Effect of the Cr₂O₃ Promoter on Pt/WO₃-ZrO₂ Catalysts for n-Heptane Isomerization

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.

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].

SO₄²⁻/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 SO₄²⁻/ZrO₂, WO₃/ZrO₂ has become increasingly important since its discovery by Arata and Hino [6] because of its good stability both in reducing and oxidizing conditions [7]. While the WO₃/ZrO₂ type catalyst shows high selectivity in the isomerization reaction of linear alkane, the catalyst activity is relatively low and the degree of cracking is too high [8]. Thus, the catalyst needs to be modified to meet the industrial production requirements. The catalyst consisting of Pt deposited on WO₃/ZrO₂ for n-heptane isomerization has attracted the attention of a large number of researchers as the most promising catalyst [9,10]. The structure of the WO₃/ZrO₂ catalyst varies depending on the WO₃ content [11,12], leading to different catalyst activities. The structure and properties of this kind of catalyst can be modified by changing the WO₃ content [13]. While most researchers believe that the best catalytic activity is obtained when WO₃ is highly dispersed on the surface of the catalyst, there has been some debate about this subject. It was shown that the catalyst has better catalyst activity with the WO₃ density of seven–eight atoms∙nm⁻² [14,15]. Many researchers have found that the addition of small amounts of metals such as Fe, Al, In, and Ga to Pt/WO₃/ZrO₂ enhances catalysis and stability of n-heptane isomerization, and it was demonstrated that Cr can be used as a modifier in Pt/WO₃/ZrO₂ catalysts. Based on economic considerations, Cr metal is a more cost-effective option, and is therefore a more suitable for industrial production than Ga metal [16,17,18]. In this work, n-heptane isomerization over Cr-promoted Pt/WO₃/ZrO₂ was studied and compared to that over Pt/WO₃/ZrO₂.

2. Results and Discussion

2.1. Effect of Cr₂O₃ Loading on Pt/ WO₃/ZrO₂

2.1.1. Characterization of the Catalysts

The XRD patterns of Cr-Pt/WZ catalysts with different Cr loading are presented in Figure 1. All 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 catalyst exhibits lower diffraction peak intensities of monoclinic ZrO₂ than the Pt/WZ catalyst. The intensities of the diffraction peaks of the tetragonal ZrO₂ phase increase at first and then decrease with increasing of Cr loading. This result shows that the appropriate amount of Cr incorporation helps to stabilize the tetragonal ZrO₂ phase of the Pt/WZ catalysts and inhibits the transformation from the tetragonal phase to the monoclinic phase during calcination at high temperature, leading to higher isomerization activity [19]. The peaks of crystalline WO₃ almost disappear when the Pt/WZ catalysts were loaded with Cr. The interaction between Cr and WO₃ reduces the energy of crystalline WO₃ on the surface of the catalyst, promoting the dispersion of WO₃. Based on the experiments, the optimal Cr content is 1.0–1.4 wt%.

The specific surface area, pore size and volume of the Pt/WZ samples with different Cr contents were characterized using BET analysis, and the results are presented in

Table 1. The specific surface area and pore size and volume of the Cr-Pt/WZ catalysts with different Cr contents.

Sample	SBET/(m2∙g−1)	VP/(cm3∙g−1)	dP/nm
Pt/WZ	77.06	0.153	6.54
0.2 wt% Cr-Pt/WZ	80.94	0.161	6.53
0.6 wt% Cr-Pt/WZ	105.26	0.168	6.14
1.0 wt% Cr-Pt/WZ	103.5	0.177	6.85
1.4 wt% Cr-Pt/WZ	92.94	0.163	5.84
1.8 wt% Cr-Pt/WZ	89.94	0.161	5.83

. The addition of Cr changes the specific surface areas of WZ. It is shown that the optimal Cr content of approximately 1.0 wt% results in larger pore diameter and specific surface area, which in turn leads to more active sites and acid sites [20]. Thus, the Cr-Pt/WZ catalyst will become more active in n-heptane isomerization. The result also shows that Cr can improve the pore structure of the catalysts, but the dispersed oxides will be found in the pores of Pt/WZ as the Cr content increases.

The surface acidity of the catalyst was characterized by NH₃-TPD. The effect of Cr loading on the catalyst acidity was studied. As shown in Figure 2, the area of the desorption peaks at 350 °C and 650 °C increase at first and then decrease with the increase of the Cr content, indicating that the medium-strong acid sites and superacid sites on the catalyst increase at first and then decrease. The Cr-Pt/WZ catalyst with 1.0 wt% Cr has the most acidic sites. As a result, it has the greatest isomerization activity [21].

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. The conversion of n-heptane gradually increases with the increase of reaction temperature, indicating the increases of the isomerization rate. However, the isomerization equilibrium constant K_p decreases, which is unfavorable to the isomerization rearrangement reaction. At the same time, 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 relatively high, and products have a higher proportion of multi-branched isomers at the reaction 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 wt% does not 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 ability, resulting in the best catalytic activity.

