60th Annual Report on Research 2015

Introduction. Steam crackers in the U.S. are shifting feedstock from petroleum-derived naphtha to shale gas-derived ethane, which will generate an abundance of ethene and a deficit of heavier alkenes (propene, butene) used as chemical precursors. Ethene dimerization provides an entry step to catalytic upgrading of ethene to heavier alkenes. The goal of this project is to study the kinetic details of ethene dimerization on Ni-exchanged molecular sieve catalysts, in order to determine how Ni²⁺ active site structures and micropore size influence rate constants for ethene dimerization. The specific objectives are to synthesize a suite of porous aluminosilicates (Al-zeolites) and zincosilicates (Zn-zeolites) with varying pore size, to exchange these supports with isolated Ni²⁺ cations (Fig. 1), and then to measure ethene dimerization turnover rates and rate constants on these Ni-zeolite materials.

Figure 1. The exchange of one extraframework Ni²⁺ site requires two framework Al³⁺ centers in a paired configuration (left; aluminosilicate) or one framework Zn²⁺ center (right; zincosilicate).

Current Progress. In year 1, we synthesized one aluminosilicate, Al-BEA (0.7 nm diam. pores), and three zincosilicates, Zn-MFI (0.55 nm diam. pores), Zn-BEA (0.7 nm diam. pores) and Zn-MCM-41 (2.0 nm diam. pores). All zeolites gave X-ray diffraction patterns and micropore volumes (N₂ adsorption isotherms, 77 K) consistent with the expected crystal topologies. These supports were exchanged with Ni²⁺ cations using aqueous-phase ion-exchange procedures (0.01-0.3 M Ni(NO₃)₂, 348 K, 5 h). Ni-exchanged zeolites were used to study the kinetic details of ethene dimerization. Here, we report the catalytic behavior of Ni-Al-BEA and Ni-Zn-BEA during ethene conversion at 453 K (0.01-1.0 kPa C₂H₄). On all Ni-zeolites, net rates of butene formation increased with time-on-stream (activation period), reached a maximum value, and then decreased with further time-on-stream (deactivation period). Data shown here were collected at reaction times corresponding to maximum net butene formation rates.

Ethene consumption rates (per Ni²⁺, 453 K, 0.1 kPa C₂H₄) were ~1000x larger on Ni-Al-BEA than on Ni-Zn-BEA (Table 1). The molar selectivity to butene isomers was 87% on Ni-Al-BEA and 82% on Ni-Zn-BEA (Table 1, Fig. 2). Linear butenes (1-butene, cis-2-butene, trans-2-butene) were formed on both catalysts in equilibrated amounts (Fig. 2). Ni-Al-BEA also formed smaller C₁-C₃ hydrocarbons and isobutene (Fig. 2), in non-equilibrated amounts with respect to linear butenes, reflecting side reactions mediated by residual H⁺ sites.

Table 1. Catalytic data for ethene conversion on Ni-Al-BEA (space velocity = 6.4 x 10^-7 mol C₂H₄ (mol Ni)^-1 s^-1) and Ni-Zn-BEA (space velocity = 5.3 x 10^-5 mol C₂H₄ (mol Ni)^-1 s^-1)  at 453 K and 0.1 kPa C₂H₄.

^aMeasured between 0.01-0.2 kPa C₂H₄.

^bMeasured between 0.1-0.2 kPa C₂H₄.

Figure 2. Product distribution, reported in terms of molar selectivities, during ethene conversion at 453 K (Table 1) on Ni-Al-BEA and Ni-Zn-BEA.

The dependence of net butene formation rates on ethene pressure was determined to be 0.8 on Ni-Al-BEA and 1.9 on Ni-Zn-BEA (Fig. 3, Table 1). The ethene reaction order of ~1 on Ni-Al-BEA is consistent with active sites saturated with one ethene-derived intermediate (e.g., Ni-ethyl), while the reaction order of ~2 on Ni-Zn-BEA is consistent with essentially unoccupied active sites. We conclude that the orders-of-magnitude higher ethene consumption rates measured on Ni-Al-BEA than on Ni-Zn-BEA (Table 1), under the conditions studied here, correspond to rates measured in different kinetic regimes and surface coverages that preclude direct kinetic comparisons. The prevalence of different kinetic regimes likely reflect differences in the strength and reactivity of Ni²⁺ sites exchanged at two framework Al centers (in Ni-Al-BEA) or at one framework Zn center (in Ni-Zn-BEA). In both first-order and second-order kinetic regimes, apparent rate constants for ethene dimerization (per Ni²⁺ site) should depend on the surrounding pore environment, which preferentially stabilize larger ethene dimerization transition states over smaller adsorbed precursors that are kinetically-relevant in these regimes.

Figure 3. Apparent reaction orders in ethene measured for Ni-Al-BEA and Ni-Zn-BEA (Table 1).

Future work will study apparent ethene reaction orders in wider ranges of pressures and temperatures and on all Ni-zeolite catalysts, in order to determine rates and rate constants that can be compared on different catalysts in equivalent kinetic regimes. We will investigate methods to titrate or exchange residual H⁺ sites in Ni-Al-BEA to suppress their catalytic contributions and isolate those arising solely from Ni²⁺ exchanged at two framework Al centers. We will also focus on spectroscopic characterization (infrared, X-ray absorption) and quantification (temperature-programmed reduction and titration/desorption techniques) of Ni and Zn sites in these materials.

Impact on Career and Participating Students. This award has enabled the PI to start a new research direction in his early career by providing full support for one graduate student (R. Joshi), and partial support for two additional graduate students and two undergraduate students. Based on this research, R. Joshi received a departmental scholarship (Phillips 66 Fellowship) for partial support in the 2015-16 academic year (year 2 of this grant). The PI presented this work at an invited seminar at the 2015 Chicago Catalysis Club Annual Meeting, and graduate and undergraduate students have presented this work in conferences at Purdue. This award has helped us generate preliminary results in the area of designing catalysts with improved selectivity for other chemistries relevant for shale gas conversion. For example, we are studying the zincosilicate materials developed in this project as catalysts for selective dehydrogenation of light alkanes sourced from shale gas. These results will be used in future grant proposals in the area of shale gas upgrading to be submitted to government funding agencies. In this manner, the research supported by this ACS PRF DNI grant will be leveraged to secure additional funds, sustain long-term research directions in our group, and continue the education of the next generation of scientists and engineers.