Article pubs.acs.org/EF
Enhancing Sulfur Tolerance of Pd Catalysts by Hydrogen Spillover with Two Different Zeolite Supports for Low-Temperature Hydrogenation of Aromatics Hyun Jae Kim and Chunshan Song* Clean Fuels and Catalysis Program, EMS Energy Institute, Department of Energy and Mineral Engineering, and Department of Chemical Engineering, The Pennsylvania State University, 209 Academic Projects Building, University Park, Pennsylvania 16802, United States ABSTRACT: Pd catalysts supported on zeolite Y and zeolite A (Pd/Y and Pd/A) were prepared separately and combined for improving sulfur tolerance based on our proposed catalyst design concept for low-temperature hydrogenation of aromatics in distillate fuels. After the external surface of Pd/A was passivated by chemical vapor deposition with silica, the resulting SiO2−Pd/ A catalyst does not show any catalytic activity for hydrogenation of tetralin. However, adding SiO2−Pd/A to Pd/Y by physical mixing significantly enhanced sulfur tolerance of the Pd catalyst for tetralin hydrogenation in the presence of 100 ppmw sulfur as benzothiophene. The small-pore system (Pd/A) in the hybrid catalyst plays a critical role to maintain the catalytic activity of metal sites in the large-pore system (Pd/Y) in the presence of sulfur, via hydrogen spillover from metal inside the zeolite A pore channel protected by size-selective exclusion of sulfur compounds.
1. INTRODUCTION Because a high aromatic content in distillate fuels lowers the fuel quality and contributes to the formation of environmentally harmful emissions in exhaust gases from combustion engines, reducing aromatic content has been a target for the transportation fuel regulation since 1993 and the diesel requirement limiting aromatics has been gradually extended from on-road vehicles to locomotives and marine engines.1−5 Noble metal catalysts are promising candidates for deep hydrogenation, owing to their high catalytic activity at low temperature, but they possess a serious problem, i.e., rapid deactivation, when they are exposed to sulfur even at a trace level.6 Previously, a sulfur-tolerant catalyst design concept was proposed by our group, which mainly focused on a noble metal catalyst supported on a shape-selective zeolite for aromatic hydrogenation in the presence of sulfur.5,7 According to the proposed concept, the catalysts are supported on zeolite material possessing bimodal pore size distributions. The large pores (>6 Å) allow for hydrogenation of multi-ring aromatics and hydrodesulfurization of organosulfur compounds, while bulky aromatics and organic sulfur are selectively excluded from the small pores (1 over the Pd catalyst with the large-pore openings of zeolite Y. Although welldispersed Pd metal active sites inside zeolite might be less affected by sulfur at a low sulfur concentration, they were seriously poisoned by a large amount of BT entering through the aperture of zeolite Y. On the other hand, the catalytic activity of the Pd/A + Pd/Y hybrid catalyst surpassed that of Pd/Y at high sulfur concentrations (S/Pd atomic ratio of >1), presenting significantly higher naphthalene conversion even at a S/Pd atomic ratio of >4 but a little less than Pd/Y at a low sulfur concentration. It should be noted that the mixture of zeolite Y (without Pd) and Pd/A did not show any conversion, even at a low sulfur concentration (56 ppmw). It was previously reported that the support without metal did not have any significant catalytic activity for hydrogenation,13,17 indicating that Pd metal sites on the zeolite Y support are the main active sites for naphthalene hydrogenation under this reaction condition. Furthermore, little or no catalytic activity was observed on the Pd/A catalyst, even at low sulfur concentrations. It seems that the metal sites on A-type zeolite are excluded from contacting with bulky naphthalene because they were located mainly inside the small-pore channel of zeolite A. Although some metal sites were present on the external surface of zeolite particles, they were immediately deactivated upon exposure to BT. Because the Pd/A catalyst did not have any significant catalytic activity for naphthalene hydrogenation, the higher sulfur tolerance of the Pd/A + Pd/Y hybrid catalyst can be mainly attributed to the synergistic interaction between Pd/A and Pd/Y catalysts. It may be possible that H2S, the product of BT hydrodesulfurization, diffuses into the smaller pore channels of zeolite A because the kinetic diameter of H2S is 3.6 Å, which is smaller than the pore opening of the Pd/A catalyst (4.0 Å). However, we could observe the evidence from this study that H2S is less detrimental to sulfur poisoning of the Pd catalyst than BT, which was previously proposed as “the thiophenic