2.2. Effect of WO₃ on Pt/WZ

2.2.1. Characterization of the Catalysts

The XRD pattern of the Cr-Pt/WZ catalyst is shown in Figure 4. It is known that a characteristic 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.

The specific area, pore size, and volume of the Pt/WZ samples with different WO₃ contents were analyzed, and the results are presented in

Table 2. The specific surface area and pore size and volume of the Cr-Pt/WZ catalyst with different WO₃ contents.

Sample	SBET/(m2∙g−1)	VP/(cm3∙g−1)	dP/(nm)
Cr-Pt/12 wt%WZ	102.8	0.180	6.86
Cr-Pt/15 wt%WZ	128.3	0.182	6.90
Cr-Pt/18 wt%WZ	103.5	0.177	6.85
Cr-Pt/21 wt%WZ	89.54	0.166	6.42
Cr-Pt/24 wt%WZ	75.21	0.124	6.57

. The results show that the catalyst with approximately 15 wt% WO₃ has a relatively large pore diameter and higher specific surface area. The highly dispersed WO₃ on the surface of the catalyst prevents the catalyst from agglomeration at high temperatures [22]. In addition, WO₃ can also increase the exposed structure of the catalyst, resulting in a high specific surface area. However, as the WO₃ content increases, the WO₃ is found as coke 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 decreases when the content of WO₃ increased to 24 wt%.

The surface acidity of the catalyst was characterized by NH₃-TPD, and the effect of WO₃ loading on the catalyst acidity was studied. The results are shown in Figure 5. The desorption peak area of NH₃ at 350 °C increases at first and then decreases with the increase in WO₃ content, indicating that the medium-strong acidic sites increased at first and then decreased. The Cr-Pt/WZ catalyst has the most acidic sites when the WO₃ content is 18–21 wt%, and therefore, the catalyst has the greatest isomerization activity. However, the desorption peak area of NH₃ at 650 °C increases with the increase of WO₃ content, implying that the superstrong acid sites increase with WO₃ content increasing. An appropriate WO₃ content prevents the transformation of ZrO₂ from the tetragonal 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 and blocks the acidic sites of the catalyst. Thus, the activity of the catalyst decreases due to the decrease of the acid sites number on the catalyst. This is approved by the results of the XRD and BET characterizations.

2.2.2. Catalytic Activity of the Catalysts

The performance of the Cr-Pt/WZ catalyst with 12–24 wt% WO₃ was evaluated and the results are shown in Figure 6. It can be observed that the isomer yield of n-heptane increases at the beginning and then decreases with the increase of the WO₃ content. At the reaction temperature of 220 °C, the isomer yield of n-heptane increases to 73.7% when the WO₃ content increases to 18 wt%. However, the isomer yield of n-heptane decreases sharply when the WO₃ content increases further. According to the XRD characterization and the state diagram of WOx on the WO₃/ZrO₂ surface [23], the catalyst is in the growth zone of tungstate when the loading of WO₃ is 12–18 wt%. The catalyst activity increases as the WO₃ content increases in this range. Then, the catalyst is found in the tungstate and WO₃ region when the WO₃ content is 24 wt%. As a result, the catalyst activity decreases. When the WO₃ content was 12 wt%, and no WO₃ peak was observed in the catalyst sample, the formation of the monoclinic ZrO₂ phase was not well suppressed, and the catalytic activity was low. The characteristic peak of WO₃ begins to appear in the catalyst, and the characteristic peak of the monoclinic ZrO₂ phase is weak for the WO₃ content of 18 wt%. In this case, WO₃ on the catalyst surface effectively prevents 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.

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.

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 monoclinic ZrO₂ phase and WO₃ crystals are detected when the calcination temperature is lower than 600–700 °C. This indicates that WO₃ has good dispersibility on the surface of the catalyst at the calcination temperature range. The highly dispersed WO₃ may effectively inhibit the transformation of the metastable tetragonal ZrO₂. It could also be that the tetragonal ZrO₂ phase is more stable and 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 at the 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 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.

The specific surface area and pore size and volume of Cr-Pt/WZ catalysts, prepared at different calcination temperatures, are shown in

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	129.8	0.246	5.81
700	107.9	0.189	6.20
800	103.5	0.177	6.85
900	73.2	0.120	5.20

. It is shown that the specific surface area of the catalyst is gradually reduced from 129.8 to 73.2 m²∙g⁻¹ when the calcination temperature is increased from 600 °C to 800 °C. Meanwhile, the pore volume is gradually reduced, and the pore size decreases and then increases. It is observed that WO₃ crystals increasingly form on the catalyst surface at first with the increase of the calcination temperature so that the catalyst pores and the pore diameter decrease. Sharp characteristic peaks of WO₃ and monoclinic ZrO₂ appear, and the catalyst pores collapse at the calcination temperature of 900 °C. As a result, the specific surface area, pore volume, and pore diameter of the catalyst decrease sharply, and the active sites and acidic sites in the catalyst decrease so that the catalytic activity decreases.

