Missouri inmate Earl Ringo Jr. was put to death early Wednesday at the state penitentiary in Bonne Terre despite several last-minute appeals to overturn his death sentence, including a claim that the prison was using a controversial sedative drug linked to three recent botched executions in Ohio, Oklahoma and Arizona. Ringo’s execution marks Missouri’s eighth execution this year and the most of any state in 2014, the nonprofit Death Penalty Information Center said. Texas is scheduled to carry out its eighth execution Wednesday.

Ringo, 40, was convicted of murder 15 years ago after he and an accomplice killed two people while robbing a Ruby Tuesday restaurant in Columbia in the early hours of July 4, 1998. Ringo quoted a passage from the Quran about the afterlife as his final words before receiving a lethal injection at 12:22 a.m. Wednesday. He was pronounced dead at 12:31 a.m., the Associated Press reported.

Ringo’s attorneys had tried to stop the execution over concerns that Missouri’s use of a pre-execution sedative called midazolam would prevent Ringo from expressing pain or suffering during the execution process. Midazolam was the common link in three executions this year in which the inmates appeared to express pain. The state has claimed the sedative relieves pre-execution anxiety, but Ringo’s lawyers maintain the sedative is used to "mask the symptoms of the lethal injection drug,” NBC News reported Tuesday.

In January, Ohio inmate Dennis McGuire showed signs of distress, including gasping for air and snorting, for 26 minutes during the execution process before he died. A few months later, Oklahoma inmate Clayton Lockett took 43 minutes to die after administrators improperly inserted the needle that delivered the lethal injection, the AP reported. Joseph Wood’s execution in Arizona in July lasted two hours.

Ringo’s lawyers also raised questions over his being sentenced to death by an all-white jury. However, Missouri Gov. Jay Nixon and several courts ruled that the execution should continue.