3. RESULT AND DISCUSSION 3.1. Sulfur Tolerance of the Hybrid Catalyst. Table 1 shows the surface area and metal dispersion of zeolites Y- and A-supported Pd catalysts. Figure 1 presents naphthalene conversion in the presence of sulfur as BT at different concentrations. Because naphthalene conversions over all catalysts without BT reached 100%, only naphthalene conversions with BT-containing model fuel were plotted in Figure 1 to make a clear distinction among the different tests. Pd/Y shows the highest naphthalene conversion on the test with 56 ppm sulfur concentration. The high catalytic activity of Pd/Y can be mainly attributed to its high metal dispersion (42%), as shown in Table 1. However, the naphthalene 6789
dx.doi.org/10.1021/ef501541j | Energy Fuels 2014, 28, 6788−6792
Energy & Fuels
Article
genation. The preliminary test of tetralin hydrogenation was conducted over the Pd/Y catalyst, but no deactivation of the catalyst was observed for 20 h with sulfur-free feed, ensuring that the sulfur poisoning is the main reason for catalyst deactivation with BT-containing feed. It should be noted that trans- and cis-decalin were the only products found in the liquid product; neither the ring-opening reaction nor dehydrogenation of tetralin to naphthalene was observed under the conditions employed. Both the uncoated Pd/A and the SiO2-coated Pd/A were tested along with Pd/Y and hybride catalyst in the flow reactor for hydrogenation of sulfur-free tetralin in the first 3.5 h of flow tests, followed by that of 100 ppm sulfur-containing tetralin feed in the subsequent 6.5 h. As shown in Figure 2, tetralin conversion over an uncoated Pd/A catalyst drastically decreased after the introduction of 100 ppm sulfur feed. The Pd metal particles on the external surface of Pd/A as the active sites for tetralin hydrogenation were easily and immediately poisoned by introducing BT. Upon silica coating, SiO2−Pd/A did not show any catalytic activity even on the sulfur-free tetralin feed, indicating that the Pd metal particles on the external surface of zeolite A were completely passivated by silica coating and that the Pd sites inside zeolite A are not accessible for tetralin. Tetralin conversion with Pd/Y was 100% for the sulfur-free feed but decreased significantly upon switching to 100 ppmw sulfur feed and down to as low as 20% in 10 h time on stream (TOS). To investigate the effect of the addition of the small-pore Pd system to the large-pore Pd system, the hybrid catalyst was prepared by mixing SiO2−Pd/A (0.16 g) with the Pd/Y catalyst (0.34 g) in the same amount as that used for the test with single Pd/Y (0.34 g). Therefore, the amounts of Pd metal active sites for tetralin hydrogenation on both Pd/Y and SiO2−Pd/A + Pd/Y catalysts were the same considering that the SiO2−Pd/A catalyst alone shows no catalytic activity for tetralin conversion. The results in Figure 2 clearly demonstrate that adding SiO2− Pd/A increased the sulfur tolerance of the Pd/Y catalyst with 100 ppmw sulfur feed, maintaining a high catalytic activity with over 80% tetralin conversion even after 10 h TOS. This represents a dramatic improvement of sulfur tolerance, with a 4 times higher tetralin conversion at 10 h TOS (over 80% versus 20% observed on Pd/Y). It is clear that the Pd/Y catalyst could retain its catalytic activity in the presence of the silica-coated Pd/A, although the latter itself shows no catalytic activity for tetralin conversion. This greatly enhanced sulfur tolerance of Pd/Y is likely made possible by hydrogen spillover from SiO2− Pd/A to Pd/Y. Figure 3 depicts the trans- and cis-decalin yields from hydrogenation of tetralin over Pd/Y and SiO2−Pd/A + Pd/Y. In the absence of sulfur (at 2 h TOS), both catalysts showed consistently high yields of trans-decalin (>90%) as well as low yields of cis-decalin (