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.

2.3.2. Catalytic Activity of the Catalysts

The activity of Cr-Pt/WZ catalysts calcined at 600~800 °C is shown in Figure 10. It is observed that when the reaction temperature is 220 °C and the calcination temperature increases from 600 °C to 800 °C, the conversion of n-heptane increases from 13.3% to 81.5%. However, the heptane conversion rate drops sharply to 3.2% when the calcination temperature is increased to 900 °C. The yield of isomeric heptane has the same trend as the conversion of n-heptane. The yield increases from 13.3% to 73.7% in the range of 600 to 800 °C and then decreases sharply at 900 °C. According to catalyst characterization results, the catalyst pore size gradually increased with the increase in the calcination temperature. The isomeric heptane intermediate product is more easily diffused in the larger channel so that the catalyst activity gradually increases. The characteristic peak of the tetragonal ZrO₂ has a high intensity at the calcination temperature of 600–700 °C. However, Pt loses some metallicity on the tetragonal ZrO₂ phase and maintains high metallicity on the monoclinic phase [24]. Therefore, the activity of the Cr-Pt/WZ catalyst calcined at 600–700 °C is lower than the activity of the catalyst calcined at 800 °C.

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 heterogeneous heptane are relatively stable. Compared to the Pt/WZ catalyst, the conversion of n-heptane and the yield of heterogeneous heptane over the Cr-Pt/WZ catalyst are significantly higher. 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.

It can be seen from SEM images of the catalyst before and after reaction (shown in Figure 12) that the particles on the surface of the catalyst are more uniformly dispersed before the reaction, and some particles of the catalyst are aggregated after the reaction. The agglomeration of catalyst causes a decrease of the catalyst activity and the catalyst is gradually deactivated with time.

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.

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	0.25	0.18	0.00	0.36	0.00	0.79
C-6	4.55	4.04	0.04	2.82	0.02	11.47
C-7	9.92	14.40	0.62	1.33	0.11	26.38
C-8	11.39	16.87	0.00	2.46	0.61	31.33
C-9	8.95	8.51	0.00	3.25	0.10	20.81
C-10	1.84	5.16	0.00	0.36	0.08	7.44
C-11	0.26	1.34	0.00	0.00	0.00	1.60

,

Table 5. The product of light naphtha Isomerization (48 h).

CNum	N-P	I-P	O	N	A	Total
C-3	0.37	0.00	0.00	0.00	0.00	0.37
C-4	1.55	6.72	0.00	0.00	0.00	8.27
C-5	2.11	10.93	0.00	0.00	0.00	13.04
C-6	6.04	14.36	0.00	1.55	0.00	21.95
C-7	7.14	32.84	0.00	3.19	0.01	43.18
C-8	1.32	6.21	0.00	3.21	0.08	10.82
C-9	0.32	0.79	0.00	1.18	0.00	2.29
C-10	0.03	0.08	0.00	0.00	0.00	0.11

and

Table 6. The product of light naphtha Isomerization (100 h).

CNum	N-P	I-P	O	N	A	Total
C-3	0.22	0.00	0.00	0.00	0.00	0.22
C-4	1.16	4.76	0.00	0.00	0.00	5.92
C-5	1.91	9.96	0.00	2.01	0.00	13.88
C-6	6.58	12.96	0.00	1.70	0.00	21.24
C-7	8.05	32.79	0.00	3.91	0.01	44.76
C-8	1.35	6.46	0.00	3.54	0.09	11.44
C-9	0.26	0.89	0.00	1.30	0.00	2.45
C-10	0.02	0.06	0.00	0.00	0.00	0.08

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.

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. Cr₂O₃/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/Cr₂O₃/WO₃/ZrO₂ was prepared by Cr₂O₃/WO₃/ZrO₂ infiltration with an H₂PtCl₆ 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).

3.3.2. Product Analysis

The reaction products were qualitatively analyzed by gas chromatography-mass spectrometry (6890/5973N, Agilent Technologies, Santa Clara, CA, USA). A gas chromatograph (GC-14C, Shimadzu Corporation, Tokyo, Japan) was used and the n-heptane conversion and its isomer yield in the experiment were analyzed. The catalyst evaluation is based on the conversion of n-heptane and the total yield of isomeric heptane (referred to as iC₇) as the catalyst activity evaluation standard. The specific calculation method is as follows:

N-heptane conversion rate:

$$X=\frac{\omega \left(n{C}_7\right){}_{Reactants}-\omega \left(n{C}_7\right){}_{Products}}{\omega \left(n{C}_7\right){}_{Reactant}}\times 100\%$$

(1)

Total selectivity of isomeric heptane:

$$S=\frac{\sum \omega \left(i{C}_7\right)}{\sum \omega \left(i{C}_7\right)+\omega (C_1-C_6)}\times 100\%$$

(2)

Total yield of isomeric heptane:

$$Y=X\times S\times 100\%$$

(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, 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.