and pore volume of the catalysts were obtained using an Autosorb-1 instrument (Quantachrome, FL, USA) using N2 as the adsorbent. The thermogravimetry-differential heat (TG-DSC) analysis for the samples was carried out using an SDT-Q600 thermal analyzer made by the American TA Company, (New Castle, KY, USA). The air was used as the carrier gas, the flow rate was 100 mL·min−1 , and the temperature range was 50~860 ◦ C. The temperature increasing rate was 10 ◦ C·min−1 . Detailed hydrocarbon analysis (DHA) of petroleum samples was obtained by an Agilent 7890 GC/FID (Santa Clara, CA, US) equipped with an Agilent 7683B Automatic Liquid Sampler, (Santa Clara, CA, USA), pre-fractionating inlet, ultra-high resolution column, flame ionization detector, and a data acquisition system. These testing methods are also referred to as PIONA because they can be used to characterize components by their hydrocarbon compound class (paraffins, isoparaffins, olefins, naphthenes, aromatics, and oxygenates). The acidity of the catalyst was analyzed by NH3 -programmed desorption using Micromeritics Autochem HP2920 (Atlanta, GA, USA) automatic multi-function temperature-programming analyzer. The samples were heated in a helium flow with 10% vol O2 . up to 700 ◦ C and were kept at that temperature for one h. Then, the samples were cooled in flowing helium at 100 ◦ C and ammonia adsorption was carried out for one h using a mixture gas of 10% vol. NH3 and 90% vol. helium. Weakly bounded ammonia was removed by blowing-off using helium gas at a temperature of 100 ◦ C for one h. The temperature was then raised to 700 ◦ C at a rate of 10 ◦ C·min−1 in a helium gas flow, and desorption was recorded to obtain a NH3 -TPD curve. 3.3. Catalytic Tests and Product Analysis 3.3.1. Catalytic Tests The diagram of n-heptane isomerization reaction devices is shown in Figure 13. The isomerization of n-heptane was performed in a fixed-bed flow reactor at a pressure of 1.0 MPa and a temperature of 200–260 ◦ C. Before starting the reaction, the catalyst was pretreated in flowing air at 450 ◦ C for three h and then reduced in flowing hydrogen at 250 ◦ C for two h. Then, the reactor was cooled to the reaction temperature and n-heptane was fed at a liquid hourly space velocity (LHSV) of 0.7 h−1 and an H2 /n-heptane ratio of nine (mol/mol).
The diagram of n-heptane isomerization reaction devices is shown in Figure 13. The isomerization of n-heptane was performed in a fixed-bed flow reactor at a pressure of 1.0 MPa and a temperature of 200–260 °C. Before starting the reaction, the catalyst was pretreated in flowing air at 450 °C for three h and then reduced in flowing hydrogen at 250 °C for two h. Then, the reactor was cooled to the reaction temperature and n-heptane was fed at a liquid hourly space velocity (LHSV) Catalysts 2018, 8, 522 13 of 15 of 0.7 h−1 and an H2/n-heptane ratio of nine (mol/mol). PI
PI
4 MFC
3
Hydrogen PI
PI
4
PI
PI
4
TIC
TI
TIC
5
TI
TIC
MFC
3
Air
TI
7
5
MFC
3
Nitrogen
PI
5
PI
9
Vent
8 5
6
LG
2
Product
PI
1
Figure 13. PID (Piping & Instrument Diagram) diagram of the experimental device of n-heptane Figure 13. PID (Piping & Instrument Diagram) diagram of the experimental device of n-heptane isomerization. 1 feedstock tank, 2 feed pump, 3 pressure reducing valve, 4 mass flowmeter, 5 check isomerization. 1 feedstock tank, 2 feed pump, 3 pressure reducing valve, 4 mass flowmeter, 5 check valve, 6 reactor, 7 loop condenser, 8 products tank, and 9 back pressure valve. valve, 6 reactor, 7 loop condenser, 8 products tank, and 9 back pressure valve.
3.3.2. Product Analysis 3.3.2. Product Analysis The reaction products were qualitatively analyzed by gas chromatography-mass spectrometry The reaction products were qualitatively byAgas spectrometry (6890/5973N, Agilent Technologies, Santa Clara,analyzed CA, USA). gaschromatography-mass chromatograph (GC-14C, Shimadzu (6890/5973N, Agilent Technologies, Santa Clara, CA, USA). A gas chromatograph (GC-14C, Corporation, Tokyo, Japan) was used and the n-heptane conversion and its isomer yield in the Shimadzu Corporation, Tokyo, Japan) was used and the n-heptane conversion and its isomer experiment were analyzed. The catalyst evaluation is based on the conversion of n-heptaneyield and in the experiment were analyzed. The catalyst evaluation is based on the conversion of n-heptane the total yield of isomeric heptane (referred to as iC7 ) as the catalyst activity evaluation standard. and the total yield of method isomericis heptane (referred to as iC7) as the catalyst activity evaluation The specific calculation as follows: standard. The specific calculation method is as follows: N-heptane conversion rate: N-heptane conversion rate: ω (nC7 ) − ω (nC7 ) X = 𝜔 𝑛𝐶 Reactants − 𝜔 𝑛𝐶 Products × 100% (1) (1) 𝑋= × 100% ω (nC7 ) Reactant 𝜔 𝑛𝐶
Total selectivity of isomeric heptane: heptane: S=
∑ ω (iC7 ) × 100% ∑ ω (iC7 ) + ω (C1 − C6 )
(2)
Total yield of isomeric heptane: Y = X × S × 100%
(3)
The ω(nC7 )Reactants and ω(nC7 )Products in the formula are the raw materials and the n-heptane mass fraction in the product, and ω(iC7 ) and ω(nC1 − C6 ) are the mass fractions of the isomeric heptane and C1 − C6 cleavage products in the product. 4. Conclusions Different amounts of the Cr transition metal were loaded to the Pt/WZ catalyst. The Pt/WO3 / ZrO2 catalyst with 1.4 wt% Cr loading shows stronger characteristic peaks of the tetragonal ZrO2 phase, stronger acidity, and larger specific surface area and demonstrates better catalytic performance in n-heptane isomerization. The WO3 loading strongly affects the activity of the catalyst. The optimal catalyst activity was obtained at the WO3 loading of 18 wt%. The calcination temperature has an influence on the structure of the catalyst, and in turn, affects the activity of the catalyst. When the catalyst is calcined at 800 ◦ C, WO3 has good dispersibility over the surface of the catalyst; meanwhile,
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WO3 and ZrO2 form a relatively stable acidic structure, and the catalyst has better activity at lower reaction temperatures. The conversion of n-heptane reached 81.5%, and the isomeric heptane yield reached 73.7% over Cr-Pt/WO3 /ZrO2 catalyst under the conditions of 220 ◦ C, 1.0 MPa, n(H2 ):n(C7 ) = 9 mol/mol, and LHSV = 1.0 h−1 . The catalyst shows no deactivation in the n-heptane isomerization for 500 h and shows better stability than Pt/WO3 /ZrO2 . At the same time, Cr-Pt/WO3 /ZrO2 catalysts have good isomerization performance in industrial naphtha raw materials, and the chromatographic octane number increases by approximately 28 units through the isomerization process. Author Contributions: Conceptualization, H.J., and X.G.; Methodology, G.H.; Validation, R.Z., and Q.Z.; Writing-Original Draft Preparation, G.H.; Writing-Review & Editing, H.J. and X.G.; Visualization, S.Y.; Funding Acquisition, S.Y. Funding: This research was funded by Financial supported by the National Natural Science Foundation of China, grant number 91634101, 21601016; the Beijing Municipal Natural Science Foundation, grant number 2174073; and The Project of Construction of Innovative Teams and Teacher Career Development for Universities and Colleges under Beijing Municipality, grant number IDHT20180508. Conflicts of Interest: The authors declare no conflict of interest.
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5. 6. 7. 8. 9.
10. 11.
12. 13.
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